The Biology of Human Survival: Life and Death in Extreme Environments

In the early history of diving, the lack of underwater technology limited descent to the depth that could be reached with a single breath of air. As diving technology appeared and improved, the limit was pushed downward until new physiological limits were encountered. As interest grew in deeper diving, biomedical research overcame apparent limits, but new limits appeared, until ultimately the effects of hydrostatic pressure per se seemed to stop humans from descending farther. Thus, the history of diving is a paradigm for the relationships among technology, physiology, and human factors in the effort to conquer extreme environments (Phillips, 1998). This chapter describes how two important physiological limits have been dealt with in diving from scientific and engineering perspectives: decompression sickness and the effects of pressure on the nervous system. Before discussing these problems it may be helpful to review the natural limits of breath-hold diving.

Breath-hold diving was practiced for centuries primarily to collect sponges and pearls and recover valuable objects lost in shallow water. Even today, practiced divers routinely descend to sixty feet and hold their breath for two minutes. The record for human breath-hold diving, with descent rate artificially assisted by weights, is in excess of 500 feet of seawater. However, this assisted maximum breath-hold depth and the maximum breath-hold time are modest compared to diving mammals.

All diving mammals hold their breath, and their breath-hold time is limited by the amount of O₂ available in the body at the start of the dive. In diving mammals

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