Physiological Functions of Water

After oxygen, water is the most essential nutrient needed to sustain human life. In healthy individuals, water comprises between 45 and 70% of total body weight and is responsible for connecting the diverse physiological functions of the body (Table 1).

Water is necessary to maintain homeostasis of the internal environment. The most obvious roles of water in the human body are to provide an aqueous medium for transport of material in blood, to dissolve and pass nutrients between blood and cells, to serve as a medium for intracellular reactions, and to transfer metabolic products for redistribution or excretion via urine. Since both the quantity of reac-tants and the volume of fluid in which they are dissolved influence chemical reaction rates, imbalances in hydration status can alter cellular and tissue function.

Dehydration also adversely affects the body's ability to regulate temperature. Energy transformations during digestion, absorption, and metabolism as well as muscular contraction generate heat. The heat released from the digestion of a mixed meal (thermic effect of food) equals 10-15% of the caloric content of the food ingested. Muscular contraction is dependent on the transformation of chemical energy (ATP) to mechanical energy. Nearly three-fourths of the energy used for muscular contraction is released as heat. Unless localized heat production from metabolism and muscular contraction is dissipated, the heat burden can be structurally damaging

Table 1 Major physiological functions of water



Waste product removal

Urea excretion by kidneys

Solvent for chemical

Glycolysis in the cell cytosol


Transport medium



Synovial fluid of joints

Shock absorber

Disks between vertebrae of spinal


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).

S 300

Core temperature (°C)


Figure 2 The influence of water loss by dehydration (hypohydration) on the sweating response to exercise following normal hydration (0%) and dehydration equal to 3, 5, and 7% of body weight. The primary stimulus for sweating is the increase in core temperature (thermal drive). Note that dehydration reduces the sweating rate at any given level of thermal drive. Hypohydration compromises exercise by reducing sweat rate and evaporative cooling and increasing body core temperature. (From Sawka MN, Young AJ, Francesconi RP et al. (1985) Thermoregulatory and blood responses during exercise at graded hypohydration levels. Journal of Applied Physiology 59: 1394-1401, with permission.)

the skin considerably reduce the potential for evaporative heat dissipation; therefore, actual evaporative cooling is usually less than the theoretical maximum. Since water is the main component of sweat, it is not surprising that dehydration affects the sweat response. The relationship between body water loss by dehydration and the rate of sweating achievable during exercise is shown in Figure 2, which illustrates that dehydration reduces sweating rate at any given level of thermal drive (core temperature) during exercise. A diminished sweating response can lead to a dangerous heat buildup unless thermal strain is curtailed by other mechanisms.

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