The gastrointestinal tract is a hollow tube stretching from the oral cavity through the oesophagus, stomach, small intestine, colon, and rectum to the anal sphincter. Its function is the transport, digestion, and elimination of ingested material to supply nutrients, vitamins, minerals, and electrolytes that are essential for life, together with the protection of the rest of the body from injurious or allergenic material. The stomach acts as a storage, sterilizing, and digestive tank; the small intestine is the major site of digestion and absorption; the colon’s function is to salvage water and electrolyte from the small intestinal effluent; and the rectum provides a storage function, enabling the elimination of colonic residue (defecation) to be restricted to times of personal convenience.
This chapter provides a brief overview of the structure and function of the gastrointestinal tract (excluding the liver and pancreas). Emphasis has been placed on those aspects of gastrointestinal anatomy and physiology which illuminate understanding of the nature of gastrointestinal symptoms and inform treatment.
The gastrointestinal tract is a hollow tube of approximately 5 to 6 m in length, stretching from the oral cavity to the anal sphincter (Fig. 15.1.1). It is arbitrarily divided into a series of organs which serve different functions, and is joined to the liver and pancreas, the major organs of digestion.
The gastrointestinal tract possess a broadly similar structure throughout its length (Fig. 15.1.2) with an innermost epithelium, a subepithelial lamina propria, and two muscle layers, an inner circular and an outer longitudinal layer, between which lies the myenteric plexus, the intrinsic neural control system of the musculature. While this description most accurately describes the small intestine, the other organs of the gastrointestinal tract differ only subtly from this stereotype.
In the oesophagus, the innermost layer is a squamous rather than columnar epithelium. The musculature in the upper one-third is striated and controlled directly via extrinsic neural pathways, unlike the lower two-thirds which has smooth muscle and a myenteric plexus.
The anatomy of the stomach differs from that of the intestine, possessing an additional oblique muscular layer and at either end, a sphincter—specialized musculature designed to act as a unidirectional valve to control the flow of luminal contents. The sphincter between the oesophagus and stomach (the lower oesophageal sphincter) lies at the level of the diaphragm. The sphincter between the stomach and small intestine is known as the pylorus.
The small intestine is arbitrarily divided into duodenum, jejunum, and ileum. The duodenum (so named because it is 12 fingers’ breadth in length) is retroperitoneal, and possess on its medial aspect the ampulla of Vater which connects the pancreatic and common bile ducts to the duodenal lumen. The jejunum (Latin, empty, after death) is mobile and free on a mesentery. The ileum (Greek, twisted) begins indistinctly from the jejunum and ends at the caecum.
Colon and rectum
The colon differs from the small intestine in its muscular structure—the inner circular layer is similar but the outer longitudinal layer is condensed into three ‘wormlike’ structures, the taeniae coli. At the proximal end of the colon, the caecum (Latin, blind ending) arises the vermiform appendix, named because of its worm-like appearance. The ascending and descending colon are retroperitoneal whereas the transverse and sigmoid colon are freely mobile on a mesentery, extending to the pelvic floor, after which it expands into the rectum.
The epithelium lies in contact with the luminal contents and ranges in permeability from being largely impermeable (oesophageal squamous epithelium) to highly permeable (intestinal epithelium). The absorptive function of the epithelial layer is modulated by a network of neurons, the submucous plexus, which receive input from the central nervous system. In addition, the neurons of the submucous plexus and the nerve terminals of extrinsic afferent nerves, particularly those running in the vagus trunk, are modulated by signals arising from the epithelium.
Neuromusculature of the gut
The striated muscle in the gastrointestinal tract (upper oesophagus and anus) is directly innervated by second-order (lower motor) neurons (arising from the brainstem and spinal cord respectively) and therefore under direct central nervous system control, whereas smooth muscle is largely autonomous, being controlled ‘locally’ by the enteric nervous system without direct innervation from the central nervous system. The central nervous system can, however, indirectly influence the muscular function of the gastrointestinal tract via its innervation of the myenteric plexus.
Immune system of the gastrointestinal tract
Throughout the gastrointestinal tract lie discreet clusters of immune cells which provide immunosurveillance and immune protection. These immune cell clusters form the so-called Peyer’s patches in the small intestine and the appendix (see Chapter 15.5).
Function of the gastrointestinal tract
The function of the gastrointestinal tract is the transport, digestion, and elimination of ingested material to supply nutrients, vitamins, minerals, and electrolytes which are essential for life, together with the protection of the rest of the body from injurious or allergenic material.
The gastrointestinal tract is responsible for movement of very large volumes across its lumen (Fig. 15.1.1). Overall, more than 8 litres enter the lumen per day. In contrast, only 200 to 300 ml is expelled per day as stool, the remainder being efficiently absorbed by the small intestine and proximal colon. The major digestive/absorptive organ of the gastrointestinal tract is the small intestine. Without the small intestine life is impossible, whereas possession of the small intestine without oesophagus, stomach, or colon is still compatible with reasonable nutrition. The various organs of the gastrointestinal tract subserve different functions to ensure that ingested nutrients are adequately digested or eliminated.
The oesophagus functions as a conduit to transport ingested food masticated by the mouth and salivary glands, through the thoracic cavity and into the proximal stomach.
The stomach acts as a storage, a sterilizing, and a digestive tank. Its receptive function enables large quantities of food to be eaten rapidly and stored and processed until adequately prepared for delivery to the small intestine. The presence of pathogens in food is reduced by the secretion of hydrochloric acid upon meal ingestion while the production of peptidases and lipase capable of operating in a low pH commence the process of digestion.
Small intestinal function
The small intestine is the major site of digestion and absorption. It regulates the speed of delivery of gastric contents via a sensing mechanism located in the epithelium, comprising endocrine cells sensitive to the pH, osmolarity, and chemical composition of the luminal contents, and signals both to intrinsic neurons and to extrinsic neurons of the vagus to delay gastric emptying. This sensory signal also stimulates the delivery of bile and the production of pancreatic secretion ensuring that these major digestive materials are delivered to the intestine in the presence of nutrients.
The absorption of digested material is achieved through the intestinal mucosa. While some passes between the intestinal cells, most is actively transported through the epithelial cells via specific transporters (e.g. peptide and hexose transporters). The small intestinal is also a major fluid absorptive organ, retrieving more than 6 litres of fluid per day from the lumen (Fig. 15.1.1), the end result of which is the delivery of a small quantity of unabsorbed food (1.5 litres) into the caecum.
Regional variation in intestinal absorption
The intestine shows regional differences in its absorptive function. The jejunum is responsible for the majority of nutrient and fluid absorption, whereas the ileum has additional, specific absorptive functions, in particular the absorption of vitamin B12 and the absorption of bile salts. Surgical resection of the ileum may thus be associated with development of vitamin B12 deficiency and of diarrhoea resulting from passage of bile salts into the colon where they induce secretion.
The colon’s function is to salvage water and electrolyte from the small-intestinal effluent, converting over 1 litre of material arriving from the intestine into small pellets for elimination via the anus. In addition to its water and electrolyte absorptive function, the colon also salvages unabsorbed calories from the lumen, particularly undigested carbohydrate, e.g. starch polysaccharides. These are incompletely digested in the small intestine and pass to the colon where the anaerobic bacteria of the lumen ferment the carbohydrate to short chain fatty acids, which are absorbed to provide a secondary energy source.
The rectum provides a storage function, enabling the elimination of colonic residue (defecation) to be restricted to times of personal convenience.
Neural control of gastrointestinal function
For the greater part of the time, the gastrointestinal tract is controlled by its own nervous system—the enteric nervous system. The enteric nervous system is not entirely autonomous, however, and requires some local and central nervous system ‘reflexes’ for adequate coordination of functions along its length. For example, the coordination of the passage of luminal contents into the small intestine from the stomach requires sophisticated control, which is provided by a vagally mediated reflex operating via the brainstem. This circuitry alters the function of the gut from its fasted state to the fed state, initiating gastric relaxation and the induction of gallbladder emptying and pancreatic secretion, thus ensuring the provision of digestive enzymes at the appropriate time. An additional relay function is provided by prevertebral ganglia where visceral afferent neurons synapse with efferent relay neurons to integrate contractile patterns and control contraction force.
Intrinsic nervous system
The intrinsic nervous system acts as a local control system with its own ‘programmes’, examples of which are the peristaltic reflex and the migrating motor complex.
This basic programme responds to local luminal distension by inducing a pattern of ascending muscular excitation and descending inhibition (Fig. 15.1.3) which ensures aboral propulsion of luminal contents. This reflex is best seen in the oesophagus where it is known as secondary or non-swallow-related peristalsis. Although the reflex can be induced in the small intestine or colon, it is not a major factor for luminal transit.
Migrating motor complex
This comprises a triphasic pattern of aborally propagating contractions in the distal stomach and small intestine during the fasted state which probably serve to maintain an empty lumen and reduce bacterial growth. Periods of quiescence are followed by irregular contractile activity, which then terminates in a aboral migrating burst of regular contractions that migrate slowly from the distal stomach down to the terminal ileum. This pattern, which characterizes the fasted state, is interrupted on food ingestion by a vagally mediated reflex that converts the pattern into a fed one.
Immune function of the gut
Being the major route of nutrient absorption, the gastrointestinal tract is also a potential portal for pathogen entry. The gastrointestinal tract therefore requires a sophisticated immune surveillance system together with a process for eliminating intestinal pathogens and the ability to either tolerate or eliminate ingested antigens. Details of this process are more fully dealt with in Chapter 15.5.
Disturbances of local neuromuscular function are associated by the disturbances in transit and elimination or secretion and absorption. Example of disturbed transit resulting from disturbed neuromuscular function are achalasia or slow transit constipation. Examples of disturbance of secretion and absorption are secretory diarrhoeas or the hyperacidity associated with Helicobacter pylori infection. Examples of symptoms which follow damage to extrinsic neural control are exemplified by the symptoms of truncal vagotomy, i.e. rapid transit and impaired nutrient–enzyme mixing, result in poor digestion and an osmotic diarrhoea.
The relationships between gastrointestinal symptoms and the central nervous system are relevant to the understanding of functional gastrointestinal disorders. It is well recognized that psychological disturbances, e.g. anxiety or depression, combined with local disturbances of gastrointestinal physiology produce pain, nausea and vomiting, and altered bowel habit.