DIVING
Humans are poorly adapted to the underwater environment. Breath-hold diving is limited by limits on the capacity of the thorax to compress during descent, and ability to withstand hyperoxia and hypercapnia. Using compressed gas technology humans can breathe underwater, although with effects on pulmonary statics, dynamics, and gas exchange. Breathing compressed gas causes an increase in density, leading to impairment of both inspiratory and expiratory flow, and hence a reduction in maximum exercise ventilation. In addition, there is an increased Bohr dead space, which can further lower alveolar ventilation. Hypercapnia is common during diving. Immersion causes blood redistribution from the periphery into the lung, and a decrease in compliance. Gaseous compounds exert their physiological effects in proportion to their partial pressures. Thus, gases that are in concentrations that are safe to breathe at 1 ATA can become toxic during diving, including oxygen. Pulmonary O 2 toxicity can occur with prolonged breathing of gas with a P O 2 that exceeds 0.5-0.6 atmospheres (ATA). Other diving hazards include pulmonary barotrauma, and pulmonary edema due to venous gas embolism or the hemodynamics of immersed exercise. Mild long-term reductions in pulmonary function have been observed in professional divers and may be due to effects other than diving. Some lung conditions preclude safe diving.
