Space physiology

Space physiology is concerned with the structure and functioning of the body under the conditions encountered by space travelers. To date, these conditions have been confined to the environment of the spacecraft that houses the astronauts. In the future, however, as travel to other bodies in the solar system is undertaken, space physiology will include the atmospheric and gravitational conditions found on these planets, moons, or other stellar bodies.

Aside from the lunar missions of the 1960s, man's extraterrestrial voyages have been confined to orbital forays aboard space capsules or space stations. But even orbiting around Earth poses difficulties for the astronauts. The reduced gravity of a spacecraft makes it difficult for the body to distinguish "up" from "down." On Earth, such distinction by the vestibular organ of the inner ear is easy, because of the orienting power of gravity. In the space shuttle and the developing international space station , all writing on the walls is oriented in the same direction, to provide the brain with a reference point.

Low gravity (also known as microgravity) affects other body systems besides the vestibular system. The propioceptive system—the system of nerves in the joints and muscles that tell us where the arms and legs are without any visual inspection—can also be affected. Low gravity reduces or eliminates the tensions impinging on the joints and muscles, which can make the appendages appear invisible to the brain.

Such confusion between what the eye sees and the brain perceives can result in what has been termed space sickness. This is somewhat analogous to the feeling of nausea experienced by someone trying to read in a moving car. The inner ear detects the motion of the car, or the spacecraft, but the eyes staring at the page of the book or the space outside the spacecraft do not detect motion. Space sickness is usually transient, and astronauts acclimate soon after going into orbit.

Microgravity also affects the skeletal structure of astronauts. The absence of stress-bearing activity and the loss of components of the bones, particularly calcium, have produced shortening and weakening of bones (essentially the development of osteoporosis) and the atrophy (wasting away) of muscles in astronauts who have been orbiting the earth for just several months. Even the heart becomes smaller. So far these conditions have reversed upon return to Earth. The extended missions of the future will need to incorporate more Earth-like gravitation conditions, or an exercise regimen, or both.

Space flight also affects the cardiovascular system of astronauts. The weakened muscles and bones cannot support the maintenance of the same rate of flow of blood as on Earth. Also, the diminished downward pull of gravity affects the ability of the body to pump blood to extremities like the legs. Fluid flow to the upper regions of the body is not affected, however. As a result, faces of astronauts often appear puffy. In a very real sense, astronauts become out of shape—so much so that Russian cosmonauts who spend months in orbit around the earth are sometimes carried away from the spacecraft on a stretcher upon return.

A physiological parameter that will become important when manned travel to other parts of the solar system begins is exposure to higher levels of radiation that will be encountered on planets where atmospheric constituents do not absorb the harmful energies. Genetic material can be damaged by high-energy (ionizing) cosmic radiation and high-energy particles, with adverse effects on the functioning of the body. Thus far, the relatively short-term voyages into space have not proven to be harmful. But the hazards posed by extended voyages of years or even decades are as yet unknown.

See also History of manned space exploration