Water constitutes 50-55% of a woman's body weight and 55-60% of man's weight. Water gives structure and form to the body, helps maintain body temperature, and also creates the necessary environment for cell metabolism and

There are two compartments of body water, extracellular and intracellular fluid. Extracellular fluid (ECF) is water found outside of cells. Making up approximately 20% of total body weight, the ECF consists of blood plasma, interstitial fluid surrounding the cells, secretory fluid, which is water in transit, and dense tissue fluid, which is water located within dense connective tissue such as cartilage and bone. Intracellular fluid (ICF) is the water inside the cells. It makes up 35-40% of total body weight.

Water Balance

The maintenance of correct proportions of ECF and ICF is vital to proper functioning of the body, as can be seen below in Clinical Problems. The body maintains fluid balance by calibrating water intake, excretion, and internal processes controlling water distribution.

Water Intake (Thirst)

The thirst mechanism in animals is a complex interaction of control centers in the brain and hypothalamus; though the details of that mechanism go well beyond the scope of this SparkNote, it is important to know that the thirst mechanism is linked to water loss. When total body fluid volume decreases by 0.5-1.0%, the thirst mechanism asserts itself. Approximately 55% of water intake is derived directly from fluids, 35% from food, and 10% from water produced as a byproduct of metabolism.

Water Output

Kidneys excrete approximately 1 to 2 liters of urine per day. Approximately 900 milliliters (ml) of this amount is obligatory water excretion that gets rid of solutes and is constant from day to day. The remainder is excreted according to the fluctuating needs of the body and the changing renal tubular reabsorption rate.

The process by which the body loses water through the skin is called insensible water loss. Approximately 350 ml of water is excreted by diffusion through the skin, while another 100 ml is lost through normal perspiration. Heavy perspiration may cause a greater loss.