The amount of body fluid is related to age, sex and body shape. Adult male body fluids account for about 60% of body weight, female 55% and infant 70%. Two-thirds of the total adult water is in cells, and 1/3 is outside cells. About 3/4 of extracellular fluid exists in intercellular space, which is called intercellular fluid (interstitial fluid); 1/4 in blood vessels, called plasma. Intercellular fluid is divided into functional intercellular fluid and non-functional intercellular fluid. Functional intercellular fluid refers to the part of fluid that can be rapidly exchanged with intravascular fluid or intracellular fluid to maintain body fluid balance. Cerebrospinal fluid, joint fluid and digestive tract secretions are non-functional intercellular fluids, which constitute the third space and have little effect on maintaining body fluid balance. However, under pathological conditions, the effusion in the third space increases, such as a large amount of exudate in the abdominal cavity of patients with peritonitis, which will also lead to imbalance of body fluids. The distribution of liquids in various parts of normal human body is relatively constant, and they are constantly exchanged, maintaining a dynamic balance. In general, the daily intake and discharge of water are relatively stable.

Usually about 850ml of water is excreted through breathing and skin evaporation every day. The discharge of this part of the water is imperceptible and uncontrollable, which is called unobvious water loss. In order to digest food, the digestive juice secreted by gastrointestinal tract is about 8200ml per day, but most of it is reabsorbed at the end of ileum and right colon, and only about 150ml of water is excreted by feces. Adults usually excrete not less than 35g of solid waste from the kidneys every day, and each gram needs at least 15ml of urine to be dissolved and discharged, so the daily urine volume should generally be maintained at 1000 ~ 1500ml. As can be seen from the above, the minimum daily intake of normal people is 1500ml, and the reasonable daily intake is about 2500ml.