The human kidney is comprised of an outer capsule and a cortex and a medulla.
The kidney is the site of formation of the urine. The functional unit of the kidney is called nephron and is composed of millions of such units.
The nephron is composed of a filtration unit that is called the glomerulus. In addition to the glomerulus, the nephron is composed os several types of tubules that are specialized in reabsorption of nutrients that are filtered in the glomerulus.
The main compounds filtered in the glomerulus are urea and water. In addition, some other essential compounds are also filtered in the glomerulus. However they are reabsorbed into the blood in the tubules of the kidney.
An example of such nutrients that are filtered and reabsorbed are amino-acids. In addition to amino-acids, electrolytes, such as sodium and potassium and hydrogen ions, are actively filtered in the glomerulus. But are also actively reabsorbed in the kidneys tubules.
They are actively transported using special mechanisms that are called pumps. These pumps use ATPase enzyme in order to drive the transport of the electrolytes against their concentration gradient. an example of such a pump is the sodium/potassium pump which functions by conserving potassium in exchange for sodium which is pumped out of the cell. Potassium is in turn pumped inward the cell.
The regulation of the kidney function is in part controlled by hormones that are secreted by the adrenal gland and the pituitary gland.
Sodium secretion and conservation is regulated by a hormone that is called aldosterone. Aldosterone is a hormone that is secreted by the adrenal gland. Its excess secretion leads to excessive conservation of sodium and with it water as well. This contributes to high blood pressure. Diuretics work on this hormone which leads to sodium and water depletion and thus lowering the blood pressure. Low levels of aldosterone or hypo-aldosteronemia leads to to depletion of water and sodium. Thus contributing to hypotension and dehydration. In case of hypoaldosteronism sodium is excreted while potassium is conserved. In case of hyperaldosteronism the levels of sodium is increased while the potassium is being depleted.
Water conservation and excretion is also controlled by another hormone, which is called antidiuretic hormone or arginine vasopressin. This hormone is secreted by the pituitary gland in response to hypovolemia or low blood levels. Hypovolemia signals the pituitary gland to secrete ADH or anti diuretic hormone and this in turn acts on the kidney to conserve water and prevent its depletion in the urine.
Hypervolemia or high levels of blood signal the pituitary gland not to secrete ADH and this in turn helps with the excessive depletion of water.
The kidney secretes a hormone that is called erythropoietin; its deficiency can contribute to anemia. Erythropoietin stimulates the sythesis of red blood cells. This condition can happen in chronic renal failure, in which the kidneys are not able to synthesize enough erythropoietin thus contributing to anemia.
The kidney is the site of also partial production of vitamin D, which has a role in facilitating the absorption of calcium to the blood. Kidney impairment leads to hypocalcemia and in turn to osteomalacia ( a bone disease that happens as a result of vitamin D deficiency) in adults and rickets ( osteomalacia in children ) in children.