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


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