Historical perspectives: Physiology in microgravity

Studies of physiology in microgravity are remarkably recent, with almost al l the data being obtained in the past 40 years. The first human spaceflight did not take place until 1961. Physiological measurements in connection wi th the early flights were crude, but, in the past 10 years, an enormous amo unt of new information has been obtained from experiments on Spacelab. The United States and Soviet/Russian programs have pursued different routes. Th e US has mainly concentrated on relatively short flights but with highly so phisticated equipment such as is available in Spacelab. In contrast, the So viet/Russian program concentrated on first the Salyut and then the Mir spac e stations. These had the advantage of providing information about long-ter m exposure to microgravity, but the degree of sophistication of the measure ments in space was less. It is hoped that the International Space Station w ill combine the best of both approaches. The most important physiological c hanges caused by microgravity include bone demineralization, skeletal muscl e atrophy, vestibular problems causing space motion sickness, cardiovascula r problems resulting in postflight orthostatic intolerance, and reductions in plasma volume and red cell mass. Pulmonary function is greatly altered b ut apparently not seriously impaired. Space exploration is a new frontier w ith long-term missions to the moon and Mars not far away. Understanding the physiological changes caused by long-duration microgravity remains a daunt ing challenge.