Human models of space physiology

Maintaining the health and fitness of astronauts is essential to the successful exploration of space. Consequently, the development of methods (i.e., countermeasures) designed to minimize potential adverse clinical effects of space flight depends on an understanding of the physiological mechanisms that underlie human adaptation to microgravity. However, uncontrolled conditions and mission logistics associated with spaceflight present significant limitations to the scientific study of human physiology in the space environment. Although no one model precisely simulates the actual space environment, the quantitative as well as qualitative comparisons of space physiology with the 6 head-down bed rest (HDBR) model are striking. Selective comparisons presented in this chapter demonstrate distinct qualitative and quantitative similarities in the underlying physiology of body fluids, cardiovascular and autonomic functions, muscle, bone, and metabolism in humans between HDBR and space. Because of these similarities, the use of the HDBR model has provided critical direction for the investigation of space physiology that otherwise could not be addressed adequately in the space environment. HDBR has proven to be one of the most effective and valuable models for assessing the effects of prolonged exposure to microgravity on human physiological functions by inducing physical and physiological changes similar to those that occur when humans are exposed to the actual environment of space.