Kidneys are the principal excretive organs Not only excrete waste metabolic products to remove various harmful substances But also regulate a variety of material in plasma to maintain the homeostasis of internal environment (osmolality and acid-base balance Besides, kidneys also produce some bioactive substances renin, prostaglandins(regulation of blood pressure erythropoietin(formation of matured RBCs)3 But also regulate a variety of material in plasma to maintain the homeostasis of internal environment (osmolality and acid-base balance) Kidneys are the principal excretive organs Not only excrete waste metabolic products to remove various harmful substances, Besides, kidneys also produce some bioactive substances renin, prostaglandins (regulation of blood pressure) erythropoietin (formation of matured RBCs) active vitamin D (metabolism of calcium and phosphorus)