Weightlessness
Significance of gravitation forces in regulating homeostasis is discussed, along with weightlessness effects on humans and a state of reduced weight (subgravity), such as on the moon. Biomedical effects of weightlessness adaptation to zero G and readaptation to terrestrial gravitation are described for the nervous system, cardiovascular system, metabolism, and musculoskeletal system. Reactions caused primarly by: (1) changes in the afferent nervous system, (2) lack of hydrostatic blood pressure, (3) lack of weight on the musculoskeletal system, and (4) exposure limits derived from the effects of prolonged weightlessness on humans are reviewed. Protection of humans from adverse effects of weightlessness is considered; Skylab missions are also summarized.