The kidneys have several functions. These are: Regulation of blood electrolytes composition. The kidneys regulate the blood level of some electrolytes. among them are sodium ion, potassium ion, calcium ion, chloride ion and biphosphate ion and water. sodium and water are reabsorbed by the proximal convoluted tubule of the nephron.
The mechanism of sodium reabsorption occurs via sodium pumps. Other filtered compounds such as glucose, amino acids and vitamins are also reabsorbed and are not lost in the urine. There is a sodium/glucose symporter ( passage in the same way) in the proximal convoluted tubules that reabsorbs both sodium and glucose. Other symporters reabsorb biphosphate and sulphate in the same way. Sodium/hydrogen antiporter( in different directions reabsorbs sodium and excrete hydrogen.
The second function of the kidneys is the regulation of PH or the acidity of the blood. The kidneys secrete protons in exchange for sodium throgh the sodium/hydrogen antiporter. In addition it conserves bicarbonate which participates in the neutralizing excess hydrogen thus controlling the PH. By this way the PH remains in a narrow range. After the hydrogen is filtered from the nephron it reacts with filtered bicarbonate which is a moderate base. Carbonic acid in formed as a result of this reaction. Carbonic acid then dissociates into water and carbon dioxide.
Another compound that is excreted by the kidney is urea. It is excreted in the proximal convoluted tubules. Urea is formed from ammonia in the hepatocytes of the liver. Ammonia is formed as a product of the deamination of amoni acids in the liver. The third function of the kidneys is to regulate blood volume. They do this by conserving or excreting water in the urine depending on the blood pressure status. This is monitored by the anti-diuretic hormone in the posterior hypophysis.
Low blood volume stimulates the release of anti-diuretic-hormone (ADH). This stimulates the kidneys to open water channels which in turn leads to preservation of water in the blood. Excessive amount of ADH renders high blood pressure as a result of the excessive conservation of water. Diabetes insipidus is a disease in which there is a lack of ADH secretion, and the patient suffers an excessive lost of water in the urine.
Another important function of the kidneys is the regulation of blood pressure. In addition to ADH there is an enzyme that it called renin which participates in raising blood pressure when it falls down. Renin acts on angiotensinogen and transfoms it to angiotensin I by cleaving 10 amino acids from angiotensinogen. An enzyme that is called angiotensin converting enzyme converts angiotensin I to its active form angiotensin II. Angiotensin II functions by increasing the blood pressure by decreasing the glomerular filtration at the nephron.
In addition angiotensin II enhances the reabsorption of sodium and water and consequently raising the blood pressure. Angiotensin can in addition stimulate the adrenal cortex to secrete aldosterone which functions by stimulating the reabsorption of sodium in exhange for potassium. this also contributes to raising the blood pressure. In addition to these important functions of the kidneys, it has an important function by producing important hormones. These hormones are vitamin D which its active form is called calcitriol which helps in absorbing calcium from the intestine, and therefor helps in the metabolism of bone formation.
The lack of vitamin D has a consequence of weak bones which can fracture easily. The other hormone that the kidneys produce is erythropoietin. This hormone has a function in the stimulation of red blood cells formation. Chronic renal failure has a clinical forms of anemia as a result of the deficiency of erythropoietin and of bone problems as a rsult of vitamin D deficiency. The kidneys also with the liver participates in glucose level monitoring.
The kidneys can make gluconeogenesis in the case of glucose shortage to synthesize new glucose from amino acids. The last function of the kidneys is to expel waste products from the body through the urine. The waste products are formed normally as by products of metabolism. Among these wastes are ammonia and bilirubin which is formed from the degradation of red blood cells.