Medical Physiology, 3rd Edition

Flight and Space Physiology

Acceleration in one direction shifts the blood volume in the opposite direction

To accelerate a rocket from rest, we must apply enough force to overcome its inertial force (i.e., its weight, the product of its mass, and the acceleration caused by gravity), as well as the frictional forces of the environment. This requirement is merely a restatement of Newton's second law of motion. imageN61-10 With the rocket accelerating vertically, astronauts inside experience an inertial G force (see p. 1225), as required by Newton's third law, a force that presses the astronauts into their seats in the direction opposite that of the rocket's acceleration.

Before liftoff, an astronaut experiences only the force of gravity, +1G. As a rocket blasts off from earth, the astronaut experiences higher G forces. In early rockets, astronauts sometimes experienced G forces as high as +10G. Maximal G forces in the space shuttle were only approximately +4G (Fig. 61-7). Similarly, pilots of high-performance aircraft experience positive G forces as they pull out of a dive, and we all experience negative G forces when an aircraft hits turbulence, suddenly loses altitude, and lifts us out of our seats. Although G forces can frequently have potentially large effects on aircraft pilots, they affect astronauts only during the liftoff and re-entry phases of space flight. To ensure that acceleration effects have a minimal influence on body function, astronauts sit with their backs perpendicular to the direction of the accelerating force, so the G force acts across the chest from front to back. imageN61-24

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FIGURE 61-7 G forces during ascent into space on the space shuttle. Before liftoff, astronauts experience +1G, the acceleration that is due to earth's gravity. After liftoff, the solid rockets burn for ~2 minutes, during which time the G force ramps up to slightly more than +3G. After the solid-rocket burn, the G force falls back to +1G. Thereafter, the main engine gradually builds up the G force to about +4G before engine cutoff. These G-force data were generated in a human centrifuge to simulate the profile of a shuttle launch. (Data from Buckey JC, Goble RL, Blomqvist CG: A new device for continuous ambulatory central venous pressure measurement. Med Instrum 21:238–243, 1987.)

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Effects of Acceleration on Astronauts

Contributed by Arthur DuBois

If we sit in a human centrifuge with acceleration directed from head to foot, and if the seat pushes headward by the same force, blood drains from our heads and we nearly pass out at +4G. Fighter pilots wear tight leggings with air bladders inside inflated to support the circulation, which extends tolerance an extra +2G. Some pilots in an aircraft undergoing a tight turn can withstand up to +8G or +9G by straining to provoke vasoconstriction. Astronauts in takeoff or landing mode lie transversely to the acceleration or deceleration to be able to tolerate up to 9G. But above 10G, they can hardly move the chest wall to breathe.

G forces propel the body's tissues in the direction opposite that of acceleration; these forces compress soft tissues against underlying structural elements (e.g., bone) or pull these tissues away from overlying structural elements. In addition, G forces tend to shift the blood volume away from the direction of acceleration, thereby adding to the other component forces that determine blood pressure (see p. 414).

In high-performance aircraft, the rapid motions associated with changes in flight direction or altitude produce G forces that can be considerable for several minutes, exceeding 8G. Even in relatively primitive aircraft, aerobatic maneuvers can shift blood volume away from the head, resulting in transient reductions in cerebral blood flow and O2 delivery. If these reductions are sufficiently large, they can result in loss of consciousness. The early warnings of such an event are narrowing of the visual field (i.e., loss of peripheral vision) and loss of color perception as the retina is deprived of O2, a phenomenon called gray-out. The term blackout describes a total loss of consciousness that occurs during acceleration that lasts for tens of seconds or minutes. Pilots experiencing gray-out or blackout are at extreme risk. As early as World War II, fighter pilots used G suits that provided counterpressure to the lower extremities during repeated tight maneuvers during dogfights. The counterpressure opposed the pooling of blood in the extremities and maintained sufficient cardiac filling, cardiac output, and blood flow to the brain, thereby eliminating the tendency toward gray-out.

“Weightlessness” causes a cephalad shift of the blood volume and an increase in urine output

An astronaut in an orbiting spacecraft experiences “weightlessness,” a state of near-zero G force, also called a microgravity environment. Although an astronaut at an altitude of 200 km still experiences ~94% of the force of the earth's gravity at sea level (i.e., the astronaut truly has weight), the centrifugal force of the spacecraft's orbital trajectory balances the earth's gravitational force, and the astronaut experiences no net acceleration forces and thus has the sensation of weightlessness. This weightlessness, however, differs from the true near-zero-gravity environment in “outer space.”

We are adapted to life at +1G, and arteriolar tone in the lower extremities prevents pooling of blood in the capacitance vessels, thereby ensuring adequate venous return to the right heart (see p. 576). The acute effects of microgravity on the circulatory system are exactly what one would expect for a system designed to oppose the effect of gravity in a standing person: blood volume redistributes toward the head. This cephalad shift of blood volume—away from the capacitance vessels of the legs—expands the central blood volume, increasing the cardiac preload and increasing the filtration of plasma water into the interstitium of the facial region. The resulting edema explains the dramatically bloated facial appearance of astronauts in microgravity within 24 hours of the launch. From this discussion, one would think that the central venous pressure (CVP) is higher in space. However, such an increase in CVP has been difficult to confirm.

In laboratory studies involving prolonged head-down tilt (i.e., a model intended to simulate microgravity exposure), the cephalad shift of blood volume produces the expected increase in CVP and rapid reflex responses to the apparent volume overload. First, the increased stretch on the right atrium causes release of atrial natriuretic peptide (ANP; see p. 547). Second, stimulation of the low-pressure baroreceptors inhibits secretion of arginine vasopressin, or antidiuretic hormone (see p. 547), from the posterior pituitary. These two events increase excretion of salt and water by the kidneys (see pp. 838–840), which tends to correct the perceived volume overload and explains the tendency for astronauts to remain relatively underhydrated during space flight.

In orbiting spacecraft, the cephalad shift of blood volume, even without an increase in CVP, causes a small increase in cerebral arterial pressure and thus in blood flow to the brain. Such regional alterations in blood volume and flow do not substantially impact total peripheral resistance in space. Thus, mean arterial pressure and cardiac output are not significantly different from their values on the earth's surface.

Space flight leads to motion sickness and to decreases in muscle and bone mass

Despite training (e.g., in three-dimensional motion simulators), more than half of all astronauts experience motion sickness during the initial days of microgravity. Motion sickness (i.e., nausea and vomiting) results from conflicting sensory input to the brain regarding the position of the body. In space flight, motion sickness is the consequence of altered inertial stimulation of the vestibular system in the absence of normal gravitational forces. Nearly all cases of motion sickness resolve within the first 96 hours of microgravity exposure as the vestibular system or the CNS accommodates to the novel input.

The increased cerebral blood flow and blood volume in microgravity, accompanied by increased capillary filtration of fluid from the intravascular space, contribute to the increased incidence of headache, nausea, and motion sickness, at least during the period of transition to microgravity. These symptoms reduce performance. Astronauts attempt to minimize these effects by restricting water intake before launch.

Numerous other changes occur during prolonged residence in microgravity, many of which are related to the markedly diminished aerobic power output in space, where the force of gravity does not oppose muscle contraction. The major physiological alterations include reductions in body water content, plasma and RBC volume, total-body nitrogen stores, muscle mass, and total-body calcium and phosphate (associated with a loss in bone mass). The bone loss appears to be continuous during time in a weightless environment, whereas the other changes occur only during the first weeks in space. The reductions in plasma and RBC volumes result in a marked decrease in maximal cardiac output (see pp. 1214–1215), a determinant of maximal aerobic power. The reduction of muscle mass decreases the maximal force developed by muscle. The reduction in bone mass similarly decreases bone strength. Although these changes are appropriate adaptations to a microgravity environment, in which great strength and high aerobic capacity have little inherent value, they are decidedly disadvantageous on return to the earth's surface. Prolonged bed rest simulates weightlessness by causing loss of calcium from the bones and protein from the muscles.

Exercise partially overcomes the deconditioning of muscles during space flight

The intermittent loading of muscles, bone, and the cardiovascular system prevents—to some extent—the deconditioning effects of space flight on muscle mass and performance. Astronauts have used bungee (i.e., elastic) cords and ergometric (i.e., work-measuring) stationary bicycles to provide resistance against which to exert force. The most effective exercise regimen appears to be walking on a motor-driven treadmill with the lower body encased in a negative-pressure chamber. Reducing the chamber pressure to 100 mm Hg lower than ambient pressure creates transmural pressure differences across the blood vessels in the feet that are similar to pressure differences when standing upright on the earth's surface. However, this arrangement greatly exaggerates transmural pressure differences near the waist. For this reason, the astronauts also wear positive-pressure pants that compress the tissues by 70 mm Hg at the level of the waist and decrease the compression decrementally to 0 mm Hg at the feet. The net effect of the negative-pressure chamber and the graded positive-pressure pants is to create a physiological toe-to-waist gradient of transmural pressures across the blood vessels of the lower body. The aerobic activity, the impact of the feet on the treadmill, and the generation of physiological transmural pressure gradients appear to be sufficient to simulate exercise at +1G. This regimen can reduce or even eliminate the deconditioning effects of space flight.

Return to earth requires special measures to maintain arterial blood pressure

The problems associated with re-entry reflect a return to full gravity on earth's surface. The most dramatic effects result from reduced blood volume and decreased tone of the leg vessels. Both factors contribute to reductions in cardiac preload, orthostatic tolerance, and exercise capacity. It has been common practice to shield astronauts from public view immediately after return to the earth's surface, until they have regained a good orthostatic response.

In recent years, astronauts have employed various strategies just before re-entry to counter the adaptations to microgravity. The countermeasure to orthostatic intolerance is restoration of blood volume before re-entry. One means of attenuating the reduction of blood volume in space flight is an exercise program. Even a brief period (e.g., 30 minutes) of intense exercise expands plasma albumin content (see p. 1220), increasing plasma oncotic pressure and plasma volume by 10% within 24 hours. The problems with exercise programs are difficulties in logistics and the astronauts' lack of motivation. A second means of minimizing the reduced blood volume is increasing salt and fluid intake. However, this practice has proven difficult to implement because of the consequent increase in urine flow. Currently, astronauts are educated about the effects of prolonged space flight and then are maintained under continuous observation after re-entry until they have regained a normal orthostatic response. This usually occurs within hours, and certainly within 1 day, of re-entry.