The main components of body fluid are water and electrolytes. It is divided into two parts: cellular and extracellular fluid, and its amount varies with gender, age and fatness. The amount of body fluid in adult males is generally 60% of body weight; in adult females, the amount of body fluid is about 55% of body weight. Children have less fat, so the proportion of body fluid volume to body weight is higher, and in newborns, it can reach 80% of body weight. The amount of body fat increases with age, and after the age of 14, the proportion of body fluid to body weight in children is similar to that in adults. Imbalance of fluid balance can cause disturbance of the balance of the body. The body mainly through the kidney to maintain the balance of body fluids, to maintain the stability of the internal environment. The regulatory function of the kidney is influenced by neurological and endocrine responses. The normal pressure of body fluids is restored and maintained by the hypothalamic-posterior pituitary-antidiuretic hormone system, and then the blood volume is restored and maintained by the renin-aldosterone system. However, when blood volume is sharply reduced, the body will prioritize the maintenance and restoration of blood volume at the expense of maintenance of body fluid osmolarity, so that perfusion of vital organs is ensured and life is maintained. When the body loses water, there is an increase in extracellular fluid osmolality, which stimulates the hypothalamic-posterior pituitary-antidiuretic hormone system, producing thirst and increased water intake, as well as contributing to increased secretion of antidiuretic hormone. The epithelial cells of the distal tubule and collecting duct, under the effect of antidiuretic hormone, enhance water reabsorption, so that urine volume decreases and water is retained in the body, resulting in a decrease in extracellular fluid osmolality. Conversely, when water increases in the body, the extracellular fluid osmolality decreases, inhibiting the thirst response and decreasing the secretion of antidiuretic hormone. The reabsorption of water by the epithelial cells of the distal tubules and collecting ducts decreases and excess water is excreted from the body, increasing the extracellular fluid osmolality. This response of antidiuretic hormone secretion is very sensitive. When the plasma osmolality increases or decreases by less than 2% from normal, there is a change in the secretion of antidiuretic hormone, which keeps the body’s water dynamic and stable. On the other hand, when the extracellular fluid decreases, especially the blood volume, the intravascular pressure decreases, and the blood pressure in the small arteries of the renal entry also decreases accordingly, and the pressure receptors located in the wall of the tubule are stimulated by the decrease in pressure, which causes the paraglomerular cells to increase the secretion of renin; at the same time, with the decrease in blood volume and blood pressure, the glomerular filtration rate also decreases accordingly, so that the amount of Na+ flowing through the distal tubule decreases significantly. The decrease in sodium stimulates the sodium receptors located in the dense patch of the distal tubule, causing the glomerular parietal cells to increase renin secretion. In addition, a decrease in systemic blood pressure can also excite sympathetic nerves and stimulate renin secretion by paraglomerular cells. Renin catalyzes the conversion of angiotensinogen present in plasma to angiotensin I and then to angiotensin II, which causes constriction of small arteries and stimulates the adrenal cortex globular zone, increasing aldosterone secretion, promoting the reabsorption of Na+ in the distal tubule and promoting the secretion of K+ and H+. With the increase in sodium reabsorption, there is also an increase in CI- reabsorption and an increase in reabsorbed water. The result is an increase in the amount of extracellular fluid. After the return of circulating blood volume and the gradual return of blood pressure, that in turn inhibits the release of renin, the production of aldosterone decreases, so that the reabsorption of Na+ decreases, and thus the amount of extracellular fluid no longer increases and remains stable.