Weightlessness and gravitational physiology.

Physiologic responses to the weightless environment have been documented in Skylab astronauts. Significant changes in cardiovascular responses, calcium metabolism, fluid distribution, and red blood cell mass are described. Although the data are limited and the sample size small, several hypotheses can be formulated. Future studies in a space laboratory setting offer the potential for understanding the effects of weightlessness on human physiology and the implications to medicine.