The physiological functions of the kidney are mainly to excrete metabolites and regulate water, electrolyte and acid-base balance to maintain the stability of the internal environment of the body. 1.Glomerular filtration is the main form of metabolite excretion. Among them, nitrogenous wastes such as urea and creatinine are mostly excreted by glomerular filtration, and some organic acids such as hippuric acid, benzoic acid, various amines and uric acid are also partly excreted by glomerular filtration. Glomerular filtrate must be filtered through the glomerular capillary wall. The capillary wall is composed of perforated endothelial cells, glomerular basement membrane (GBM) and pedicle cells (dirty layer epithelial cells). In addition to size selectivity, the glomerular filtration membrane can restrict the passage of macromolecules; it also has charge selectivity, which can restrict the filtration of negatively charged substances. The glomerular filtration rate (GFR) depends mainly on the hydrostatic pressure and colloid osmotic pressure in the capillaries and capsule of the glomerulus, as well as the area of the filtration membrane and the capillary ultrafiltration fraction (the latter two are collectively called the filtration coefficient). Renal blood flow and GFR remain relatively constant under different renal perfusion pressures, which is the self-regulation of renal blood flow and glomerular filtration rate. This self-regulation has important physiological significance, on the one hand, it ensures that glomerular filtration can still be carried out stably during hemodynamic changes, and metabolic waste can continue to be excreted from the body, on the other hand, it also ensures the balance of body fluids. 2, renal tubular reabsorption and secretion function The daily glomerular filtration of primary urine can reach 180L, in which the electrolyte composition is basically similar to that of blood plasma. However, normal people excrete only about 1500ml of urine per day, of which more than 99% of water and many substances are reabsorbed by the renal tubules. 3. Kidney and hormones The kidney is not only a target for hormone action, but it also synthesizes, regulates and secretes hormones that affect non-renal functions, such as erythropoiesis and bone metabolism. The hormones secreted by the kidney can be divided into vasoactive peptides and non-vascular active hormones. The former acts on the kidney itself, mainly regulating renal hemodynamics and water and salt metabolism, including renin, angiotensin, prostaglandins, kinin-releasing enzyme-kinin system, endothelin, natriuretic peptide, and arachidonic acid-like substances; non-vasoreactive hormones include 1α-hydroxylase and erythropoietin.