Anatomically speaking, the kidney is composed of millions of nephrons which are also the functional units of the kidney. Each nephron is composed of a glomerulus and attached to it are tubules. In the glomerulus of the nephron, filtering of the blood occurs and formation of urine occurs also.
In the kidney tubules, two processes occur. One of them is collection of the urine which is then passed through the two ureters to the urinary bladder, and the other process that occurs in the kidney tubules is the process of reabsorption of important molecules to the blood circulation some of them by active transport and others by diffusion such as occurs with water.
The function of the kidney is mainly to filter the blood and form the urine. In addition, it has other important functions such as blood cells formation and calcium homeostasis. These two functions will be mentioned shortly. The function of the kidney is tightly regulated by hormones which are secreted from the pituitary gland and the adrenal gland.
The amount of blood that is filtered and reabsorbed in the kidney tubules are regulated by these hormones. This way also the blood pressure inside the arteries will be affected by the function of these hormones also. The first hormone that has effect on the water balance in the body by affecting the kidney function is called anti-diuretic hormone. This hormone is secreted by the posterior pituitary gland. Its secretion is affected by the volume of the blood in the circulation and the extent of osmolality of the blood also.
The kidney usually reabsorbs more water to the blood circulation in the case of hypovolemia and hyperosmolality of the blood. These two conditions stimulate the release of of the antidiuretic hormone to the blood circulation causing excessive conservation of water in the kidney tubules. The opposite effect happens in the case of hypervolemia and hypoosmolality of the blood in which case excessive amount of water in excreted in the urine. The amount of water that is either excreted or reabsorbed to the blood circulation depends on the amount of antidiuretic hormone in the blood.
Another hormone which has an effect on the kidney function to a minor degree is called atrial natriuretic peptide. This hormone is secreted by the heart cells and stimulates the kidney to secrete water in the urine. This hormone is usually functional in a state of hypertension.
The second major hormone to affect kidney function is called aldosterone. This hormone is usually secreted by the adrenal gland from its cortical area. Its secretion is induced by hypovolemic states indirectly by activating another hormone and which is called renin. This hormone is secreted by the kidney cells itself in response to hypovolemia or poor blood perfusion of the renal artery.
This hormone in turn stimulates the secretion of the hormone aldosterone by the adrenal cortex. This hormone signals the kidney to increase the conservation of sodium ions to the circulation with water which is also reabsorbed extensively in the kidney tubules. The lack of this hormone can cause the kidney to increase its secretion of sodium ions and water. Thus it can cause a state of hypotension.
In addition, to water and electrolytes regulation by the effect of hormones on the kidney, the kidney has two other important functions that are related to calcium ions homeostasis in the body and another function that is related to to blood cells formation. The kidney is the site of vitamin D synthesis from precursor molecules.
Deficiency of this vitamin as occurs in chronic renal failure can cause a state of hypocalcemia or decreased amount of calcium in the blood and bone tissue the thing that can predispose to to easily fractured bones and osteomalacia. The other important function of the kidney is to synthesize the hormone erythropoietin. This hormone is a stimulant for red blood cells synthesis. Its deficiency as it occurs in chronic kidney failure can cause anemia.