(i) constitutes an important part of cells and body fluids. The water content in adults accounts for about 65% of body weight, and the water content in blood accounts for more than 80%. Water is widely distributed inside and outside tissues and cells, which constitutes the internal environment of human body.

(2) Participate in substance metabolism in human body. Water has strong dissolving power and great electrolytic power, which can make water-soluble substances exist in dissolved state and electrolyte ion state; Water has great fluidity. In the process of digestion, absorption, circulation and excretion, it helps to speed up the transport of nutrients and the excretion of waste, so that metabolism and physiological and chemical reactions in the human body can be carried out smoothly.

(3) lubrication. There is a certain amount of water in joints, chest cavity, abdominal cavity and gastrointestinal tract. It can cushion, lubricate and protect organs, joints, muscles and tissues.

(4) regulating body temperature. The specific heat value of water is large. It takes about 4.2J energy to raise or lower 1℃ in 19 water. A large amount of water can absorb the energy generated in the metabolic process, so that the body temperature will not increase significantly. Water has a great heat of vaporization. At the temperature of 37℃, evaporation 19 water can take away 2.4kJ of energy. Therefore, at high temperature, body heat can evaporate with water through the skin to dissipate heat, thus maintaining a constant body temperature.