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CHAPTER 3 — Underwater Physiology and Diving Disorders 3-53 Thermal problems, arising from exposure to various temperatures of water, pose a major consideration when planning operational dives and selecting equipment. Bottom time may be limited more by a diver’s intolerance to heat or cold than his exposure to increased oxygen partial pressures or the amount of decompression required. The diver’s thermal status affects the rate of inert gas uptake and elimination. Divers who are warm on the bottom will absorb more inert gas than divers who are cold. No ­ decompression dives in warm water, therefore, may carry a greater risk of DCS than comparable dives in cold water. Given identical exposures on the bottom, divers who are warm during decompression stops will lose more inert gas and have a lower risk of DCS than divers who are cold. 3-10.1 Regulating Body Temperature. The metabolic processes of the body constantly generate heat. If heat is allowed to build up inside the body, damage to the cells can occur. To maintain internal temperature at the proper level, the body must lose heat equal to the amount it produces. Heat transfer is accomplished in several ways. The blood, while circulating through the body, picks up excess heat and carries it to the lungs, where some of it is lost with the exhaled breath. Heat is also transferred to the surface of the skin, where much of it is dissipated through a combination of conduction, convection, and radiation. Moisture released by the sweat glands cools the surface of the body as it evaporates and speeds the transfer of heat from the blood to the surrounding air. If the body is working hard and generating greater than normal quantities of heat, the blood vessels nearest the skin dilate to permit more of the heated blood to reach the body surfaces, and the sweat glands increase their activity. Maintaining proper body temperature is particularly difficult for a diver working underwater. The principal temperature control problem encountered by divers is keeping the body warm. The high thermal conductivity of water, coupled with the normally cool ­ to ­ cold waters in which divers operate, can result in rapid and excessive heat loss. 3-10.2 Excessive Heat Loss (Hypothermia). Hypothermia is a lowering of the core temperature of the body. Immersion hypoth ermia is a potential hazard whenever diving operations take place in cool to cold waters. A diver’s response to immersion in cold water depends on the degree of thermal protection worn and water temperature. A water temperature of approxi mately 91°F (33°C) is required to keep an unprotected, resting man at a stable temperature. The unprotected diver will be affected by excessive heat loss and become chilled within a short period of time in water temperatures below 72°F (23°C). 3‑10.2.1 Causes of Hypothermia. Hypothermia in diving occurs when the difference between the water and body temperature is large enough for the body to lose more heat than it produces. Exer cise normally increases heat production and body temperature in dry conditions. Paradoxically, exercise in cold water may cause the body temperature to fall more rapidly. Any movement that stirs the water in contact with the skin creates turbu lence that carries off heat (convection). Heat loss

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