Prior to the first flights of Salyut and Skylab, the effects of long-term exposure to a weightless environment were a matter of speculation. Aerospace engineers and space medicine teams from both the Soviet Union and the United States understood that unless humans could adequately adapt to weightlessness, hopes for more sophisticated space stations and long-duration spaceflights would never be realized. To this end, many of the experiments conducted aboard space stations involved determining, testing, measuring, and assessing changes in the conditions of the astronauts and cosmonauts themselves. In a very real sense, the researchers were the subjects of their own experiments. As one might expect, the early flights of Salyut and Skylab established the first thresholds for tolerance of weightlessness; Mir and the ISS have since tested those limits.

Blood and Fluid Distribution

The principal concern among space physicians regarding the functioning of the heart in...