Other Functions Of The Kidneys
In addition to cleaning the blood, the kidneys perform several other essential functions. One such activity is regulation of the amount of water contained in the blood. This process is influenced by antidiuretic hormone (ADH), also called vasopressin, which is produced in the hypothalamus (a part of the brain that regulates many internal functions) and stored in the nearby pituitary gland.
Receptors in the brain monitor the blood’s water concentration. When the amount of salt and other substances in the blood becomes too high, the pituitary gland releases ADH into the bloodstream. When it enters the kidney, ADH makes the walls of the renal tubules and collecting ducts more permeable to water, so that more water is reabsorbed into the bloodstream.
The hormone aldosterone, produced by the adrenal glands, interacts with the kidneys to regulate the blood’s sodium and potassium content. High amounts of aldosterone cause the nephrons to reabsorb more sodium ions, more water, and fewer potassium ions; low levels of aldosterone have the reverse effect. The kidney’s responses to aldosterone help keep the blood’s salt levels within the narrow range that is best for crucial physiological activities.
Aldosterone also helps regulate blood pressure. When blood pressure starts to fall, the kidney releases an enzyme (a specialized protein) called renin, which converts a blood protein into the hormone angiotensin. This hormone causes blood vessels to constrict, resulting in a rise in blood pressure. Angiotensin then induces the adrenal glands to release aldosterone, which promotes sodium and water to be reabsorbed, further increasing blood volume and blood pressure.
The kidney also adjusts the body's acid-base balance to prevent such blood disorders as acidosis and alkalosis, both of which impair the functioning of the central nervous system. If the blood is too acidic, meaning that there is an excess of hydrogen ions, the kidney moves these ions to the urine through the process of tubular secretion. An additional function of the kidney is the processing of vitamin D; the kidney converts this vitamin to an active form that stimulates bone development.
Several hormones are produced in the kidney. One of these, erythropoietin, influences the production of red blood cells in the bone marrow. When the kidney detects that the number of red blood cells in the body is declining, it secretes erythropoietin. This hormone travels in the bloodstream to the bone marrow, stimulating the production and release of more red cells.
KIDNEY DISEASE AND TREATMENT
Urine, pale yellow fluid produced by the kidneys, composed of dissolved wastes and excess water or chemical substances from the body. It is produced when blood filters through the kidneys, which remove about 110 liters (230 pints) of watery fluid from the blood every day. Most of this fluid is reabsorbed into the blood, but the remainder is passed from the body as urine. Urine leaves the kidneys, passes to the bladder through two slender tubes, the ureters, and exits the body through the urethra. A healthy adult can produce between 0.5 to 2 liters (1 to 4 pints) of urine a day, but the quantity varies considerably, depending on fluid intake and loss of fluid from sweating, vomiting, or diarrhea.
Water accounts for about 96 percent, by volume, of the urine excreted by a healthy person. Urine also contains small amounts of urea, chloride, sodium, potassium, ammonia, and calcium. Other substances, such as sugar, are sometimes excreted in the urine if their concentration in the body becomes too great. The volume, acidity, and salt concentration of urine are controlled by hormones. Measurements of the composition of urine are useful in the diagnosis of a wide variety of conditions, including kidney disease, diabetes, and pregnancy.
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