Water makes up about 2/3 of our body weight. The amount of water in the body depends on age, in infants it is 75%, in adolescents 65%, in adults 60% and in the elderly 55% of body weight.

The amount of water in the body is inversely proportional to the amount of adipose tissue, with severe obesity, its amount drops even to 40% of body weight. Approximately 2/3 of all water in the body is contained inside the cells and 1/3 – outside them. Regulation of water balance is closely related to electrolytic balance. If there is too much water in the body, thinner urine is excreted, and if the concentration of electrolytes in body fluids becomes too high, the thirst center in the brain is stimulated, which leads to a feeling of thirst, and the excretion of water by the kidneys decreases. Fluid loss occurs in urine and feces, and it also evaporates through the skin and respiratory tract. In a healthy adult, the daily amount of urine exceeds 600 ml and is normally 1–2.5 liters. With feces, 100-200 ml per day is usually excreted, but this amount increases markedly with diarrhea. Due to evaporation in a temperate climate, a person loses an average of 300–500 ml of water per day per 1 m2 of body surface. Losses through sweat are usually small, but they increase to several liters per day in a hot and humid environment, or even in moderate conditions during heavy physical work.