Space Physiology

Spacecraft, the space environment, and weightlessness itself all impact human physiology. Clean air, drinkable water, and effective waste collection systems are required for maintaining a habitable environment. Without the Earth's atmosphere to protect them, astronauts are exposed to a much higher level of radiation than individuals on the Earth. Weightlessness impacts almost every system in the body, including those of the bones, muscles, heart and blood vessels, and nerves.

Bone. Bone loss, especially in the legs, is significant during spaceflight. This is most important on flights longer than thirty days, because the amount of bone lost increases as the length of time in space increases. Weightlessness also increases excretion of calcium in the urine and the risk of forming kidney stones. Both of these conditions are related to bone loss.

Many nutrients are important for healthy bone, particularly calcium and vitamin D. When a food containing calcium is eaten, the calcium is absorbed by the intestines and goes into the bloodstream. Absorption of calcium from the intestines decreases during spaceflight. Even when astronauts take extra calcium as a supplement, they still lose bone.

On Earth, the body can produce vitamin D after the skin is exposed to the sun's ultraviolet light. In space, astronauts could receive too much ultraviolet light, so spacecraft are shielded to prevent this exposure. Because of this, all of the astronauts' vitamin D has to be provided by their diet. However, it is very common for vitamin D levels to decrease during spaceflight.

Sodium intake is also a concern during spaceflight, because space diets tend to have relatively high amounts of sodium. Increased dietary sodium is associated with increased amounts of calcium in the urine and may relate to the increased risk of kidney stones. The potential effect of these and other nutrients on the maintenance of bone health during spaceflight highlights the importance of optimal dietary intake.

Bone is a living tissue, and is constantly being remodeled. This remodeling is achieved through breakdown of existing bone tissue (a process called resorption) and formation of new bone tissue. Chemicals in the blood and urine can be measured to determine the relative amounts of bone resorption and formation. During spaceflight, bone resorption increases significantly, and formation either remains unchanged or decreases slightly. The net effect of this imbalance is a loss of bone mass.

It is not clear whether bone mass lost in space is fully replaced after returning to Earth. It is also unclear whether the quality (or strength) of the replaced bone is the same as the bone that was there before a spaceflight. Preliminary data seem to show that some crew members do indeed regain their preflight bone mass, but this process takes about two or three times as long as their flight. The ability to understand and counteract weightlessness-induced bone loss remains a critical issue for astronaut health and safety.

The changes in bone during spaceflight are very similar to those seen in certain situations on the ground. There are similarities to osteoporosis, and even paralysis. While osteoporosis has many causes, the end result seems to be similar to spaceflight bone loss. Paralyzed individuals have biochemical changes very similar to those of astronauts. This is because in both cases the bones are not being used for support. In fact, one of the ways spaceflight bone loss is studied is to have people lie in bed for several weeks. Using this approach, scientists attempt to understand the mechanisms of bone loss and to test ways to counteract it. If they can find ways to successfully counteract spaceflight bone loss, doctors may be able to use similar methods to treat people with osteoporosis or paralysis.

Muscle. Loss of body weight (mass) is a consistent finding throughout the history of spaceflight. Typically, these losses are small (1 percent to 5 percent of body mass), but they can reach 10 percent to 15 percent of preflight body mass. Although a 1 percent body-weight loss can be explained by loss of body water, most of the observed loss of body weight is accounted for by loss of muscle and adipose (fat) tissue. Weightlessness leads to loss of muscle mass and muscle volume, weakening muscle performance, especially in the legs. The loss is believed to be related to a metabolic stress associated with spaceflight. These findings are similar to those found in patients with serious diseases or trauma, such as burn patients.

Exercise routines have not succeeded in maintaining muscle mass or strength of astronauts during spaceflight. Most of the exercises performed have been aerobic (e.g., treadmill, stationary bicycle). Use of resistance diet: the total daily food intake, or the types of foods eaten osteoporosis: weakening of the bone structure paralysis: inability to move biochemical: related to chemical processes within cells fat: type of food molecule rich in carbon and hydrogen, with high energy content metabolic: related to processing of nutrients and building of necessary molecules within the cell aerobic: designed to maintain adequate oxygen in the bloodstream exercise, in which a weight (or another person) provides resistance to exercise against, has been proposed to aid in the maintenance of both muscle and bone during flight. Ground-based studies (not done in space) of resistance exercise show that it may be helpful, not only for muscle but also for bone. Studies being conducted on the International Space Station are testing the effectiveness of this type of exercise for astronauts.

Blood. A decrease in the mass of red blood cells (i.e., the total amount of blood in the body) is also a consistent finding after short- and long-term spaceflight. The actual composition of the blood changes little, because the amount of fluid (blood plasma) decreases as well. The net result is that the total volume of blood in the circulatory system decreases. While this loss is significant (about 10 percent to 15 percent below preflight levels), it seems to be simply an adaptation to spaceflight, with no reported effect on body function during flight.

The initial loss of red blood cells seems to happen because newly synthesized cells (which are not needed in a smaller blood volume) are destroyed until a new steady state is reached. One consequence of the increased destruction of red blood cells is that the iron released when they are destroyed is processed for storage in the body. Too much iron may be harmful, and is thus a concern for long space missions.

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