Effects of Long-Term SpaceflightExposure to
weightlessness for time periods exceeding about two weeks results in degenerative
physiological effects similar to those of prolonged bed rest on Earth. If left unchecked, these changes could result in severe and perhaps even dangerous weakness. Because of this, several countermeasures are employed on long-duration flights to arrest the body's adaptation to the effortless weightless environment and to keep it fit enough to return safely to Earth. For example, certain muscles tend to atrophy through lack of use during
weightlessness, and animal tests show that muscle tissue can also lose blood vessels and nerve associations. Studies also suggest that deterioration in muscle tissue may be accompanied by decreases in metabolic efficiency. An increase in food intake, combined with vigorous inflight
exercise using a variety of equipment, can at least partially counteract muscle deterioration.The redistribution of body fluids during weightlessness causes the body to eliminate some fluids in the urine. Tissues dehydrate to some degree, and the volume of blood plasma drops by about 10 percent. The mass of red blood cells drops by 15 percent in two weeks, and the shape of the cells also changes. Their function, however, is not impaired. In fact, red-blood-cell mass appears to stabilize and recover after about 60 days of weightlessness, in most cases. These changes appear to be normal body adaptations to space conditions; countermeasures include increased water intake and exercise during the flight. After the return to Earth, body fluids return to preflight norms rapidly and blood counts return to normal within a few weeks.The shift in body fluids and the decrease in blood volume, along with the absence of gravity, also reduce the size and pumping capacity of the heart. Exercise and adequate fluid intake are, again, the usual countermeasures used to avoid serious cardiac and circulatory problems. Most cardiovascular responses return to normal within a few weeks.Loss of calcium from the weight-bearing bones of the body is apparently continuous throughout a flight. The loss is significant, ranging in various individuals from 0.3 percent to 1.5 percent of total body calcium per month. Although several countermeasures have been tried, none have halted the decalcification process; exercise, however, does appear to slow down the rate of loss. Postflight recovery of lost calcium is a lengthy process, taking at least as long as the flight itself. Some bone loss may be irreversible, particularly in the beamlike network (trabeculae) of spongy bone.On very-long-duration Soviet flights, such changes have been observed as a rise in steroid hormones, damage to the T-lymphocytes, heightened sensitization to allergens, and increased vulnerability to staphylococcus and streptococcus infections. Such changes indicate possible changes in the body's immune system over long periods of time.Finally, some indications exist that drugs commonly used to treat illnesses on Earth may not be absorbed properly by the body in weightless conditions. In addition, many drugs can have unpredictable and unwelcome side effects and can even lose their effectiveness in space.