The main function of the kidney is the maintenance of the interior composition of the body. Due to continued digestion and utilisation of food in the body, construction and destruction of the various body tissues, waste products are generated daily.
If allowed to collect in the body, the function of the body is affected. These waste products are thrown out from the body by liver and intestines, lungs, skin and kidneys.
Kidneys excrete urea, creatinine and similar products in a concentrated form in urine. The amount of excretion closely balances the amount of production of these materials.
Kidneys maintain salt and water balance in the body. When healthy, a person may drink upto 10 liters of water or as low as ½ litre in a day. The kidneys increase and decrease urine appropriately to maintaine the total body water.
About 1 litre of blood flows through kidney. Of this about 180 litres of water, some salts and small particle are filtered daily in the kidney. Of this 180 litres, only 1 to 2 litre is finally excreted as urine. The rest is absorbed back to the blood from urinary tubules. Thus kidneys can dilute or concentrate urine.
The body water in humans depends to a large extent on salt content of the body. In case of reduced intake of salt , kidneys limit salt wastage in urine and similarly if intake is high , kidneys try to excrete extra salt.
Kidneys also excrete phosphate, ammonia products, urates and various drugs ingested by humans.
Urine is acidic in nature as it has to carry the daily acids produced from metabolism and activities of daily living, exercise etc. by excreting acid the pH of the body tissue and blood is maintained in a narrow range.
Kidneys produce a hormone called erythropoietin or EPO in short. EPO stimulates bonemarrow to produces RBCs and hemoglobin. Lack of EPO production in the kidneys leads to anemia or decreased hemoglobin.
Vitamin D produced from the ultraviolet radiation from sun or ingested in food is inactive. The liver and kidneys transform this inactive form to active form. Lack of activation leads to various bone abnormalities, fractures, bone pains and muscle weakness.