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Temperature regulation

Evaporative sweat loss

to enzymes or other proteins. Water absorbs heat produced at the cellular level and transfers it to the surface of the skin, where it can be dissipated to the external environment (Figure 1).

The evaporative dissipation of heat through sweating is a two-phase, water-dependent mechanism. Water is removed from capillary blood perfusing sweat glands to produce a thin layer of sweat over the surface of the skin. Simultaneously, the water component of blood carries heat produced from cellular metabolic processes to capillary beds located near the surface of the skin. Heat is transferred by conduction to the skin surface, where it vaporizes sweat coating the skin, thus transferring body heat to the external environment. The heat of vaporization of water is 586 kcal/l (2453 kJ/l) at 20 °C. Approximately 500 ml of sweat is lost per day under average ambient environmental conditions. Such obligatory water loss occurs without visible or tactile sensations and is termed 'insensible' sweat. However, given a sufficient thermal challenge, humans are capable of producing approximately 101 of 'sensible' sweat per day. Theoretically, if the entire 10 l of sweat was evaporated, more than 5000 kcal (20 930 kJ) of heat per day would be dissipated via the sweating mechanism. Humidity of the air and sweat that drips from the surface of

586 kcal (2453 kJ)/liter of sweat

Outer Skin Layer

586 kcal (2453 kJ)/liter of sweat

Outer Skin Layer

Blood Vessel

Blood Vessel

Figure 1 Metabolic heat transfer to the skin and dissipation of heat by evaporation of sweat. The body has more than 2 million sweat glands that secrete sweat to the surface of skin. Blood-perfusing skin capillary beds transfer heat by convection to the surface of the skin. Heat is dissipated by vaporizing the water in sweat. The heat of vaporization of water at 20 °C is 586 kcal/l (2453 kJ).

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