At The Beginning of this book, you learned that there are 3 main sources of pressure in physiology: atmospheric, hydrostatic, and mechanical. One of the benefits of studying odd applications or environments is that it forces us to reconsider what we already know about physiology. When the envelope is pushed by high altitude, extreme exercise, the duress of disease states, or by being under water, physiologists have opportunities to see whether theory holds up against reality. In the twenty-first century, another unusual opportunity beckons: manned space flight to Mars (Figure 19.1). Although no firm date has been set, the National Aeronautics and Space Administration (NASA) has already drawn up preliminary plans for a manned space flight to Mars or perhaps to one of its moons, Phobos or Deimos.
Unless a feasible method to generate “artificial gravity” is developed (such as in-flight rotation of all or part of the spacecraft), the astronauts on board will experience about 888 days of weightlessness or reduced gravity during a Mars mission. Even if the flights have the benefit of rotational gravity, the time spent on the Martian surface (approximately 1.5 years) will test the limits of human adaptation to gravitational change, given that Mars has only 38% of the g force of Earth.
What does this mean physiologically? Let’s take the case of prolonged exposure to zero-gravity conditions during a nonrotational interplanetary flight. If the NASA spacecraft is pressurized to the equivalent of 1 atmosphere, then the atmospheric and alveolar pressures that we encountered in Chapter 2 will be unchanged. The alveolar air equation will still be relevant because the barometric pressure is 760 mm Hg. What about mechanical pressures generated by the heart, blood vessels, and the muscles of respiration? They will remain similarly unchanged, except for whatever atrophy or deconditioning occurs during the mission. Remember that the muscles of the cardiovascular system are smooth muscles, whereas the muscles of respiration are skeletal (or voluntary) muscles, even though they are under the control of the autonomic nervous system. (That’s why you don’t have to lay awake at night and concentrate on breathing, although you can consciously override autonomic ventilation and hold your breath or hyperventilate.)
One interesting and apparently intractable problem of reduced gravity is muscle wasting. Regardless of their exercise or diet regimens, astronauts lose muscle mass and muscle strength while aloft. They don’t come bounding out of space shuttles right after returning from a mission—they can’t stand without help because they’re simply too weak and have orthostatic intolerance. Presumably, skeletal muscle wasting is caused partly because the astronauts’ antigravity postural muscles aren’t under constant use the same way they are at home. Hopefully, the skeletal muscles of respiration will not atrophy because they still will be constantly used and exercised. There are no iron lungs in space.
What about hydrostatic pressures? Here is where everything changes. Remember that hydrostatic pressure is determined by ρgh, where g is acceleration due to gravity. In weightless environments, g=0 by definition (thus, the term “zero-g”. . . . See, this isn’t really rocket science, is it?). Without gravity, there is no possibility of a hydrostatic pressure gradient. What is the practical effect of losing the hydrostatic pressure gradient? Apparently, it has very little effect because astronauts seem to survive and their brains seem to be perfused just fine during exposure to weightless environments. This reconfirms what you’ve already been taught about cerebral perfusion pressure (inlet pressure minus outlet pressure; or mean arterial pressure minus right atrial pressure) being independent of gravity.
If you recall our discussion of giraffes, the circulatory system is a closed loop. This relieves the heart of the need to work against gravity; it just has to generate enough fluid energy to drive blood beyond the resistance of the blood vessels. In space, that point is no longer controversial. Remember that each point in a circulatory loop has its own transmural pressure (inside pressure minus outside pressure), and transmural pressure does depend on distance above or below the level of the heart. In a weightless environment, though, only the mechanical pressure generated by the heart contributes to transmural pressure. For example, when you examine cerebral circulation in weightlessness (Figure 19.2), you will have a mechanical pressure (P) curve and an intracranial pressure (ICP) component, but there will be no hydrostatic pressure (ρgh) line (see Chapter 4 [Doctor Dolittle Visits a Sitting Case]). This means that at any given point in the cerebral circulation, the transmural pressure will be the simple sum of mechanically generated heart pressure (which decays as it flows through the resistance of the cerebral blood vessels) and intracranial pressure (only when the vessels are exposed to it).
If you’ve been paying attention, you’ll know why I predict that giraffes will still have very high arterial blood pressure in space. The giraffe’s cerebral perfusion pressure hasn’t changed, nor has its cerebral vascular resistance changed. Only the distribution of transmural pressures along the cerebral circulatory path has changed, and those pressures are irrelevant to flow.
There is another physiologic challenge in space, however, and the concepts that you’ve learned in these chapters might make a very practical difference. One of the problems that astronauts experience is a decrease in total blood volume, which results in orthostatic intolerance upon returning to Earth. We don’t know just how severe those alterations in volume will be during a prolonged space flight (eg, to Mars), but indications from other American and Russian missions suggest that they may be quite serious. Similar to the loss of skeletal muscle in spite of exercise, normal thirst mechanisms and the homeostatic feedback loops that you learned about in the previous chapter may be inadequate to maintain healthy hydration status.
It is unlikely that astronauts can or should be monitored with central venous pressure (CVP) lines during an extended mission. Such lines are maximally invasive, difficult to place, and dangerous to maintain for prolonged periods. The usefulness of CVP measurements in weightlessness also is unclear. Data from the space shuttle program indicate that CVP may rise, fall, or remain unchanged during the brief transition from 1 g to weightlessness.
One intriguing possibility for hydration status monitoring is the estimation of mean systemic pressure (Pms). As you might remember, Pms is the ratio of stressed blood volume to total vascular compliance; it is, therefore, a direct index of cardiovascular filling, independent of cardiac activity or central venous pressures. Fortunately, the estimation of Pms is quite simple—it can be assessed by measuring the transmural pressure in any peripheral vein. This measurement is minimally invasive and accounts for hydrostatic pressure gradients on Earth or their absence in space. On Earth, the pressure transducer is placed at the level of the heart to neutralize the effect of gravity on the measurement. In space, the transducer can be placed anywhere.
The estimation of Pms by peripheral venous pressure (Figure 19.3) may provide a simple way of monitoring the astronauts’ hydration status in space and also may provide healthy target values, should intravenous hydration be necessary. Close attention to Starling’s lessons from the nineteenth century may pay dividends in the twenty-first.
Questions and Answers
19.1 When measuring peripheral venous pressure in a zero-g environment, why doesn’t the position of the pressure transducer relative to the heart matter?
19.2 It’s unlikely that any astronaut will undergo emergency brain surgery during space travel, but let’s say that one does. If you remember from Chapter 4 (Doctor Dolittle Visits a Sitting Case), a perforation of a cerebral sinus or vein can cause venous air embolism (air sucked into the blood vessel) if the brain is elevated above the heart. Can venous air embolism occur in a zero-g environment?
Extra credit At the surface of the Earth, the theoretical maximal height of a water column sustained by a vacuum above the column is about 33 feet (equivalent to 1 atm of pressure). Nonetheless, giant sequoia trees can reach heights exceeding 300 feet, and they bring water from the roots to the highest leaves. A major mechanism for moving water through a plant is transpiration, which involves evaporative water loss from small openings in the leaves that connect to thin water tubes (xylem tubes) that run all the way down to the roots. The evaporation creates a vacuum at the top of the xylem tubes. How does the giant sequoia apparently violate the 33-foot limit described above?
19.1 The position of the pressure transducer doesn’t matter because, in the absence of gravity, there is no hydrostatic pressure component. The column of fluid in the pressure measurement tubing exerts no force (it has no weight in space).
19.2 No. For venous air embolism to occur, subatmospheric pressure must exist within the perforated blood vessel. Subatmospheric pressure can be generated within a blood vessel in 2 ways. One, blood inside the vessel must be elevated above the heart; however, this generates subatmospheric pressure only in the presence of gravity. Two, the patient makes strong inspiratory efforts and the subatmospheric pressure in the thorax is transmitted into the blood vessels that drain into the chest. If the patient is mechanically ventilated (a likely scenario during brain surgery), the thoracic pressures will always be atmospheric or greater. Thus, a “vacuum effect” cannot be exerted on the blood vessels draining into the thorax.
Extra credit There is no theoretical height limit for a water column sustained by positive pressure from below (as opposed to suction from above). That’s how well pumps work when the well is more than 33 feet deep. In the case of the giant sequoia, though, positive pressure is not generated from below. Instead, water is raised through capillary action because the xylem tubes are very narrow. Capillary action, nature’s way of reducing surface tension, is not limited by atmospheric pressure.