The review substantiates the idea of a kidney as a homeostatic organ that continuously recreates the ideal composition and volume of fluids of the internal environment. The kidney restores the protein composition of the blood serum, which determines its oncotic pressure and the ratio of intra- and extravascular fluid. Analyzed data on the proportion of the text of the chapter on the physiology of the kidneys from the total volume of the textbook or manual on physiology for universities. Characterized kidney function in the human body and the processes underlying the urine formation. Presents data on the molecular mechanisms of transport from the renal tubules into the fluid of the internal environment of ions, amino acids, carbohydrates, vitamins. The significance and mechanisms of regulation of fluid reabsorption in the proximal and distal segments of the nephron are substantiated. Analyzed the role of the kidneys in maintaining short-term endocrine mirrors of blood. Excretion of substances is considered as