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.
Experiments showed that the major components of body composition undergo significant changes. About 60% of weight loss observed was attributed to the loss of lean body mass and the rest of the loss was done to loss in fat stores.
An obligatory loss of atleast one litre of body water occurs within the first few days of space flights. The initial water deficits are primarily a result of reduced fluid intake because urine and evaporative water losses were below preflight levels. The crew members exhibiting severe space motion sickness symptoms reduced their fluid intake by the greatest amounts and lost the greatest amounts of body water.
The reduction of body water was maintained throughout the flight inspite of adlibitum drinking.
On the whole 0.82L of water was found to be lost.
Body fat is the material preferentially used to compensate for energy deficits resulting from inadequate energy intake. The longer the space flight, the greater the extent of fat loss. 50% was the fat lost in two week space mission. In general the changes in body fat can be explained by the balance between calorie intake and energy expenditure and do not appear to have been influenced by weightlessness per se.
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.
Atrophy of skeletal muscle occurs in response to disuse, inadequate functional load, insufficient food intake and lack of exercise. Protein loss has been found to begin immediately after entering weightlessness. Protein loss stabilizes after the first month of flight.
Postural muscles are virtually unused in weightlessness with resultant atrophy.
Interventions such as diet and exercise, skeletal muscle atrophy continues to some extent throughout the flight, manifesting progressive muscle loss and negative nitrogen balance.
A decrease in skeletal muscle also lead to a decrease in potassium levels. This persists for a week after flight.
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.
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.
- Bone loss during space flight occurs invariably.
- Supplementation of calcium lactate (1.8g) and 3.0g of phosphorus lead to less negative calcium balance.
- The fundamental mechanism in bone loss is presumed to be increased bone resorption rather than decreased bone formation.
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.
During flight, plasma levels of cortisol, insulin and human growth hormone were found to increase.