There are many causes of water retention, the most common ones being impaired renal function, hypertension and hyperkalemia. Also, water retention is one of the common side effects of immunosuppressive drugs, with glucocorticoids being the most common. Hormones can lead to sodium retention in the body, which is always accompanied by sodium, resulting in sodium-water retention. Many of the drugs taken after surgery have varying degrees of nephrotoxicity, and water retention is a sign of severely impaired renal function. Studies have shown that if there is severe water retention before surgery, the chance of water retention after surgery is often significantly higher. In normal people, sodium and water intake and excretion are in dynamic balance, so the amount of body fluid remains constant. Sodium is excreted mainly through the kidneys, so the basic mechanism of sodium retention is renal dysregulation. If the normal glomerular filtration of sodium and water is 100%, the final discharge only accounts for 0.5-1% of the total amount, of which 99-99.5% is reabsorbed by the renal tubules, 60-70% is actively absorbed by the proximal tubules, and the reabsorption of sodium and water by the distal tubules and collecting ducts is regulated by hormones, maintaining the above state as the ball and tube equilibrium, and the renal regulation is impaired, that is, the ball and tube imbalance. Glomerular filtration rate decreases Glomerular filtration rate mainly depends on effective filtration pressure, permeability of the filtration membrane and filtration area, any of which can lead to a decrease in glomerular filtration rate. In the case of heart failure, cirrhosis ascites and other decreases in effective circulating blood volume, on the one hand, arterial blood pressure decreases and reflexively excites sympathetic nerves; on the other hand, due to renal vasoconstriction and decreased renal blood flow, the renin-angiotensin-ketosteroid system is activated, which further constricts the small arteries into the glomerulus, causing a decrease in glomerular capillary blood pressure and a decrease in effective filtration pressure; in acute glomerulonephritis, due to inflammatory exudates and glomerular capillary endothelial swelling, glomerular filtration membrane permeability is reduced; in chronic glomerulonephritis, a large number of kidney units are destroyed and the glomerular filtration area is reduced, all these factors lead to a decrease in glomerular filtration rate and sodium and water retention. 2, proximal tubular reabsorption of sodium and water increased It is currently believed that when the effective circulating blood volume decreases, in addition to reduced renal blood flow, sympathetic excitation, renin-angiotensin-aldosterone activation, increased angiotensin II makes the small glomerular outflow artery contraction more pronounced than the small inlet artery contraction, glomerular capillary blood pressure increases, the result is a decrease in renal plasma flow, more significant than the decrease in glomerular filtration rate, that is Glomerular filtration rate is relatively higher and filtration fraction is increased. Thus, the blood flowing out from the glomerulus, because of the increased filtration in the glomerulus, its hydrostatic pressure decreases, while the colloid osmotic pressure increases (blood viscosity), and the blood with the above characteristics is distributed in the proximal tubule, so that the proximal tubule reabsorption of sodium and water increases. 3. Increased sodium and water reabsorption in the distal convoluted tubules and collecting ducts The ability of the distal convoluted tubules and collecting ducts to reabsorb sodium and water is regulated by ADH and aldosterone. The decrease in effective circulating blood volume caused by various reasons and the decrease in blood volume are the main reasons for the increased secretion of ADH and aldosterone. The inactivation of aldosterone and ADH is inactivated in the liver, and when liver dysfunction occurs, the inactivation of both hormones decreases. increased levels of ADH and aldosterone in the blood lead to increased reabsorption of sodium and water by the distal convoluted tubules and collecting ducts, and sodium and water retention.