The kidneys are regarded as complex purification machinery; they have the function of both elimination and regulation of internal fluids. They excrete water, but they also conserve it; they eliminate through urine all the products of food metabolism that could be potentially harmful.

The kidneys also perform hormonal functions, since they produce a substance called erythropoietin that prevents the appearance of anemia. In addition, they are also responsible for the regulation of blood pressure and important substances in bone metabolism, such as calcium, phosphorus, and vitamin D.