The earth and its atmosphere provide environments that are compatible with an extraordinary number of diverse life forms, each adapted to its particular ecological niche. However, not all the earth's surface is equally friendly for human survival, let alone comfort and function. Mountain climbers and deep-sea divers know the profound effects of barometric pressure (PB) on human physiology, and astronauts quickly learn how the physically equivalent forces of gravity and acceleration affect the body. Humans can adapt to changes in PB and gravity up to a point, but survival under extreme conditions requires special equipment; otherwise, our physiological limitations would restrict our occupancy of this planet to its lowland surfaces.
Much can be learned from exposure to extreme environmental conditions. Although most people do not seek out these extreme environments, the same physiological responses that occur under extreme environmental conditions may also occur, to a lesser extent, in everyday life. In this chapter, we first discuss general principles of environmental physiology and then focus on extreme environments encountered in three activities: deep-sea diving, mountain climbing, and space flight.