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Cardiac physiology 

Cardiac physiology

Chapter:
Cardiac physiology
Author(s):

Rhys Evans

, Kenneth T. MacLeod

, Steven B. Marston

, Nicholas J. Severs

, and Peter H. Sugden

DOI:
10.1093/med/9780199204854.003.160103_update_002

Update:

This chapter has been extensively revised, and now includes content from retired Chapter 16.1.2.

Updated on 29 Oct 2015. The previous version of this content can be found here.
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Essentials

The function of the heart is to provide the tissues of the body with sufficient oxygenated blood and metabolites to meet the moment-to-moment needs as dictated by physical activity and postural and emotional changes.

Functional anatomy of the cardiac myocyte

Cardiac myocytes are the contractile cells of the heart and constitute the bulk of heart mass. There are differences between the myocytes of the ventricles, the atria, and the conduction system: ventricular myocytes are elongated cells, packed with myofibrils (the contractile apparatus) and mitochondria (for ATP production). Myofibrils are repeating units (sarcomeres) made up of thin actin filaments anchored at the Z-discs at either end of the sarcomere, and thick myosin filaments which interdigitate and interact with the thin filaments. Contraction results from sarcomere shortening produced by the ATP-dependent movement of the thin and thick filaments relative to one another. Transverse (T-) tubules facilitate extracellular Ca2+ entry into the cytoplasm (sarcoplasm) for signalling and contraction. Atrial myocytes differ from ventricular myocytes, having few T-tubules but more abundant caveolae. Myocytes of the conduction system are small and possess only a rudimentary myofibrillar structure.

Myocytes are attached to their neighbours and to the extracellular matrix to allow transmission of force. At some regions of contact (the intercalated discs), specialized structures (the gap junctions) contain channels which form contiguous electrical connections between a myocyte and its neighbours, and allow passage of ions and small molecules.

Cardiac action potential

A potential difference (the membrane potential) is maintained across the plasma membrane (sarcolemma) such that the inside of the cell is negative compared to the outside by about 90 mV. This is caused largely by the efflux of K+ from the cell through K+ channels and down its concentration gradient until the electronegative force retaining K+ in the cell balances the tendency for efflux.

The sarcoplasmic reticulum (SR) surrounds the myofibrils and is a reservoir of the Ca2+ which participates in myofibrillar contraction. T-tubules are deep, finger-like indentations of the sarcolemma that abut the SR at junctional regions in register with the Z-discs of the superficial sarcomeres.

When a myocyte is electrically excited, Na+ channels open and Na+ enters the cell down its own concentration gradient, producing a rapid inward current and depolarizing the cell towards its equilibrium potential: the initial phase (phase 0) of the action potential. As the myocyte depolarizes, L-type Ca2+ channels in the sarcolemma and T-tubules open and Ca2+ enters the cell down its concentration gradient. The Na+ channels close rapidly, but the L-type Ca2+ channels remain open for longer, maintaining depolarization: phases 1 and 2 of the action potential, where the tendency to depolarize is balanced by repolarizing outward current flow carried by a variety of K+ channels. The membrane potential in phase 2 is relatively stable and hence this phase is also known as the plateau phase.

Ca2+ entry in close apposition to the junctional SR causes SR Ca2+-release channels to open, discharging about half of the SR Ca2+ reservoir into the cytoplasm (Ca2+-induced Ca2+-release). This increase in Ca2+ concentration (the Ca2+ transient) is sensed by a Ca2+-binding protein (troponin C) that is a component of the thin filament regulatory complex (the troponin–tropomyosin complex). This initiates myofibrillar contraction, which starts about halfway through phase 2.

As the L-type Ca2+ channels close, outward current flow though K+ channels predominates and the myocyte repolarizes towards the K+ equilibrium potential (phase 3). Ca2+ is removed from the cytoplasm and returned to the SR in an ATP-requiring process mediated by the sarcoplasmic/endoplasmic Ca2+-ATPase (SERCA2). Ca2+ is also expelled from the cell by the plasma membrane Na+,Ca2+ exchanger, which is electrogenic (three Na+ exchanged for one Ca2+) and tends to prolong the plateau phase. The behaviour of the Na+,Ca2+ exchanger is complex because—depending on the Na+ and Ca2+ concentrations and the membrane potential—it can reverse, thus mediating Ca2+ entry and repolarization. This occurs at depolarized potentials, and more so when intracellular Na+ is increased. In phase 4, repolarization is complete and the myocyte is electrically quiescent until the next depolarization.

Cardiac pacemaker and regulation of contractility

The sinoatrial node (‘pacemaker’) contains modified myocytes that exhibit a different form of action potential from ventricular myocytes because of differences in the expression of ion channels. The cell depolarizes spontaneously and gradually during phase 4 until an action potential is produced. This partly results from the presence of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels which are absent from ventricular myocytes and which carry an inward-depolarizing Na+ current. Depolarization is then mediated by Ca2+ channel opening. The stimulus is transmitted in a controlled manner via the conduction system to all regions of the heart.

Whole organ physiology

Cardiac contractility is controlled largely by the sympathoadrenal system and the parasympathetic nervous system. β‎-Adrenergic stimulation increases the tendency of the L-type Ca2+ channel to open (positive inotropism). β‎-Stimulation also increases relaxation (positive lusitropism) by stimulation of SERCA2 and an increased rate of release of Ca2+ from the troponin complex. The positive chronotropic effects of β‎-stimulation result from increased HCN channel opening, causing an increased frequency of pacemaker depolarization. These effects are all opposed by the (cholinergic) muscarinic receptors of the parasympathetic nervous system.

The energy requirements of the heart during rest and exertion are influenced by ventricular volume, outflow resistance (blood pressure), venous return, and the activity of the autonomic nervous system. An increase in ventricular volume increases wall tension during contraction, and an augmented myocardial oxygen supply is then required to maintain the same systemic blood pressure and stroke volume.

The normal integration of the venous return, heart rate, stroke volume, and arterial blood pressure ensures that there is an adequate supply of oxygen and nutrients to the tissues. The activities of the sympathetic and parasympathetic nervous systems contribute to the adjustment of cardiac performance to immediate needs—the former by increasing heart rate and myocardial contractility during exertion and emotion, the latter by maintaining a relatively slow heart rate at rest. Vagal fibres in the heart are distributed mainly to the sinoatrial node and the atria; sympathetic innervation is to both the atria and the ventricles. There is a normal diurnal variation in autonomic function, with an increased sympathetic outflow in the mornings, soon after wakening.

Coronary flow occurs largely in diastole. It is finely adjusted to meet metabolic requirements and may increase five- or sixfold during strenuous exercise. The inner layers of the ventricular muscle normally receive a slightly greater blood flow than the outer layers. Haemodynamic and ventilatory responses during exercise take 2 to 3 min to equilibrate and adjust to an increased workload and reach a new steady state. Regular exercise to least 60% of maximal heart rate about three times a week improves effort tolerance. Measurement of the cardiovascular response to exercise provides an objective assessment of cardiac function.

Introduction

The function of the heart is to pump sufficient oxygenated blood containing nutrients, metabolites, and hormones to meet moment-to-moment metabolic needs and preserve a constant internal environment. The heart has two essential characteristics—contractility and rhythmicity. The nervous system and neurohumoral agents modulate relationships between the venous return to the heart, the outflow resistance against which it contracts, the frequency of contraction, and its inotropic state; there are also intrinsic cardiac autoregulatory mechanisms. An understanding of the molecular mechanisms governing cardiac cell behaviour and the mechanical, electrical, and hormonal control of the heart at a whole organ level is essential for the understanding of cardiac pathophysiology.

Cardiac myocytes

Cardiac myocytes are the contractile cells of the heart, and include ventricular and atrial myocytes, as well as cells specialized to provide the electrical impulse and conduction system. Myocytes constitute the bulk of the cellular volume, but because they are large cells they are fewer in number, being outnumbered by endothelial cells, smooth muscle cells of the vasculature, and fibroblasts. Ventricular myocytes are believed to be terminally differentiated cells in mammals, incapable of replication; this is less clear for the atrial myocyte. Terminal differentiation has important consequences for the heart in terms of its limited ability to survive haemodynamic insults or stresses, but also means that the myocardium is essentially resistant to malignant transformation.

Morphology of the ventricular myocyte and its contractile machinery

The ventricular myocyte is an elongated cell (100–150 µm long and 20–35 µm wide) and is packed with striated myofibrils (the contractile elements) that alternate with rows of mitochondria (Fig. 16.1.3.1). Each myofibril is roughly cylindrical (2–3 µm in diameter), stretches the length of the cell, and is anchored at each end in a fascia adherens junction. The myofibril comprises sarcomeres arranged in series. Sarcomeres consist of two arrays of filaments: thin filaments, comprised predominantly of the protein actin, interdigitated with thick filaments of myosin. The characteristic striated appearance arises from the organization of these filaments within the myofibril (Fig. 16.1.3.1). The thick filaments are confined to the A-band at the centre of the sarcomere. The thin filaments extend out from either side of the Z-disc (Z-line), crossing the I-band, and penetrate partially into the A-band, where they overlap and interact with the thick filaments. Each Z- to Z-disc repeat constitutes a sarcomere, and the distance between consecutive Z-discs (the sarcomere length) is a measure of the contractile state of the myofibril. At the centre of the sarcomere lies the M-line. Each myofibril contains 70–80 sarcomeres. Myocytes have an irregular ‘branched’ morphology; through these branches, each ventricular myocyte typically connects to 10 or more of its neighbours to form the three-dimensional branching, syncytium-like structure of the myofibre.

Fig. 16.1.3.1 Upper panel: Electron micrograph of ventricular myocyte showing the structure of the myofibrils. Portions of two myofibrils are shown in the field, with a row of mitochondria (mito) between. Lower panel: Diagrammatic representation of the arrangement of the thick and thin filaments in relation to the striated pattern seen in microscopy.

Fig. 16.1.3.1
Upper panel: Electron micrograph of ventricular myocyte showing the structure of the myofibrils. Portions of two myofibrils are shown in the field, with a row of mitochondria (mito) between. Lower panel: Diagrammatic representation of the arrangement of the thick and thin filaments in relation to the striated pattern seen in microscopy.

Structure of the contractile apparatus

Thick filaments

The myosin molecule comprises two heavy chains (molecular mass ~200 kDa) and two pairs of light chains (mass 18–28 kDa). The myosin heavy chains are arranged as dimers, with a tail and two heads (Fig. 16.1.3.2). The tails are packed together to form the shaft of the thick filament, while the heads protrude from the filament and lie close to the thin actin filaments. The myosin heads are the motor units of muscle: they bind and hydrolyse ATP to ADP and convert the free energy of hydrolysis into mechanical work through their interaction with actin in the thin filaments (for details see ‘The mechanism of myocyte contraction’).

Fig. 16.1.3.2 Structural arrangement of contractile proteins in the filament overlap zone of the sarcomere.

Fig. 16.1.3.2
Structural arrangement of contractile proteins in the filament overlap zone of the sarcomere.

Reproduced from Spirito et al. N Engl J Med 1997;336:775–785. Copyright © 1997 Massachusetts Medical Society. All rights reserved.

Thin filaments

Each thin filament comprises about 300 globular actin subunits (mass 42 kDa). The actin monomers have sites for interaction with the myosin heads and with a regulatory protein complex that confers Ca2+ sensitivity. The latter consists of the troponin complex and the elongated protein α‎-tropomyosin (Fig. 16.1.3.2). Troponin complexes are located at intervals along the actin filament. Tropomyosin forms two continuous strands along the thin filament and is responsible for cooperative propagation of regulatory signals.

Other structural components of the sarcomere

The thin filaments are attached to the Z-discs in a regular array with filaments on each side in opposite orientation (Fig. 16.1.3.1). The main structural component of the Z-disc is the actin cross-linking protein α‎-actinin. Z-discs are also associated with the T-tubules and costameres (see below), and contain a number of additional proteins believed to be associated with cell signalling. The M-line (Fig. 16.1.3.1) contains the protein myomesin that cross-links the thick filaments to maintain their orientation. In addition, the giant protein titin (connectin) extends from the M-line to the Z-disc. Titin contains multiple binding sites for multiple sarcomeric proteins, including myosin-binding protein-C (MyBP-C; Fig. 16.1.3.2). It contributes to elasticity, passive tension, and thick filament positioning in the sarcomere.

Intermediate filaments, costameres, and the plasma membrane skeleton

The myofibrils are held in position by scaffold-like webs of intermediate filaments made from a (non-contractile) protein, desmin. Desmin filaments are anchored to costameres, which circumscribe the lateral plasma membrane. Apart from maintaining the spatial organization of the contractile apparatus, the costameres mechanically couple the cells laterally to the extracellular matrix. Associated with the costameres, but closely applied to the entire cytoplasmic aspect of the lateral plasma membrane, is the membrane skeleton, a peripheral membrane protein network of dystrophin and spectrin. The costameres, membrane skeleton, and intermediate filaments are linked to the glycocalyx and extracellular matrix by sets of integral plasma membrane proteins, notably the integrins and the components of the dystrophin–glycoprotein complex.

Coupling of the plasma membrane to the SR

The plasma membrane (sarcolemma) contains openings of transverse (T)-tubules and the caveolae, which are cholesterol-enriched pits in which signal-transducing and water-channel proteins are concentrated (Fig. 16.1.3.3a). T-tubules are long invaginations of the plasma membrane adjacent to the costameres and myofibril Z-discs, and penetrate deeply into the cell. T-tubules mediate extracellular Ca2+ (Ca2+o) entry into the cell through the L-type Ca2+ channels. Each myofibril is surrounded by a network of interconnecting membranous tubules and cisternae known as the sarcoplasmic reticulum (SR) (Fig. 16.1.3.3a). At multiple sites within this network, the membranes form flattened sacs, the junctional SR (JSR) cisternae, which press tightly against the peripheral plasma membrane and T-tubules (Fig. 16.1.3.3b). The plasma membrane and T-tubule domains facing the JSR membrane contain clusters of L-type Ca2+ channels, while the apposing domains of the JSR are packed with Ca2+-release channels (Fig. 16.1.3.3), also known as ‘ryanodine receptors’ because of their sensitivity to interference by the plant alkaloid ryanodine. The JSR is the major reservoir of cytoplasmic Ca2+. This close spatial arrangement is important in control of the Ca2+ transient required for myofibrillar contraction. Following contraction, the sarcoplasmic/endoplasmic reticulum Ca2+ATPase 2 (SERCA2) pumps Ca2+ back into the SR lumen, causing myofibrillar relaxation.

Fig. 16.1.3.3 EC coupling in the heart. (a) L-type Ca2+ channels allow Ca2+ influx across the plasma membrane, creating ICa. This influx increases the local Ca2+ concentration around a cluster of SR Ca2+-release channels (ryanodine receptor) in sufficient amounts to open them (Ca2+-induced Ca2+-release). (b) The opening of clusters of SR Ca2+-release channels allows SR Ca2+ reservoir to be discharged into the cytoplasm. Ca2+ fluxes combine to initiate contraction. The contraction process is terminated (1) by SERCA2 (regulated by phospholamban and dependent on phospholamban phosphorylation state), which pumps Ca2+ back into the SR, and (2) by the plasma membrane Na/CaX which expels Ca2+ from the cell.

Fig. 16.1.3.3
EC coupling in the heart. (a) L-type Ca2+ channels allow Ca2+ influx across the plasma membrane, creating ICa. This influx increases the local Ca2+ concentration around a cluster of SR Ca2+-release channels (ryanodine receptor) in sufficient amounts to open them (Ca2+-induced Ca2+-release). (b) The opening of clusters of SR Ca2+-release channels allows SR Ca2+ reservoir to be discharged into the cytoplasm. Ca2+ fluxes combine to initiate contraction. The contraction process is terminated (1) by SERCA2 (regulated by phospholamban and dependent on phospholamban phosphorylation state), which pumps Ca2+ back into the SR, and (2) by the plasma membrane Na/CaX which expels Ca2+ from the cell.

Connections between cardiac myocytes

The myocyte can function as an autonomous contractile unit. To produce a heartbeat, the contractile capabilities of the ~3 billion myocytes that constitute the human heart have to be electromechanically synchronized. This requires both an orderly spread of the wave of electrical activation and the effective transmission of contractile force from one cell to the next, throughout the heart. This is achieved by the intercalated discs, formed from specialized regions of the plasma membrane where adjacent cells interact.

Intercalated discs are situated at the blunted ends of the main body of the myocyte and its side branches (Fig. 16.1.3.4). Three types of cell junction—the gap junction, the fascia adherens, and the desmosome—connect the adjacent membranes at the disc. The fascia adherens and desmosome are forms of anchoring junction; gap junctions contain clusters of connexons (Fig. 16.1.3.4). These junctions are clusters of intercellular channels which span two closely apposed plasma membranes and directly link adjacent cytoplasmic compartments of neighbouring cells. They form the sites of electrical coupling between individual cardiac myocytes and permit direct cell-to-cell transmission of chemical signals (ions and small molecules of <1 kDa). The combination of connexin isoforms constituting a gap junction channel is a major determinant of its functional properties, and varies in different cardiomyocyte subsets. This arrangement renders the myocardium into a functional syncytium.

Fig. 16.1.3.4 The intercalated disc and cardiac gap junction organization and structure. (a) Clusters of gap junctions at the intercalated discs revealed in a single ventricular cardiac myocyte by immunoconfocal microscopy. (b) One disc-cluster of gap junctions viewed en face (reconstruction from a stack of serial optical sections). One of these immunolabelled spots corresponds to a single gap junction. (c) Electron micrograph illustrating the three types of cell junction of the intercalated disc. Gap junctions occur where the adjacent plasma membrane profiles run in close contact. The fascia adherens and the desmosome are characterized by a much wider intermembrane space (c.25 nm). (d) Viewing the membrane en face by freeze-fracture reveals the gap junction as a cluster of particles (connexons). (e) The gap junction channel consists of a pair of connexons (hemichannels), one contributed by each of the adjacent plasma membranes. Each connexon is itself formed from six connexin molecules. The specific connexin type or types within the connexon is a major determinant of the functional properties of the gap junction channel.

Fig. 16.1.3.4
The intercalated disc and cardiac gap junction organization and structure. (a) Clusters of gap junctions at the intercalated discs revealed in a single ventricular cardiac myocyte by immunoconfocal microscopy. (b) One disc-cluster of gap junctions viewed en face (reconstruction from a stack of serial optical sections). One of these immunolabelled spots corresponds to a single gap junction. (c) Electron micrograph illustrating the three types of cell junction of the intercalated disc. Gap junctions occur where the adjacent plasma membrane profiles run in close contact. The fascia adherens and the desmosome are characterized by a much wider intermembrane space (c.25 nm). (d) Viewing the membrane en face by freeze-fracture reveals the gap junction as a cluster of particles (connexons). (e) The gap junction channel consists of a pair of connexons (hemichannels), one contributed by each of the adjacent plasma membranes. Each connexon is itself formed from six connexin molecules. The specific connexin type or types within the connexon is a major determinant of the functional properties of the gap junction channel.

Reprinted from Severs NJ (2000). The cardiac muscle cell. BioEssays, 22, 188–99 (Fig. 5). With permission from Wiley-Blackwell.

Cardiac myocyte subtypes

Atrial myocytes are significantly different to the ventricular myocytes described above; they are long and slender, with few or no T-tubules but more abundant caveolae. By producing the peptide hormone atrial natriuretic peptide (ANP; atrial natriuretic factor), they also function as secretory cells. Natriuretic peptides participate in the control of sodium and water balance and hence of blood pressure.

A third, heterogeneous, group of modified and morphologically distinct myocytes makes up the pacemaker and conduction system. These cells show some resemblance to ventricular and atrial cells, but their primary function is impulse generation and its timed distribution to the contractile myocytes at the appropriate point in the cardiac cycle.

Myocytes of the sinoatrial and atrioventricular nodes are typically small (c.5 µm diameter), containing just a few rudimentary myofibrils, and small, sparse, gap junctions. These features contribute to poor coupling, which in the atrioventricular node is essential to slowing of conduction to ensure time for atrial ejection. The cell population is morphologically heterogeneous: cells of the compact atrioventricular node, and those of the surrounding areas (the transitional cells and posterior nodal extension), are distinctive, and myocytes of the His–Purkinje system show a range of morphologies according to their location, progressively increasing in size and myofibril content, and with more developed intercalated discs distally, towards the ventricular myocardium.

The cardiac action potential

The membrane potential

Electrical activity of the heart (membrane potential; action potential) is based on differential distribution of ions across the cellular membranes. This distribution is achieved by the action of ion-transport proteins, including ion pumps and channels.

Ion channels

Electrical excitation of myocytes involves the movement of ions through specific channels. These are ‘excitable’ proteins embedded in membranes that contain pores capable of opening or closing in response to a stimulus, which could be a change in membrane potential, a neurotransmitter or hormone, an intracellular second messenger or ion, or mechanical stretch of the membrane. On opening, a channel becomes selectively permeable to a restricted series of ions. There are many different types of channel, often named after the most permeant ion they pass, e.g. Na+, Ca2+, and K+ channels. Ions move down their electrochemical gradients through the channel at high rates (>106 ions/s), distinguishing them from other ion-transport proteins (e.g. the Na+,K+-ATPase or pump, and the Na+,Ca2+ exchanger (Na/CaX); see below ‘The Na+,Ca2+ exchanger (Na/CaX) and the Na+,K+-ATPase’) which move ions across plasma membranes several orders of magnitude more slowly. Hence cardiac excitation provides a means of coordinating the contractile activities of the four chambers and is the basis for the electrocardiograph (ECG) (see Chapter 16.3.1).

Origin of the membrane potential

Cardiac membrane potential is determined by three factors: (1) ionic concentrations across the sarcolemma; (2) the permeability (conductance) of the sarcolemma to specific ions; and (3) the activity of electrogenic pumps that maintain the ionic concentration gradients. When a ventricular myocyte is at rest (diastole), there is a potential difference of about −90 mV across the plasma membrane, the inside of the cell being negative with respect to the outside. This is principally caused by plasma membrane permeability to K+. The extracellular concentration of K+ (K+o) is about 4 mmol/litre, and the intracellular (cytoplasmic) concentration (K+i) is about 140 mmol/litre, so K+ tends to diffuse out of the cell down its concentration gradient, resulting in the interior becoming negatively charged. An equilibrium is thus established where the electronegative force retaining K+ inside the cell (mostly derived from negatively charged proteins) balances its tendency to diffuse out of the cell down its concentration gradient. This is termed the equilibrium potential (E) and can be calculated from the Nernst equation (see Table 16.1.3.1 for E values of relevant ions). At this potential, there will be no net flux of K+ ions through K+ channels and, if the membrane is only permeable to K+, then the membrane potential will be equal to EK.

Table 16.1.3.1 Ion concentrations in the quiescent myocyte, and their calculated equilibrium potentials (E). E is calculated from the Nernst equation, E = (RT/zF) ln(ao/ai), where E is in volts, T is the absolute temperature, R is the gas constant, F is the Faraday constant, z is the valency, and ao and ai are the extracellular and intracellular activities of the ion in question

Ion

Intracellular concentration (mmol/litre)

Plasma concentration (mmol/litre)

Calculated E (mV)

Na+

10

140

+70

K+

140

4.5

–91

Ca2+

0.0001

2.3

+131

Cl–

20

110

+45

The membrane potential at any moment is dependent upon the equilibrium potentials for all permeant species and their relative permeabilities. The actual transmembrane potential difference at rest and the calculated EK are rarely the same owing to a small leakage, mainly of Na+ into the cell down its concentration gradient (Na+o = 140 mmol/litre, Na+i = 7–10 mmol/litre). To counteract this leak and to maintain the concentration gradients of Na+ and K+ upon which the generation of the membrane potential depends, the plasma membrane Na+,K+-ATPase uses free energy derived from the hydrolysis of ATP to pump these ions against their concentration gradients. This process is electrogenic (three Na+ extruded for two K+ entering) and generates 3 to 10 mV of the membrane potential.

The action potential

The action potential is divided into five phases (Fig. 16.1.3.5). The currents that flow are described in Table 16.1.3.1, Table 16.1.3.2, and Fig. 16.1.3.5. Depolarization from the resting potential is mediated by inward current flow.

Fig. 16.1.3.5 Ca2+ transient, membrane potential, K+ currents, and Ca2+-related currents during a ventricular myocyte action potential. The inward Na+ current that produces the rapid upstroke of the action potential is not shown Top panel: Changes in cytoplasmic Ca2+ concentration during the action potential (Ca2+ transient). Upper middle panel: Phases of the ventricular myocyte action potential. For a period between phase 0 and about midway through phase 3, cardiac muscle cannot be excited with another stimulus: the absolute refractory period. From about halfway through phase 3 until just before the end of phase 3, cardiac muscle is in its relative refractory period, when a stronger stimulus than normal is required to initiate an action potential. The states of refractoriness are related to the ability of ion channels to recover from a stimulus. This recovery is both voltage- and time-dependent. Lower middle panel: K+ currents during one action potential. All K+ currents (ITO, IKs, IKr, and IK1) repolarize the myocyte because of outward K+ movement. Bottom panel: Ca2+-related currents during one action potential. Because of the inward movement of Ca2+, Ca2+ current (ICa) is depolarizing. The Na/CaX produces both outward and inward current (INa,Ca) depending the phase of the action potential. The inward Na+ current is roughly 8 to 10 times the size of the Ca2+ current and has largely inactivated by the time of the peak Ca2+ current.

Fig. 16.1.3.5
Ca2+ transient, membrane potential, K+ currents, and Ca2+-related currents during a ventricular myocyte action potential. The inward Na+ current that produces the rapid upstroke of the action potential is not shown Top panel: Changes in cytoplasmic Ca2+ concentration during the action potential (Ca2+ transient). Upper middle panel: Phases of the ventricular myocyte action potential. For a period between phase 0 and about midway through phase 3, cardiac muscle cannot be excited with another stimulus: the absolute refractory period. From about halfway through phase 3 until just before the end of phase 3, cardiac muscle is in its relative refractory period, when a stronger stimulus than normal is required to initiate an action potential. The states of refractoriness are related to the ability of ion channels to recover from a stimulus. This recovery is both voltage- and time-dependent. Lower middle panel: K+ currents during one action potential. All K+ currents (ITO, IKs, IKr, and IK1) repolarize the myocyte because of outward K+ movement. Bottom panel: Ca2+-related currents during one action potential. Because of the inward movement of Ca2+, Ca2+ current (ICa) is depolarizing. The Na/CaX produces both outward and inward current (INa,Ca) depending the phase of the action potential. The inward Na+ current is roughly 8 to 10 times the size of the Ca2+ current and has largely inactivated by the time of the peak Ca2+ current.

Table 16.1.3.2 Plasma membrane currents in the cardiac myocyte

Current

Name

Activated by

Blocked by

Gene

Protein

Function

Inward currents

INa

(Fast) Na+ current

Depolarization

Tetrodotoxin, local anaesthetics

SCN5A

Nav1.5

Rapid upstroke of action potential

ICa,L

L-type Ca2+ current (‘long lasting’)

Depolarization

Verapamil, Cd2+, dihydropyridines

CACNA1C

Cav1.2

Ca2+ influx that activates CICR, provides some Ca2+ for contraction

ICa,T

T-type Ca2+ current (“transient”)

Activates on depolarization but at more negative potentials than L-type current

Ni2+, mibefradil

CACNA1G CACNA1H

Cav3.1 Cav3.2

Channel density high in pacemaker and conducting tissue so may contribute to pacemaker activity. Role in ventricular cells unclear

If

Hyperpolarization-activated, cyclic nucleotide-gated cation channel

Hyperpolarization, noradrenaline, cAMP

Cs+, ZD7288, ivabradine, zatebradine, cilobradine

HCN2 HCN4

Exists in sinoatrial node and Purkinje fibres bringing membrane potential slowly to threshold

Inward and outward (reversible) current

INa/Ca

Na/CaX current

Ca2+i

Ni2+, KB-R7943

NCX1

Expels Ca2+ from the cell, maintains inward current flow near end of action potential, at positive potentials may reverse and mediate Ca2+ influx

Outward currents

ITO

Transient outward current

Depolarization

4-Aminopyridine

KCNA4 KCND2 KCND3

Kv1.4 Kv4.2 Kv4.3

Early repolarization (notch)

ICl

Chloride current

cAMP

CFTR

Early repolarization

ICl,Ca

Ca2+-activated chloride current

Ca2+

CLCA1

Early repolarization

IKur

Ultra-rapid delayed rectifier

Depolarization

Tetraethylammonium, Cs+, Ba2+, 4-aminopyridine, flecainide, nifedipine, diltiazem, bupivacaine, propafenone, quinidine

KCNA5

Kv1.5

Repolarization of cell

IKr

Rapid delayed rectifier

Depolarization

Tetraethylammonium, Cs+, Ba2+, E-4031, dofetilide, D-sotolol, cisapride, BRL32872

KCNH2

herg, Kv11.1

Repolarization of cell

IKs

Slow delayed rectifier

Depolarization

Chromanol 293B

KCNQ1

KvLQT1

Repolarization of cell

IK1

Inward (anomalous) rectifier

Depolarization from EK Conductance of channel increases then decreases to zero at 0 mV

Cs+, Rb+, Ba2+, intracellular Mg2+, spermidine, spermine

KCNJ2 KCNJ12

Kir2.1 Kir2.2

Prolongs action potential duration, background K+ conductance,

Ip

Na+/K+ pump current

Na+i , K+o

Cardiac glycosides

Maintains low [Na+]i,

IK,ACh

Acetylcholine-activated K+ current (inward rectifier)

ACh

Ba2+

KCNJ3 KCNJ5

Kir3.1 Kir3.4

Muscarinic receptor-coupled. Activates additional K+ channels so slowing pacemaker potential

Phase 0 of the action potential

When a myocyte is electrically stimulated, Na+ channels open and allow Na+ ions to enter the cell. The channels open by sensing potential difference more positive than about −65 mV across the cell membrane (Fig. 16.1.3.5). Excitation depolarizes the cell membrane slightly and this increases the probability of Na+ channel opening. A cardiac myocyte contains many thousands of Na+ channels, hence the current (I) generated by the movement of Na+ ions into the cell (INa) is the sum of the small currents that flow though each individual channel. Positive charge is taken into the cell, the membrane potential increases towards the equilibrium potential for Na+ (ENa = +70 mV, Table 16.1.3.1), and the cell depolarizes (Fig. 16.1.3.5). The Na+ current causes the rapid upstroke (phase 0) of the action potential. The propagation velocity of the action potential across the whole heart is related to the rate of the rapid upstroke. Following activation and opening, the channels close very rapidly, even though the myocyte remains depolarized, a process termed ‘inactivation’. Inactivated channels cannot open again until the cell repolarizes, causing the refractory period during which a further stimulus cannot evoke another action potential (Fig. 16.1.3.5).

The inactivation of each channel decreases the total number of Na+ channels that are conducting such that INa almost entirely inactivates within the first 5 ms of the action potential (the overall action potential in humans at rest lasts c.350 ms). Some Na+ channels do not inactivate so rapidly, allowing a small inward current to persist during the plateau phase of the action potential (phase 2, see below).

Phase 1 of the action potential

The characteristic notch observed in phase 1 of the action potential in ventricular myocytes (Fig. 16.1.3.5) is caused by a transient outward current (ITO), carried mainly by K+ ions flowing out of the cell (ITO1), but also by some Cl− current (ITO2), that partially repolarizes the membrane. The current inactivates within 30–40 ms but is important in determining action potential duration. A component of ITO appears to be dependent upon intracellular Ca2+ concentration (raised Ca2+o increases ITO): this is the probable mechanism underlying action potential shortening during tachycardia.

Phase 2 of the action potential

Several currents flow during phase 2 (the action potential plateau), including ICa (Fig. 16.1.3.5). L-type (‘long lasting’) Ca2+ channels, which take longer to activate and inactivate than Na+ channels, open within 3–5 ms of the start of the upstroke and allow Ca2+ to flow into the cell (ICa). L-type Ca2+ channels activate at more positive voltages than Na+ channels (around −35 mV). The influx of Ca2+ maintains depolarization (Fig. 16.1.3.5, Tables 16.1.3.1 and 16.1.3.2), and initiates Ca2+-induced Ca2+-release (CICR) from the SR through the SR Ca2+-release channels, causing the myocyte to contract (see below). Hence, Ca2+ has a role in both membrane potential and signal transduction/inotropy. In addition, a slow delayed rectifier K+ current (IKS) exports K+ in phase 2.

The plateau phase (phase 2) of the action potential (Fig. 16.1.3.5) is prolonged in ventricular myocytes because of the properties of several types of K+ channel that give rise to several different K+ currents. The main repolarizing current, IK, is composed of two distinct currents, one activating more rapidly (IKr) than the other (IKs) (see Table 16.1.3.2). Both channels open at positive membrane potentials and close (deactivate) at negative potentials. The plateau of the action potential is the result of a balance of inward (Ca2+) and outward (K+) current flow.

Phase 3 of the action potential

The final phase of repolarization begins with the termination of ICa and progressively increasing K+ current (IKr and IKs) (Fig. 16.1.3.5). As repolarization proceeds, the Na/CaX responds to the increase in cytoplasmic Ca2+ concentration and produces an inward current (INa,Ca) through the exchange of three Na+ entering the cell for one Ca2+ expelled; by producing an inward current, the Na/CaX slows repolarization and prolongs the plateau. In ventricular myocytes, complete repolarization and a return to a negative resting membrane potential is eventually achieved by (the large) IK1 (the ‘inward rectifier’; Fig. 16.1.3.5; Table 16.1.3.2). The channel through which this current flows possesses peculiar characteristics. Normally, because of the relative concentrations of K+ inside and outside the cell, there is outward movement of K+ ions that becomes larger the more positive the displacement from EK. However, the IK1 current flows through a channel that first increases its conductance but then decreases it as the cell depolarizes away from EK (anomalous rectification). Thus, there is outward flow of repolarizing current only over a narrow voltage range (around −30 to −80 mV)—another reason for the prolonged cardiac action potential because a large, rapid, outward K+ current does not flow despite the membrane potential approaching 0 mV during the plateau phase.

IK1 underlies the main flow of K+ giving rise to the membrane potential. The channels through which IK1 flows are numerous in ventricular cells, fewer in atrial cells, and absent in pacemaker cells. The current is therefore large in ventricular cells and this is the reason that the resting membrane potential of ventricular myocytes lies near EK, whereas atrial cells have a more positive resting membrane potential, and SA nodal cells do not have a stable resting potential.

Phase 4 of the action potential

This phase relates to the membrane potential during the electrically silent period between excitatory events in ventricular myocytes (Fig. 16.1.3.5); phase 4 is stable in these cells.

Regional variations in action potential

The configuration of the cardiac action potential differs regionally within the heart (Fig. 16.1.3.6) because ion-channel expression varies between cells. In the sinoatrial node, INa is very small and the main current responsible for the depolarizing upstroke is ICa, carried mainly by L-type Ca2+ channels. The only repolarizing current is IK. IK1 is absent and, as mentioned above, this partially explains why sinoatrial node cells have a more depolarized ‘diastolic’ potential than ventricular myocytes. Sinoatrial node cells depolarize spontaneously during phase 4 (Fig. 16.1.3.6), owing to the absence of IK1 and the presence of a current activated on hyperpolarization called the ‘funny’ current (If), carried mainly by Na+ through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and current (ICa,T) resulting from an influx of Ca2+ through voltage-dependent T-type (‘transient’) Ca2+ channels (abundant in these cells) (Table 16.1.3.2). Phase 4 is often termed the ‘pre- or pacemaker potential’ in nodal cells and is caused by the gradual decrease in IK and increase in If and ICa,T (Fig. 16.1.3.6). Once the cell has depolarized to a voltage at which L-type Ca2+ channels open (the threshold), a more rapid depolarization (caused by ICa,L) occurs, forming the upstroke (phase 0) of the sinoatrial node action potential. Acetylcholine (ACh) activates IK,ACh, which helps drive the membrane potential towards EK and slows the rate of depolarization, while β‎-adrenergic stimulation increases the slope of the pacemaker potential and heart rate through an effect on If, affecting heart rate (Fig. 16.1.3.7).

Fig. 16.1.3.6 Regional configurations of the action potential. In the sinoatrial (SA) and atrioventricular (AV) nodes, the cells spontaneously depolarize during diastole (phase 4 depolarization). When the membrane potential reaches a threshold, the complete action potential is initiated. Because the SA nodal cells have the fastest phase 4 depolarization, they act as the cardiac pacemaker.

Fig. 16.1.3.6
Regional configurations of the action potential. In the sinoatrial (SA) and atrioventricular (AV) nodes, the cells spontaneously depolarize during diastole (phase 4 depolarization). When the membrane potential reaches a threshold, the complete action potential is initiated. Because the SA nodal cells have the fastest phase 4 depolarization, they act as the cardiac pacemaker.

Fig. 16.1.3.7 Change in heart rate produced by altering the phase 4 slope of the pacemaker potential in the sinoatrial (SA) node. β‎-Adrenergic stimulation increases, and cholinergic stimulation decreases, the slope of the pacemaker potential, affecting the time taken to reach threshold.

Fig. 16.1.3.7
Change in heart rate produced by altering the phase 4 slope of the pacemaker potential in the sinoatrial (SA) node. β‎-Adrenergic stimulation increases, and cholinergic stimulation decreases, the slope of the pacemaker potential, affecting the time taken to reach threshold.

Atrial and ventricular myocytes do not have pacemaker potentials and spontaneously discharge only when injured or when there is abnormal ionic balance. The longest action potential is in Purkinje fibres (Fig. 16.1.3.6) and this acts as a ‘gate’ preventing retrograde activation by depolarization of adjacent ventricular myocytes.

The mechanism of myocyte contraction

Excitation—contraction coupling

The electrical events throughout the heart initiate and regulate contraction (Fig. 16.1.3.5). Coupling of the electrical excitation of the heart to contraction (termed excitation—contraction coupling or EC coupling) by Ca2+ ions involves the interaction of several proteins involved in Ca2+ homeostasis (Fig. 16.1.3.3). The T-tubules carry depolarization deeply into the cell. During diastole, when cytoplasmic Ca2+ concentrations are low (c.0.1 µmol/litre), Ca2+ is sequestered by the Ca2+-buffering protein calsequestrin within the JSR. Depolarization then opens the L-type Ca2+ channels in the T-tubule and plasma membrane allowing influx of Ca2+ (Figs. 16.1.3.3 and 16.1.3.5) and producing ICa (Fig. 16.1.3.5). Ca2+ influx increases the local Ca2+ concentration around clusters of SR Ca2+-release channels sufficiently to open them (i.e. CICR), the number of channels activated in this way being mainly, though not exclusively, determined by the size of the Ca2+ current. CICR provides amplification as the small ‘trigger’ Ca2+ influx through the L-type Ca2+ channels evokes a much larger release of Ca2+ from the SR into the cytoplasm; also, the release of Ca2+ from the SR is under precise control as it is closely matched to the amount of Ca2+ influx. Cytoplasmic Ca2+ concentration rises to between 1 and 3 µmol/litre (Fig. 16.1.3.5). The release of Ca2+ ceases because the L-type Ca channels inactivate so the trigger influx declines, leading to closure of SR Ca2+ release channels.

The mechanism of myofibrillar contraction

Ca2+ release from the SR activates the contractile apparatus of the sarcomere (Figs. 16.1.3.2 and 16.1.3.8). The temporal relationship between the action potential, the Ca2+ transient, and the subsequent development of tension is shown in Fig. 16.1.3.9. Sarcomere shortening is caused by the interaction of motor protein myosin in the thick filaments with actin in the thin filaments (Fig. 16.1.3.8). Myosin heads bind and hydrolyse ATP, retaining bound ADP and phosphate and trapping the free energy of hydrolysis within the myosin molecule. The myosin—ADP—phosphate complex then binds to actin, leading to the release of the stored energy by a conformational change that moves the actin filament by about 10 nm relative to the thick filament. This is known as the cross-bridge cycle (Fig. 16.1.3.8) and results in the sliding of the thin filament past the thick filaments, and sarcomere shortening. If the muscle is under load, the cross-bridge cycle generates force and work is done (the maximum efficiency is more than 60% in intact muscle). The mechanical characteristics of contracting muscle can be described in terms of the relationship between shortening speed and force, and between sarcomere length and force (Fig. 16.1.3.10a). Maximum force is produced under isometric conditions, while maximum shortening speed is observed in unloaded muscle. Power output is the product of force and velocity and is optimal at about 30% of maximum shortening speed (Fig. 16.1.3.10a).

Fig. 16.1.3.8 The cross-bridge cycle. Exchange of ATP with ADP (a) on either a load-bearing (b) or a resting-length myosin head (c) results in a conformational change in the myosin head, causing a rapid dissociation of the myosin head from actin ((b) to (d) and (c) to (d), respectively). Following detachment from actin, the ATP is hydrolysed to ADP and Pi, both of which remain tightly bound to the myosin head (e). Hydrolysis is accompanied by a major conformational change which represents the reversal or a repriming of the power stroke. If an actin site is within reach of the myosin head, it will bind rapidly and reversibly to the actin site (a). When the myosin head binds actin, the interaction can promote a major change in conformation (the power stroke) which is accompanied by the dissociation of Pi ((a) to (b)). This step approximates to isometric contraction (no relative movement of actin and myosin) whereas the (a) to (c) steps approximate to an isotonic contraction (relative movement, with a release of the myosin ‘spring’). This power stroke consists of a reorientation of part of the myosin head that results in the displacement of the tip by up to 10 nm.

Fig. 16.1.3.8
The cross-bridge cycle. Exchange of ATP with ADP (a) on either a load-bearing (b) or a resting-length myosin head (c) results in a conformational change in the myosin head, causing a rapid dissociation of the myosin head from actin ((b) to (d) and (c) to (d), respectively). Following detachment from actin, the ATP is hydrolysed to ADP and Pi, both of which remain tightly bound to the myosin head (e). Hydrolysis is accompanied by a major conformational change which represents the reversal or a repriming of the power stroke. If an actin site is within reach of the myosin head, it will bind rapidly and reversibly to the actin site (a). When the myosin head binds actin, the interaction can promote a major change in conformation (the power stroke) which is accompanied by the dissociation of Pi ((a) to (b)). This step approximates to isometric contraction (no relative movement of actin and myosin) whereas the (a) to (c) steps approximate to an isotonic contraction (relative movement, with a release of the myosin ‘spring’). This power stroke consists of a reorientation of part of the myosin head that results in the displacement of the tip by up to 10 nm.

Reproduced with permission from S. Weiss and M. A. Geeves.

Fig. 16.1.3.9 The relationship between the action potential, the Ca2+-transient and the generation of force. The peak of force production is not achieved until near the end of the plateau phase of the action potential and lags behind the peak of the Ca2+-transient, reflecting the time required for Ca2+-induced Ca2+-release and cross-bridge cycling.

Fig. 16.1.3.9
The relationship between the action potential, the Ca2+-transient and the generation of force. The peak of force production is not achieved until near the end of the plateau phase of the action potential and lags behind the peak of the Ca2+-transient, reflecting the time required for Ca2+-induced Ca2+-release and cross-bridge cycling.

Fig. 16.1.3.10 Force—velocity—power relationship in cardiac muscle. (a) Force-velocity relationship (red symbols, black line). Maximum force is produced under isometric conditions (velocity V = 0), while maximum shortening speed is observed in unloaded muscle (force P = 0). The force—power relationship (blue symbols, green line) is a parabola with maximum power being produced at an intermediate force and velocity. (b) Length—tension relationship in cardiac muscle. At sarcomere length greater than 2.0 µm, isometric tension depends on the amount of overlap between myosin cross-bridges and actin filaments. At shorter sarcomere lengths (down to 1.6 µm), tension is reduced because of interference of thin filaments from opposite ends of the sarcomere. Below 1.6 µm sarcomere length, myosin filaments interfere with the Z-line and tension falls rapidly. The range of sarcomere lengths during a normal cardiac cycle is shown in blue.

Fig. 16.1.3.10
Force—velocity—power relationship in cardiac muscle. (a) Force-velocity relationship (red symbols, black line). Maximum force is produced under isometric conditions (velocity V = 0), while maximum shortening speed is observed in unloaded muscle (force P = 0). The force—power relationship (blue symbols, green line) is a parabola with maximum power being produced at an intermediate force and velocity. (b) Length—tension relationship in cardiac muscle. At sarcomere length greater than 2.0 µm, isometric tension depends on the amount of overlap between myosin cross-bridges and actin filaments. At shorter sarcomere lengths (down to 1.6 µm), tension is reduced because of interference of thin filaments from opposite ends of the sarcomere. Below 1.6 µm sarcomere length, myosin filaments interfere with the Z-line and tension falls rapidly. The range of sarcomere lengths during a normal cardiac cycle is shown in blue.

The isometric force produced by a muscle depends on the sarcomere length, being optimal at 2.00–2.25 µm where the overlap of thick and thin filaments is such that all the myosin cross-bridges can interact with actin (Fig. 16.1.3.10b). In the heart, the sarcomere length is generally less than optimal, with ‘preload’ stretching the sarcomere to 2.1 µm at the end of diastole and the sarcomere shortening to 1.6 µm during systole. In this length range, stretching the cardiac muscle when it is relaxed leads to increased force in the subsequent contraction. This characteristic is responsible in part for Starling’s law of the heart.

Control of contraction by Ca2+

Muscle contraction is initiated by an increase in cytoplasmic Ca2+, which binds to the troponin complex of the thin filament. Troponin comprises three subunits. Troponin C is a Ca2+-binding protein; in cardiac myocytes, the thin filament is activated when a single Ca2+ ion binds to troponin C. Troponin I is the inhibitory subunit. In relaxed muscle, the Ca2+ concentration is low and troponin I binds to a site on actin which blocks the binding of myosin cross-bridges, thus preventing cross-bridge cycling. In the presence of activating Ca2+ concentrations, Ca2+ binds to troponin C, which binds troponin I preventing its interaction with actin, permitting actin—myosin interaction. The third component, troponin T, binds to troponin C and troponin I and also to tropomyosin, independently of the Ca2+ concentration, thereby anchoring the regulatory complex on the thin filament (Fig. 16.1.3.2).

The Ca2+-sensitivity of cardiac myocytes is increased by stretch, promoting relaxation at the start of diastole (short sarcomere lengths) and activating contraction at start of systole (long sarcomere lengths). Moreover, this ‘stretch activation’ is delayed so that the enhanced contractility is synchronized with systole, thus contributing to the Starling effect.

Termination of contraction

Sarcoplasmic/endoplasmic reticulum ATPase type 2 (SERCA 2)

Contraction is terminated predominantly by Ca2+ reuptake into the SR by activation of SERCA2, an ATP-requiring Ca2+ pump expressed in the network of non-junctional SR surrounding the myofibrils (Fig. 16.1.3.3a). SERCA2 activity is regulated by the extent of phosphorylation of the SERCA2-associated protein phospholamban.

The Na+,Ca2+ exchanger (Na/CaX) and the Na+,K+-ATPase

The sarcolemmal Na/CaX contributes to lowering cytoplasmic Ca2+ during the latter part of the action potential and during diastole (Fig. 16.1.3.5 and Table 16.1.3.2). The Na/CaX utilizes the energy associated with the concentration and electrical gradients for Na+ to expel Ca2+ from the cell. It is electrogenic, promoting depolarization under these conditions. The exchange is sensitive to Na+i concentration: when membrane potential is near its diastolic level and Na+i is at normal physiological concentration, the Na/CaX will eject Ca2+ from the cell; if Na+i increases by a few mmol/litre and the membrane potential becomes depolarized, the exchanger can reverse and mediate Ca2+ entry.

The sarcolemmal Na+,K+-ATPase is responsible for Na+i extrusion. Cardiac glycosides (e.g. digoxin) inhibit the Na+,K+-ATPase, preventing Na+ extrusion, which indirectly reverses the Na/CaX into Ca2+o uptake mode. Under these conditions, Ca2+ uptake by the SR may be increased, thereby augmenting the cardiac Ca2+ pool and facilitating CICR. The net effect of the cardiac glycosides is to increase the cytoplasmic concentration and availability of Ca2+ resulting in an increased force of contraction.

Ventricular myocytes possess other, minor, systems to decrease cytoplasmic Ca2+ concentrations, including the plasma membrane Ca2+ ATPase and mitochondrial Ca2+ uptake. SERCA2 and Na/CaX contribute about 70% and 25%, respectively, towards relaxation, though these figures vary greatly between species.

Whole organ physiology

The cardiac cycle

Electrical events initiate the cardiac cycle with depolarization of the sinoatrial (SA) node in the upper right atrium (Fig. 16.1.3.11). Cardiac muscle acts as a functional syncytium. Communication between neighbouring cells is mediated by gap junctions which form arrays of cell-to-cell channels. The generated action potential spreads from the sinoatrial node across the functional syncytium at a speed of 1.0 to 1.2 M/s. The first mechanical response is atrial systole.

Fig. 16.1.3.11 Diagram of the heart showing the impulse-generating and impulse-conducting system.

Fig. 16.1.3.11
Diagram of the heart showing the impulse-generating and impulse-conducting system.

From Junqueira LC, Carneiro J, (2005). Basic histology,11th edn. McGraw-Hill, New York.

The cell-to-cell conduction of the electrical impulse from atrium to ventricle normally occurs only through the atrioventricular (AV) node (Fig. 16.1.3.11), a region of slow conductance at 0.02 to 0.1 m/s. This delays activation of the cells of the bundle of His and allows time for completion of ventricular filling. The conduction velocity in the bundle of His is from 1.2 to 2.0 m/s. The impulse passes via the right bundle branch and the two branches of the left bundle, and spreads rapidly (2.0–4.0 m/s) through the Purkinje fibres and each muscle cell to produce an orderly sequence of ventricular contraction (Fig. 16.1.3.11). Atrial and ventricular depolarization (P wave and QRS complex) and repolarization (T wave) can be recorded on the ECG (Fig. 16.1.3.12).

Fig. 16.1.3.12 Diagram of electrocardiographic complexes, intervals, and segments. VAT, ventricular activation time.

Fig. 16.1.3.12
Diagram of electrocardiographic complexes, intervals, and segments. VAT, ventricular activation time.

From Goldschlager N, Goldman MJ (1989). Principles of clinical electrocardiography, 13th edn. Appleton and Lange, East Norwalk, CT.

The specialized cells of pacemaker tissue have an inherent rhythmicity that is shared by the sinoatrial node, the atrioventricular node, and Purkinje tissue. Unlike other myocardial cells, these cells do not maintain a diastolic intracellular potential of about –90 mV, but tend to depolarize spontaneously. Because the sinoatrial node has the fastest inherent discharge (depolarization) rate, and because there is a brief period after depolarization of the whole heart during which a further stimulus is ineffective—the absolute refractory period—the sinoatrial node is normally the pacesetter for the heart. However, if this does not occur, pacemaker tissue in the atrioventricular node, the bundle of His, or the Purkinje system will assume this role, in which case the heart rate is then considerably slower.

Mechanical events

The mechanical events following depolarization of the atrial and ventricular muscle and their timing in relation to the ECG, to pressure and flow changes, and to heart sounds are shown in five phases in Fig. 16.1.3.13. After the P wave, and coinciding with atrial systole, ‘a’ waves appear in left atrial and right atrial pressure tracings due to atrial contraction, and an ‘a’ wave can be seen in the jugular venous pulse. Atrial contraction increases ventricular filling by about 10% (phase 1).

Fig. 16.1.3.13 Events of the cardiac cycle at a heart rate of 75 beats/min. The phases of the cardiac cycle, identified by the numbers at the bottom, are: (1) atrial systole; (2) isovolumetric ventricular contraction; (3) ventricular ejection; (4) isovolumetric ventricular relaxation; and (5) ventricular filling. Note that late in systole, aortic pressure actually exceeds left ventricular pressure. However, the momentum of the blood keeps it flowing out of the ventricle for a short time. The pressure relationships in the right ventricle and pulmonary artery are similar. The jugular venous pulse is similar in form to that seen in the right atrial pressure tracing. The ‘c’ wave interrupts the ‘x’ descent of the ‘a’ wave. The decline in pressure from the peak of the ‘v’ is the ‘y’ descent; the rate of decline reflects speed of ventricular filling. Atr. syst, atrial systole; ventric. syst, ventricular systole.

Fig. 16.1.3.13
Events of the cardiac cycle at a heart rate of 75 beats/min. The phases of the cardiac cycle, identified by the numbers at the bottom, are: (1) atrial systole; (2) isovolumetric ventricular contraction; (3) ventricular ejection; (4) isovolumetric ventricular relaxation; and (5) ventricular filling. Note that late in systole, aortic pressure actually exceeds left ventricular pressure. However, the momentum of the blood keeps it flowing out of the ventricle for a short time. The pressure relationships in the right ventricle and pulmonary artery are similar. The jugular venous pulse is similar in form to that seen in the right atrial pressure tracing. The ‘c’ wave interrupts the ‘x’ descent of the ‘a’ wave. The decline in pressure from the peak of the ‘v’ is the ‘y’ descent; the rate of decline reflects speed of ventricular filling. Atr. syst, atrial systole; ventric. syst, ventricular systole.

Modified with permission from Ganong WF (2005). Review of medical physiology, 22nd edn. McGraw-Hill, New York.

The onset of ventricular contraction coincides with the peak of the R wave of the ECG; a rapid rise in intraventricular pressure closes the mitral and tricuspid valves, causing the first heart sound. During this short isovolumetric period (phase 2 of Fig. 16.1.3.13), the pressure rises rapidly in the ventricles. When ventricular pressures exceed those in the pulmonary artery and aorta, the outflow valves open and ventricular ejection follows. The highest flow rate is in early systole, and pressures in the aorta and pulmonary artery rise. Normally, between 50 and 70% of the ventricular volume is ejected during systole, and this can be seen in the volume curve included in Fig. 16.1.3.13 (phase 3).

The jugular venous pulse, during ventricular contraction, has a positive deflection in early systole, the ‘c’ wave, due to right ventricular contraction and bulging of the tricuspid valve into the right atrium. Descent of the tricuspid ring caused by ventricular contraction then produces a negative ‘x’ descent, but as atrial inflow continues the pressure rises in the atria and great veins, producing the ‘v’ wave. This reaches its peak just before the opening of the tricuspid valve, declining during early ventricular filling as the negative ‘y’ descent. The changes in the pulmonary veins and left atrium are similar.

As the strength of ventricular contraction declines, coinciding with the end of the T wave, the aortic and pulmonary valves close, producing the dicrotic notch seen on both, aortic and pulmonary, artery pressure tracings in Fig. 16.1.3.13. Aortic closure slightly precedes pulmonary closure, and together these are responsible for the two components of the second heart sound. A short period of further rapid decline in ventricular pressure ensues without change in the ventricular volume (the period of isovolumetric ventricular relaxation, phase 4), and at the end of this the mitral and tricuspid valves open. There is a pressure gradient from atrium to ventricle so that a period of rapid ventricular filling follows, which coincides with the timing of the third heart sound. The rapid ventricular filling is reflected in the shape of the ventricular volume curve, and is followed by a period of slower filling (phase 5) with a final sudden small increment from the next atrial contraction as diastole ends (phase 1).

Third heart sounds are normally audible in children and young adults, but over the age of about 40 years this usually indicates elevation of ventricular end-diastolic pressure (most frequently in the left ventricle). The myocardium and valvular structures become stiffer with ageing, and large increases in ventricular end-diastolic pressure are then required to tense valvular structures and generate audible vibrations. A fourth heart sound almost always indicates abnormal ventricular function, with increased end-diastolic pressure. A fourth heart sound precedes the Q wave of the ECG, and must be distinguished from a normal splitting of the two components of the first heart sound. The latter occurs after the Q wave (Figs. 16.1.3.12 and 16.1.3.13).

Normal volumes, pressures, and flows

The blood volume in normal adults is about 5 litres (haematocrit 45%), and, of this, about 1.5 litres are in the heart and lungs—the central blood volume. The pulmonary arteries, capillaries, and veins contain about 0.9 litres, with only about 75 ml being in the pulmonary capillaries at any one instant. The volume of blood in the heart is about 0.6 litres. Left ventricular end-diastolic volume is about 140 ml, stroke volume about 90 ml, and end-systolic volume around 50 ml, reflecting an ejection fraction (stroke volume/end-diastolic volume) of between 50 and 70%. The right ventricular ejection fraction is similar.

Of the 3.5 litres in the systemic circulation, most—at least 60% of the total blood volume—is in the veins. The systemic veins containing most of the blood volume are easily distensible, and input of blood into the contracting heart is associated with only small changes in venous pressure. By contrast, ejection of blood into the much less distensible arterial tree produces large pressure changes.

The normal values for pressures generated in the heart and great vessels during the cardiac cycle are shown in Table 16.1.3.3. Pressures are measured with referencePressures are measured with reference to a zero pressure arbitrarily set at 5 cm below the sternal angle with the patient recumbent. ‘Normal’ arterial blood pressure is considered later (see below ‘Regulation of systemic arterial blood pressure’).

Table 16.1.3.3 Normal resting values for pressures in the heart and great vessels

Site

Systolic pressure (mmHg)

Diastolic pressure (mmHg)

Mean pressure (mmHg)

Right atrium

‘a’ up to 7, ‘v’ up to 5

‘y’ up to 3, ‘x’ up to 3

Less than 5

Right ventricle

Up to 25

End pressure before ‘a’ up to 3; end pressure on ‘a’ up to 7

Not applicable

Pulmonary artery

Up to 25

Up to 15

Up to 18

Left atrium (direct or indirect pulmonary capillary wedge)

‘a’ up to 12, ‘v’ up to 10

‘x’ up to 7, ‘y’ up to 7

Up to 10

Left ventricle

120

End pressure before ‘a’ up to 7; end pressure on ‘a’ up to 12

Not applicable

Cardiac output is the product of stroke volume and heart rate. It is related to body size and is best expressed as litre/min per m2 of body surface area: the ‘cardiac index’. The mean cardiac index under resting and relaxed conditions is 3.5 litre/min per m2, and values below 2 and above 5 are abnormal. The cardiac index declines with age. In persons of average size, resting oxygen consumption is about 240 ml/min, and the difference in oxygen content between arterial and mixed venous blood is about 40 ml/litre (arteriovenous oxygen difference), giving a basal cardiac output of 6 litre/min. In normal subjects, the arteriovenous difference in oxygen content at rest is maintained within narrow limits, from 35 to 45 ml/litre; values of 55 ml/litre and above are always abnormal.

Pulmonary or systemic vascular resistance is estimated by dividing the difference between mean inflow pressure (pulmonary artery or aortic) and mean outflow pressure (left atrial or right atrial) in mmHg by the flow in litre/min through the respective circulations. In normal subjects and patients without intracardiac shunts, this flow is the cardiac output. Normal pulmonary vascular resistance is less than 2 mmHg/litre per min (16 MPa s m−3,160 dyn s cm–5). Arterial blood pressure is the product of cardiac output and total peripheral resistance.

Stroke work is the integral of instantaneous ventricular pressure with respect to stroke volume, but is usually estimated as the product of stroke volume and mean ejection pressure. The orderly sequence of contraction in the normal cardiac cycle coordinates changes in instantaneous pressure and flow, so maximizing the transfer of energy to the circulation. Normal left ventricular work output at rest is about 6 kg/m2 per min.

Myocardial mechanics

When a muscle is activated to contract, it develops a potential for doing work. In isolated skeletal and heart muscle preparations, the stretching force applied to the muscle—and therefore the length of the muscle—can be varied before contraction; this is the preload. The activated muscle will begin to shorten when it has generated a force sufficient to overcome that exerted by the attached weight or load against which it contracts. When the force exerted by the load is so arranged that it is not applied to the relaxed muscle and is applied only after the muscle has begun to develop tension, it is termed the afterload. If this load is so large that the activated muscle is unable to overcome it, and so cannot shorten, the contraction produces tension only, and the contraction is isometric. When shortening does occur, external work is done. If the load is constant during the shortening, the contraction is said to be isotonic; if it changes, it is auxotonic.

The tension produced by both skeletal and cardiac muscle during contraction depends on initial fibre length; during afterloaded isotonic contractions from a particular length, the amount and the speed of fibre shortening and the tension developed all depend upon the afterload. Over a range of loads the initial velocity of muscle shortening is most rapid and the most extensive shortening occurs when the load is smallest.

The inverse relationship between initial velocity of fibre shortening and load in an isotonic contraction is a fundamental one for both skeletal and cardiac muscle. There is, however, a major difference between the two types of muscle in that the relationship at any one length is constant in a skeletal muscle, whereas in cardiac muscle there are variations in inotropic state that are accompanied by considerable changes in the relationship between force and velocity. A positive inotropic effect produces a more extensive contraction from the same initial length and afterload, and a faster maximum velocity of shortening (Vmax). An increase in initial fibre length with no increase in inotropic state increases the force of contraction but does not, however, change the maximum velocity of shortening. This is illustrated in Fig. 16.1.3.14.

Fig. 16.1.3.14 Idealized relationships between velocity of fibre shortening and afterload or force developed during contraction of a strip of cardiac muscle under three different conditions. Curves a and b were obtained with the muscle in the same inotropic state but with a longer initial fibre length (greater preload) for curve b. Curves b and c were obtained with initial fibre length the same but with contractility increased in c by the addition of a drug producing a positive inotropic effect. The terms Vmax and P0 describe, respectively, a hypothetical maximum shortening velocity in the absence of any load (hence the broken lines), and the force developed in an isometric contraction. An increase in initial fibre length increases P0 but not Vmax; a positive inotropic change increases both P0 and Vmax.

Fig. 16.1.3.14
Idealized relationships between velocity of fibre shortening and afterload or force developed during contraction of a strip of cardiac muscle under three different conditions. Curves a and b were obtained with the muscle in the same inotropic state but with a longer initial fibre length (greater preload) for curve b. Curves b and c were obtained with initial fibre length the same but with contractility increased in c by the addition of a drug producing a positive inotropic effect. The terms Vmax and P0 describe, respectively, a hypothetical maximum shortening velocity in the absence of any load (hence the broken lines), and the force developed in an isometric contraction. An increase in initial fibre length increases P0 but not Vmax; a positive inotropic change increases both P0 and Vmax.

The contraction of the intact heart can be visualized as being similar mechanically to the afterloaded contraction of an isolated muscle strip. For the left ventricle, the preload is the distending force which stretches the muscle fibres in end-diastole, and the initial afterload is the force the ventricle must generate in order to open the aortic valve and eject blood. At the end of ejection, the ventricular muscle is isolated from the peripheral circulation, with the afterload then supported by the competent aortic valve, and the muscle relaxes against a comparatively small force. Relaxation of the heart is an active process due to withdrawal of calcium ions from the cytoplasm surrounding the myofibrils. ‘Active’ relaxation is still proceeding in the ventricular wall when the atrioventricular valves open, and, if it is delayed—as in the hypoxic heart—the slower relaxation increases the stiffness of the ventricular wall and reduces filling. Wall thickness is also a determinant of relaxation rate and compliance. For this reason, filling pressures are higher for the thicker and stiffer left ventricle than for the thinner and more distensible right ventricle (Table 16.1.3.3). When the left ventricle is hypertrophied due to chronic pressure overload, as in systemic hypertension or aortic stenosis, it becomes stiffer and filling pressures may then be abnormally high.

Regulation of cardiac function

Four essential factors determine the performance of the heart: (1) venous return, (2) outflow resistance (afterload), (3) inotropic state or contractility, and (4) heart rate. Changes in cardiac performance are accomplished by mechanisms that alter these four determinants.

Venous return, preload, and the Frank–Starling relationship

The relationship described independently by Frank and Starling between end-diastolic fibre length and force of contraction is shown in Fig. 16.1.3.15. When the ventricle ejects against a constant pressure, variations in venous return alter the degree of stretch of the muscle fibres in diastole, and this determines contraction strength and work output. The number of active force-generating sites in each fibre increases as it lengthens so that, within limits, the force of contraction and stroke work are positively related to end-diastolic fibre length. The relationship is curvilinear when stroke work is plotted against end-diastolic pressure as an index of preload, reflecting the exponential relationship between end-diastolic pressure and end-diastolic volume. When stroke work is plotted against end-diastolic volume, the relationship between stroke work and preload is linear.

Fig. 16.1.3.15 The relation between left ventricular end-diastolic fibre length and left ventricular stroke work showing displacement upward and to the left with an increase in contractility and downward and to the right with a reduction in contractility. Similar but not identical curves are obtained by plotting left ventricular stroke work as one measure of the force of contraction against ventricular end-diastolic pressure or volume (see text). Similar function curves may be obtained from both ventricles and both atria.

Fig. 16.1.3.15
The relation between left ventricular end-diastolic fibre length and left ventricular stroke work showing displacement upward and to the left with an increase in contractility and downward and to the right with a reduction in contractility. Similar but not identical curves are obtained by plotting left ventricular stroke work as one measure of the force of contraction against ventricular end-diastolic pressure or volume (see text). Similar function curves may be obtained from both ventricles and both atria.

The response of the heart at any particular time depends upon: (1) the intrinsic state of the muscle, i.e. the biochemistry and contractile machinery; (2) the prevailing neurohumoral state, e.g. increased sympathetic outflow produces a more forceful contraction at any end-diastolic fibre length; (3) extrinsic inotropic influences—drugs which have either positive or negative inotropic effects.

End-diastolic fibre length is determined by the force distending the ventricle at end-diastole, and end-diastolic pressure provides a reasonable indication of this force when the ventricle has normal distensibility or compliance; this is the preload. The systemic venous return and the elastic properties of the myocardium produce the end-diastolic distending pressure for the right ventricle, and the pulmonary venous return and myocardial elasticity that for the left ventricle. For clinical purposes, it is convenient to equate venous return with preload because, as it changes from beat to beat, it adjusts the strength of the subsequent ventricular (and atrial) contraction by varying the force stretching the relaxed cardiac muscle and changing end-diastolic fibre length.

Outflow resistance or afterload

Pulmonary and aortic valve opening pressures are determined largely by the pulmonary and systemic vascular resistances, as shown for the latter in Fig. 16.1.3.16. These resistances, together with an inertial component dependent upon the mass of blood within the vessels, the compliance (stiffness) of the vessels, and the physical characteristics of each vascular tree combined with the pulsatile nature of the flow, constitute the impedance to ventricular outflow. This is the load against which the ventricle must contract and shorten. As this load is not applied in diastole to the relaxed muscle, it then being supported by competent aortic and pulmonary valves, it is described clinically as the afterload: it becomes applied to the muscle only after the ventricle has begun to develop tension.

Fig. 16.1.3.16 Diagram of the changes in pressure as blood flows through the systemic circulation. The total cross-sectional area of the vessels (TA) increases from 4.5 cm2 to 4500 cm2 in the capillaries. The major resistance to flow is at the arteriolar level.

Fig. 16.1.3.16
Diagram of the changes in pressure as blood flows through the systemic circulation. The total cross-sectional area of the vessels (TA) increases from 4.5 cm2 to 4500 cm2 in the capillaries. The major resistance to flow is at the arteriolar level.

Modified and reproduced with permission from Ganong WF (2005). Review of medical physiology, 22nd edn. McGraw-Hill, New York.

Regulation of systemic arterial blood pressure

The regulation of the systemic circulation is well adapted to the vital function of maintaining constant, adequate cerebral perfusion. There is a need to maintain a relatively constant arterial blood pressure when there are changes in posture and circulating blood volume. The baroreceptors mediate rapid responses to alterations in aortic pressure, while a variety of hormonal and physical factors regulate the circulating blood volume.

Baroreceptors

The baroreceptor regulatory system comprises two groups of stretch receptors: one group in the carotid sinuses near the bifurcations of the common carotid arteries in the neck and a second group in the arch of the aorta. These respond to an increase in central arterial pressure by the firing of impulses, which pass by the glossopharyngeal and vagus nerves to the solitary tract nucleus in the medulla and inhibit sympathetic outflow. Efferent impulses from these central connections pass via the right vagus nerve mainly to the sinoatrial node, and via the left vagus mainly to the atrioventricular node. The effect is to decrease the heart rate and the force of atrial contraction. There is also attenuation of sympathetic discharge to arteriolar smooth muscle in the limbs and visceral circulation, resulting in a release of peripheral arteriolar constriction and, therefore, peripheral vasodilatation. Thus the immediate response to a rise in arterial pressure is slowing of the heart rate, reduced force of atrial contraction, and reduced vascular resistance. The net effect of this negative feedback system is to offset the elevation in blood pressure. Conversely, a lowering of blood pressure diminishes stimulation of the stretch receptors and reduces afferent traffic to the solitary tract nucleus, resulting in reduced inhibition of sympathetic outflow. There is, then, a quickening of the heart rate and peripheral vasoconstriction so that the blood pressure increases. The changes in heart rate take place within 1 to 2 s and changes in vasomotor control within 5 or 6 s.

Baroreceptor mechanisms effectively modulate the responses of blood pressure to postural change. Additionally, they adapt to maintain the normal circadian variation in blood pressure (see below ‘Diurnal variation in autonomic function’). They also maintain elevated arterial blood pressure in systemic hypertension. Sensory input to the reflex is reduced in disorders of the autonomic nervous system, and in the prolonged weightlessness of space flight.

Blood volume

The circulating blood volume is relatively small, and a large proportion is contained in the veins (Fig. 16.1.3.16) so that any change in blood volume will affect venous return and, therefore, cardiac output and blood pressure. When blood volume is large and the veins full, there is little reduction in venous return on standing and cardiac output is maintained. However, when effective blood volume is reduced and the veins are relatively empty, on standing there is pooling of blood in the veins of the legs and a reduction in venous return and cardiac output so that arterial blood pressure falls. Baroreceptor responses become evident within a couple of beats, the heart rate increases, and cardiac output and blood pressure are restored. Circulating blood volume is kept relatively constant by a combination of mechanisms which involve the actions of natriuretic peptides, the renin–angiotensin–aldosterone system, vasopressin, and osmolality.

Natriuretic peptides

The discovery of secretory granules in the atria of the heart, and the demonstration in 1981 that they produce a natriuretic factor that inhibits the reabsorption of sodium in the distal tubule of the kidney, enhanced understanding of the regulation of blood volume and cardiac performance. Three natriuretic peptides have subsequently been identified.

  • ANP is present in the circulation, and concentrations increase during volume expansion. The right atrium contains about 2 to 4 times as much activity as the left, and release of the hormone is mediated largely by atrial distension. The effect is to produce a diuresis and to reduce cardiac and circulating blood volume. ANP also has a vasodilator action and opposes the vasoconstricting effects of noradrenaline and angiotension II.

  • The second natriuretic peptide was identified in brain tissue and is now referred to as B-type natriuretic peptide. Large amounts are found in the ventricles of the heart, and circulating levels are increased in ventricular hypertrophy and cardiac failure. B-type and ANP have similar actions.

  • The third natriuretic peptide to be identified was C-type natriuretic peptide. It is distributed widely in tissues, circulating concentrations are low, and it appears to have actions similar to the other two peptides, but with a greater vasodilator effect on veins.

These three peptides contribute to the regulation of cardiac and circulating blood volume and of blood pressure. Both B-type natriuretic peptide and N-terminal pro-brain natriuretic peptide are useful adjuncts to the clinical evaluation of dyspnoeic patients in that levels are elevated when the dyspnoea is due to cardiac failure.

Renin–angiotensin system

This system, which is both local and systemic, is of major importance in the regulation of circulating blood volume and the maintenance of normal blood pressure. Enhanced activity of systemic renin and angiotensin increases the production of aldosterone, which promotes reabsorption of sodium by the kidney and expansion of circulating blood volume. All components of the renin–angiotensin system are distributed widely throughout tissues—including the brain and the heart—and increased activation of the system increases the risk of cardiovascular events. Angiotensin II is a potent vasoconstrictor that has a number of additional important actions on the vasculature (Fig. 16.1.3.17). The angiotensin converting enzyme (ACE) inhibitors in clinical use diminish angiotensin II production locally and in the circulating blood.

Fig. 16.1.3.17 The renin—angiotensin system. ACE, angiotensin converting enzyme; PAI, plasminogen activator inhibitor.

Fig. 16.1.3.17
The renin—angiotensin system. ACE, angiotensin converting enzyme; PAI, plasminogen activator inhibitor.

Both local and general effects appear important in mediating the benefits that accrue from the use of these drugs in the management of hypertension and congestive cardiac failure, and in the reduction in rates of recurrence of coronary events in ischaemic heart disease. The mechanisms mediating the latter include antioxidant effects and a reduction in the production of potentially damaging free radicals, anti-inflammatory effects, and augmentation of the profibrinolytic effects of bradykinin. The more recently developed angiotensin II receptor blocking drugs (ARBs) have now been shown to produce similar outcomes.

Regulation of nitric oxide production

A recently recognized contribution to endothelial function, which affects the afterload, is related to nitric oxide production, and its inhibition by asymmetric dimethylarginine (ADMA). Asymmetric dimethylarginine is produced by the physiological degradation of methylated proteins. ADMA inhibits the production of nitric oxide, which is derived directly from l-arginine, present in all cells. ADMA levels are regulated by the balance between its production and its metabolism. The balance may be disrupted in clinical situations, for example in renal impairment. Reduced renal function increases the level of ADMA and this reduces endothelial dilatation.

Ventricular volume and afterload

Ventricular volume also has a major effect on afterload, as pressure is equal to force per unit area. The force acting radially on the inner surface of the whole ventricle at any time during systole is the product of the intraventricular pressure and ventricular surface area at that time. If the left ventricle is assumed to be a sphere (surface area = π‎d2), the force opposing ejection at any time during contraction is the product of the intracavity pressure and π‎d2 at that time. Thus, a doubling in left ventricular diameter from a normal value of 5 cm to 10 cm would result in a fourfold increase in the force opposing ejection for the same intracavity systolic pressure; the ventricle would need to develop greatly increased wall tension to overcome that force. Because wall tension developed during systole is the major determinant of myocardial oxygen consumption, the contraction will clearly be much less efficient in the larger heart for the same stroke volume and ejection pressure (stroke work).

During a normal heartbeat, the afterload is greatest at the beginning of ejection (rapid rise in pressure and maximum volume; Fig. 16.1.3.13), but decreases thereafter as the pressure reaches a plateau and then declines as the ventricle becomes smaller. There is, therefore, a matching of the afterload to the declining intensity of the contraction as it proceeds to completion, and fibres shorten at a relatively constant rate. This is less obvious in a large heart where the volume change during ejection is a smaller proportion of the total ventricular volume.

The end-diastolic volume is influenced by preload, afterload, circulating blood volume, the inotropic state of the ventricle, heart rate, and neurohumoral influences. It is smaller in the erect than in the horizontal position because of reduced venous return, and it decreases with a moderate increase in heart rate because of an associated positive inotropic effect. The proportion of end-diastolic volume ejected during systole, the ejection fraction (normal 50–70%), is a useful index of overall left ventricular function and is easily measured noninvasively by gated blood-pool scanning and two-dimensional echocardiographic techniques. The ejection fraction increases with exercise and with positive inotropic interventions. Values for right ventricular ejection fraction are of the same order as those for the left side of the heart.

Role of the sympathoadrenal system in normal and failing hearts

Catecholamines have positive inototropic, lusitropic, and chronotropic effects on the normal heart. The inotropic effect of catecholamines on the force of contraction is mediated by a PKA-mediated phosphorylation of the L-type Ca2+ channel, which increases the probability of channels opening when the cell is depolarized, thus increasing ICa. Positive lusitropism (myocardial relaxation) is achieved by a PKA catalytic subunit-mediated phosphorylation of phospholamban which inhibits SERCA2 in its hypophosphorylated state (Fig. 16.1.3.8a). Phosphorylated phospholamban does not inhibit SERCA2. The effect of phospholamban phosphorylation is thus to activate SERCA2 and stimulate Ca2+ reuptake into the SR. In addition, protein kinase A phosphorylates cardiac troponin I, and this increases the rate of dissociation of Ca2+ from troponin C, increasing the rate of dissociation of myosin cross-bridges from actin (i.e. stimulating relaxation). Positive chronotropism is achieved by increasing the frequency of depolarization in the sinoatrial node. Upon stimulation of the sympathoadrenal system, If (generated by HCN channels) is activated to depolarize the membrane to the threshold level more quickly and increase the rate of production of action potentials. The positive chronotropism of the sympathoadrenal system is in part mediated by the binding of cAMP to these HCN channels. This shifts their voltage dependence of activation to more depolarized potentials and increases both the rate of channel opening and the maximal current level. The net result is an increased frequency of depolarization and the heart rate increases.

In heart failure, the myocyte becomes unresponsive to β‎-adrenergic agonists, and consequently phosphorylation of the proteins responsible for the control of contractility is diminished. The β‎1-adrenergic receptor abundance is decreased and the expression of proteins which antagonize β‎1-receptor signalling is increased, thus the efficacy of β‎-agonism is diminished. Many drugs that mitigate heart failure are targeted at the proteins that regulate the inotropic state. There is evidence also that SERCA2 expression is decreased and this may contribute to the elevated cytoplasmic Ca2+ concentrations sometimes seen in diastole during heart failure and the poorer contraction of the heart in this condition.

Myocardial function is greatly altered by changes in inotropic state or contractility. Positive inotropic effects are thought to be mediated by activation of excitation–contraction coupling mechanisms and are associated with an increased influx of calcium ions into myocardial cells and a more powerful contraction. Changes in the intensity of excitation–contraction coupling are independent of the Frank–Starling mechanism. Increases in the intensity shift the curve upwards and to the left, and decreases shift it downwards and to the right (Fig. 16.1.3.15). With a positive inotropic effect, the force of contraction, however measured, is increased for a given end-diastolic fibre length and, if the afterload is the same, the initial velocity of fibre shortening is also increased (Fig. 16.1.3.14); in the intact heart, there is more complete emptying during systole. Increased sympathetic stimulation, some drugs, and an increase in heart rate itself (the staircase or Bowditch phenomenon; postectopic potentiation, see ‘Heart rate’, next section) have positive inotropic effects. Myocardial depressants, such as hypoxia and most anaesthetic drugs, have negative inotropic effects. Increased parasympathetic stimulation produces acetylcholine-mediated negative inotropic effects that are confined almost entirely to the atria because of the anatomical distribution of vagal endings in the myocardium.

It is difficult to measure inotropic changes accurately in the human heart because changes in the intensity of excitation–contraction coupling and changes in the Frank–Starling relationship, though separate, are nevertheless closely linked. The peak rate of change of intraventricular pressure (peak dp/dt) is a useful index of change in contractility, provided that preload, afterload, and heart rate remain constant.

Heart rate

Frequency of contraction is the fourth essential determinant of cardiac performance. Heart rate during rest and exertion may vary from 45 to 200 beats/min in the healthy young adult. As changes can occur within seconds, an increase in heart rate is the usual and most effective way of producing a rapid increase in cardiac output. It plays the major role in the response to exercise, during which stroke volume does increase (more so in athletes and when in the erect, rather than the supine, position) but the changes are less marked than those of rate. In addition, an increase in contraction frequency itself produces a positive inotropic effect, whereby the force of contraction increases and reaches a new steady state within a few beats. This is termed the ‘positive staircase’, Treppe, or Bowditch effect. It may be a consequence of an augmented movement of calcium ions into myocardial cells with increased frequency of action potentials, combined with diminished time for outward movement of calcium between beats. More forceful contractions also follow premature beats—the phenomenon of postextrasystolic potentiation—and the mechanism is probably the same. The extrasystole occurring prematurely is a weak contraction because of decreased filling time and an uncoordinated activation of the ventricle when the ectopic focus is within the ventricle. The next beat is delayed because of the refractory period of the extrasystolic beat, but is a more powerful contraction because of increased filling time and ventricular volume, and increased contractility. Calcium-dependent changes similar to those of the Bowditch effect are probably responsible for the latter. Recently it has become clear that intrinsic circadian clocks are involved in the control of heart rate, with time-of-day dependent oscillations in clock gene expression. The cardiomyocyte circadian clock influences many myocardial processes, including ion channels.

Coronary blood flow

Coronary blood flow accounts for about 4% of the cardiac output. The heart extracts most (70%) of the oxygen carried in the coronary circulation; the arteriovenous difference for oxygen across the heart being about 110 ml/litre, while that for the whole body is only about 40 ml/litre under resting conditions. Therefore, large increases in myocardial oxygen requirements must be met largely by increases in coronary blood flow, and this may increase five- or sixfold during strenuous exercise. The greater part of this flow is to the left ventricle, of which at least two-thirds occurs during diastole because of the throttling effect systole has on myocardial perfusion. The main coronary arteries are on the superficial surface of the heart, and because of this, and the hindrance to coronary flow during systole, the subendocardial region of the left ventricle is more vulnerable to perfusion deficits in relation to oxygen need than the outer two-thirds of the muscle wall. Despite these mechanical problems, flow is normally evenly distributed throughout the myocardium so that when regional coronary blood flow is measured using injected radioactive microspheres (in dogs), the ratio of endocardial to epicardial flow is approximately unity. In fact, the inner layers of the heart probably receive slightly more blood (up to 10%) than the outer layers. This is consistent with the subendocardium developing more tension than the subepicardium, and is evidence for a greater rate of myocardial oxygen consumption in the inner layers.

Myocardial oxygen requirements and coronary blood flow are finely adjusted.

The nervous system and the heart

The heart is richly supplied with adrenergic nerves, whose terminals reach atrial and ventricular muscle fibres and impinge upon all pacemaker tissue, including the sinoatrial and atrioventricular nodes and Purkinje fibres. Sympathetic stimulation leads to an increase in myocardial contractility and heart rate, and in the rate of spread of the activation wave through the atrioventricular node and the Purkinje system. This is mediated by local noradrenaline release, which interacts with β‎-adrenergic receptors. The key elements in these regulatory mechanisms are calcium ions and cAMP. The activated β‎-receptor increases adenylcyclase activity and the conversion of ATP to cAMP. Peptide cotransmitters, released with noradrenaline and acetylcholine, have recently been isolated and also influence autonomic function. Neuropeptide Y is a peptide of 36 amino acids that is colocated with noradrenaline in most sympathetic nerves and is released with sympathetic stimulation. It is a powerful pressor agent with direct arteriolar vasoconstrictor action and also potentiates the pressor action of noradrenaline.

The distribution of parasympathetic fibres is much more limited, being confined to the sinoatrial and atrioventricular nodes and the atria, with few, if any, fibres reaching the ventricles in humans, except perhaps in relation to coronary arteries and Purkinje tissue. The effects of parasympathetic nerve stimulation are mediated by local acetylcholine release, which slows the heart rate and speed of conduction through the atrioventricular node and Purkinje tissue, and depresses atrial contractility. The negative inotropic effects are associated with a lowering of the concentration of intracellular cAMP.

The effect of the nervous system on the heart at any one time is the sum of the activities of these two opposing control systems. They usually vary reciprocally. Under resting conditions, vagal inhibitory effects predominate, maintaining a slow heart rate, there being virtually no sympathetic outflow. With exercise, there is withdrawal of vagal activity and an increase in sympathetic outflow. Afferents from stretch receptors in the carotid sinus and aortic arch—the baroreceptors—also have a considerable effect on cardiac performance, this effect being mediated via the adrenergic nervous system and vagal withdrawal. A fall in blood pressure reduces stretching in the carotid sinus and inhibitory afferent traffic so that the sympathetic outflow increases. As a consequence of this combined vagal and adrenergic effect, there is a quickening of the heart rate within one or two beats, a positive inotropic effect, and also a constriction of veins and arterioles that increases preload and afterload. Elevation of pressure in the carotid sinus has the reverse effects. In cardiac failure, there is a reduced variability in heart rate due to these autonomic mechanisms as there is then a predominance of adrenergic activity.

There are also mechanoreceptors in all four chambers of the heart (identified in dogs) and in the coronary vessels, which give rise to depressor reflexes. Their clinical relevance is uncertain, but they may contribute, e.g. to the bradycardia and hypotension occurring in some patients with acute myocardial infarction and to the syncope that patients with critical aortic stenosis may experience with the onset of exercise when there is sudden left ventricular distension. Vagal afferents from reflexogenic areas in the infarcting left ventricle may be responsible for the bradycardia, gastric distension, nausea, and vomiting which frequently occur with the onset of inferior or posterior myocardial infarction, but not usually of anterior infarction, which is generally associated with a marked increase in sympathetic activity. The cardiac receptors connected to afferent fibres running in cardiac sympathetic nerves, however, are very important because they are responsible for the perception of cardiac pain. Receptors have also been identified (in animals) at the junction of pulmonary veins with the atrial wall. These respond to mechanical distension with increased sympathetic outflow to the sinus node and inhibition of secretion of antidiuretic hormone from the posterior lobe of the pituitary gland. The result is a quickening of the heart rate and diuresis.

Autonomic efferent activity

The autonomic outflow to the heart is controlled by multiple integrative sites within the central nervous system, with complex interactions between afferent and central inputs. Autonomic responses are mediated through the suprapontine and bulbospinal pathways—both those arising ‘reflexively’ and those arising from various types of volitional or central ‘command’. Nevertheless, intrinsic mechanisms are sufficient for adequate cardiac function in the absence of autonomic control, as prolonged survival after cardiac transplantation has shown. But in the denervated heart, there is blunting of the normally rapid physiological adjustments mediated by the autonomic nervous system.

Diurnal variation in autonomic function

Variations in vascular tone and control of blood pressure and of hormone secretion and platelet function occur in a predictable way throughout the 24-h cycle. In normal subjects, there is a circadian rhythm of blood pressure changes that is not seen in patients, after cardiac transplantation, who have denervated hearts. There is a decline in both blood pressure and heart rate at night, and increases in both soon after wakening. This is due to a normal adrenergic surge in the early morning, which results in increased vascular tone and blood pressure. Increased forearm vascular resistance in the morning, with a reduction in the afternoon and evening, can be clearly identified in humans by assessing responses to α‎-adrenergic blockade. It is presumed that this occurs in coronary vessels as well. Measurable early morning increases in circulating catecholamines and in the propensity for platelets to aggregate can also be documented.

The circadian rhythm of autonomic function is correlated with a significant tendency for myocardial infarction and sudden cardiac death to occur more frequently in the morning, soon after wakening. There is also an increase in the occurrence of angina pectoris in the early morning, independent of the level of physical activity.

Exercise and the heart: cardiac reserve

The heart responds to exercise with an increase in cardiac output, and values of 30 litres/min may be achieved in a trained athlete. Exercising muscles extract more oxygen from the blood, but the response of the cardiac output is the ultimate determinant of delivery of oxygen to tissues and is the limiting factor for aerobic exercise.

The cardiac response to exercise involves all the mechanisms already discussed. Interaction within the central nervous system between higher and autonomic centres augments sympathetic discharge, and there is a withdrawal of parasympathetic outflow. The heart rate increases immediately, and redistribution of peripheral flow increases venous return and preload. There is venoconstriction, particularly in the large-volume splanchnic circulation, and vasoconstriction and increased oxygen extraction in inactive parts. In active parts, there is vasodilation. This is most evident in the vascular beds of the exercising skeletal muscles and of the heart. The overall effect is a marked lowering of total peripheral vascular resistance, which reduces afterload and encourages greater systolic emptying of the left ventricle. Stroke volume increases during exercise in the upright position. During light to moderate exercise (running or cycling), up to about 80% of maximum exercise capacity there is an almost linear relationship between work intensity and heart rate response, cardiac output, and oxygen uptake. With further exercise, the heart rate and cardiac output responses level off while additional increases in oxygen consumption (c.500 ml) occur by increased oxygen extraction and a greater widening of the arteriovenous difference for oxygen.

The venous return increases in relation to the elevated cardiac output. Vasodilation in the working muscles that receive the bulk of the redirected blood permits high flow rates into the capacitance vessels. Because of adrenergically mediated venoconstriction, the capacity of this system is reduced, so that blood moves rapidly into the right atrium. Venous return is also enhanced by the pumping action of the rhythmically contracting working muscles, by a decrease in intrathoracic pressure with forced inspiration, and by an increase in intra-abdominal pressure. The augmented pulmonary blood flow results in only slight increases in pulmonary artery pressure because of the distensibility of the large pulmonary arteries, an increased area of the pulmonary capillary bed due to the recruitment of more capillaries, and the low resistance offered by the normal pulmonary circulation (see Table 16.1.3.1).

The elevated cardiac output and larger stroke volume result in increased systolic blood pressure and pulse pressure, even though the afterload itself is reduced. Enhanced neurohumoral activity from adrenergic stimulation of the heart and the suprarenal glands (increased circulating adrenaline and noradrenaline) effect positive inotropic changes, to which tachycardia also contributes because of the Bowditch effect. There is a shift in the Frank–Starling relationship to the left, increased speed and force of cardiac contraction, and elevated ejection fraction and stroke volume. Peak dp/dt is increased, and there is a rapid rise in coronary blood flow to meet myocardial oxygen requirements that increase linearly with the product of systolic blood pressure and heart rate. During moderate exercise, these changes together result in a decreased or unaltered end-diastolic volume and decreased end-systolic volume. With severe exercise, end-diastolic dimensions and end-diastolic fibre length are slightly increased and the Frank–Starling mechanism then operates and further augments the force of contraction.

The haemodynamic and ventilatory responses evoked by an increase to a new steady workload take about 2 to 3 min to equilibrate and adjust oxygen supply to the greater demand. Protocols for exercise testing are therefore usually based on work increments at 3-min intervals to allow time for a new ‘steady state’ to occur, e.g. in the standard Bruce exercise protocol. A steady state becomes progressively more difficult to maintain as maximal exercise capacity is approached. Glycogen is used by the working skeletal muscles as a source of stored energy, and the anaerobic metabolism which ensues produces lactic acidosis and thereby further increases ventilation. As all cardiopulmonary transport mechanisms reach maximum levels, shortness of breath, fatigue, and muscle pain become limiting symptoms; motivation is then the final determinant of the duration of exercise. Ageing reduces the efficacy of cardiopulmonary transport mechanisms and, of course, exercise capacity. The heart rate response at peak exercise reflects this. In healthy individuals aged 20 years it is about 200 beats/min, and at 65 years about 170 beats/min.

When exercise stops, the cardiopulmonary and metabolic changes return rapidly to resting levels, the rate following an exponential pattern in the first few minutes; the excretion and metabolism of lactate and other substances, and the dissipation of heat generated take longer (time constant of about 15 min or more). Reduced circulatory function slows the recovery rate.

Training effects

Regular exercise to about 60% of maximal heart rate for 20 to 30 min three times a week is the minimum requirement for improved effort tolerance due to a training effect. The resting heart rate becomes slower, while the cardiac output is maintained by an increased end-diastolic volume and ejection fraction, and therefore stroke volume. In a ‘trained’ exercising individual, there is a reduced heart rate response to a standard submaximal workload, and systemic blood flow is more effectively distributed away from visceral and skin circulations to working muscles. Changes in muscle mitochondria permit increased oxygen consumption. There is suggestive animal evidence that prolonged endurance training increases the calibre of coronary arteries and enlarges capillary surface area relative to cardiac muscle mass. Myocardial protein synthesis increases. Adrenergic mechanisms appear to be involved in mediating this response. Rhythmic exercise (e.g. running) and isometric exercise (e.g. weightlifting) have different physiological effects. The blood pressure rises disproportionately during the latter. The mechanisms are partly reflex and partly mechanical from the contracting muscles. Isometric exercise training is not recommended for cardiac patients because of the increased afterload it imposes.

Regular exercise has other effects: it increases feelings of well-being and lowers blood pressure in normotensive and mildly hypertensive subjects. There are also diverse exercise-related hormonal changes, including the reduction of glucose-stimulated insulin secretion—of particular relevance to patients with type 2 diabetes. Regular exercise also improves the availability of nitric oxide, with its important vascular effects. These are considered elsewhere.

To summarize, changes in the four essential determinants of cardiac function—preload, afterload, heart rate, and contractility—combine to augment cardiac output and oxygen delivery during exercise. Measurement of the cardiovascular response to exercise is essential for the objective assessment of cardiac function.

Further reading

Bers DM (2001). Excitation-contraction coupling and cardiac contractile force, 2nd edition. Kluwer, Dordrecht.Find this resource:

    Durgan DJ, Young ME (2010). The cardiomyocyte circadian clock: emerging roles in health and disease. Circ Res, 106, 647–58.Find this resource:

    Ganong WF (2005). Review of medical physiology, 22nd edition. McGraw Hill, New York.Find this resource:

      Houser SR, Margulies KB (2003). Is depressed myocyte contractility centrally involved in heart failure? Circ Res, 92, 350–8.Find this resource:

        Jones NL, Killian KJ (2000). Exercise limitation in health and disease. N Engl J Med, 243, 632–41.Find this resource:

        Kaestner L (2012). Calcium signalling: approaches and findings in the heart and blood. Springer Science Media, Berlin.Find this resource:

          Kardami E, et al. (2010). Cardiac cell biology. Springer Science Media, Berlin.Find this resource:

            Katz, AM (2006). Physiology of the heart, 4th edition. Lippincott Williams and Wilkins, Philadelphia, PA.Find this resource:

              Ko Y-S, et al. (2004). Three-dimensional reconstruction of the rabbit atrioventricular conduction axis by combining histological, desmin, and connexin mapping data. Circulation, 109, 1172–9.Find this resource:

                Libby P, et al. (ed.) (2008). Braunwald’s heart disease; a textbook of cardiovascular medicine, 8th edition. Saunders Elsevier, Philadelphia.Find this resource:

                  Opie LH (2013). Stunning, hibernation and calcium in myocardial ischemia and reperfusion. Kluwer Academic Publishers, Boston, MA.Find this resource:

                    O’Rourke B (2010). Be still, my beating heart: never! Circ Res, 106, 238–9.Find this resource:

                    Severs NJ (2000). The cardiac muscle cell. BioEssays, 22, 188–99.Find this resource:

                      Severs NJ, et al. (2004). Gap junction alterations in human cardiac disease. Cardiovasc Res, 62, 368–77.Find this resource:

                        Solaro RJ, Tardiff JC (2013). Biophysics of the failing heart: physics and biology of heart muscle. Springer Science Media, Berlin.Find this resource:

                          Willis MS, Homeister JW, Stone J (2014). Cellular and molecular pathobiology of cardiovascular disease. Academic Press-Elsevier, Amsterdam.Find this resource:

                            Young ME (2006). The circadian clock within the heart: potential influence on myocardial gene expression, metabolism, and function. Am J Physiol Heart Circ Physiol, 290, 1–16.Find this resource:

                            Zipes DP, Jalife J (2013). Cardiac electrophysiology, 6th edition. Saunders-Elsevier, Philadelphia, PA.Find this resource: