Kidney

Kidney, paired organ whose functions include removing waste products from the blood and regulating the amount of fluid in the body. The basic units of the kidneys are microscopically thin structures called nephrons, which filter the blood and cause wastes to be removed in the form of urine. Together with the bladder, two ureters, and the single urethra, the kidneys make up the body’s urinary system. Human beings, as well as members of all other vertebrate species, typically have two kidneys.

Like kidney beans, the body’s kidneys are dark red in color and have a shape in which one side is convex, or rounded, and the other is concave, or indented. The kidneys of adult humans are about 10 to 13 cm (4 to 5 in) long and about 5 to 7.5 cm (2 to 3 in) wide—about the size of a computer mouse.

The kidneys lie against the rear wall of the abdomen, on either side of the spine. They are situated below the middle of the back, beneath the liver on the right and the spleen on the left. Each kidney is encased in a transparent, fibrous membrane called a renal capsule, which helps protect it against trauma and infection. The concave part of the kidney attaches to two of the body’s crucial blood vessels—the renal artery and the renal vein—and the ureter, a tubelike structure that carries urine to the bladder.

A primary function of kidneys is the removal of poisonous wastes from the blood. Chief among these wastes are the nitrogen-containing compounds urea and uric acid, which result from the breakdown of proteins and nucleic acids. Life-threatening illnesses occur when too many of these waste products accumulate in the bloodstream. Fortunately, a healthy kidney can easily rid the body of these substances.

The outermost layer of the kidney is called the cortex. Beneath the cortex lies the medulla, an area that contains between 8 and 18 cone-shaped sections known as pyramids, which are formed almost entirely of bundles of microscopic tubules. The tips of these pyramids point toward the center of the kidney. The cortex extends into the spaces between the pyramids, forming structures called renal columns. At the center of the kidney is a cavity called the renal pelvis.

The task of cleaning, or filtering, the blood is performed by millions of nephrons, remarkable structures that extend between the cortex and the medulla. Under magnification, nephrons look like tangles of tiny vessels or tubules, but each nephron actually has an orderly arrangement that makes possible filtration of wastes from the blood. The primary structure in this filtering system is the glomerulus, a network of extremely thin blood vessels called capillaries. The glomerulus is contained in a cuplike structure called Bowman’s capsule, from which extends a narrow vessel, called the renal tubule. This tube twists and turns until it drains into a collecting tubule that carries urine toward the renal pelvis. Part of the renal tubule, called the loop of Henle, becomes extremely narrow, extending down away from Bowman’s capsule and then back up again in a U shape. Surrounding the loop of Henle and the other parts of the renal tubule is a network of capillaries, which are formed from a small blood vessel that branches out from the glomerulus.

Blood enters the kidney through the renal artery. The artery divides into smaller and smaller blood vessels, called arterioles, eventually ending in the tiny capillaries of the glomerulus. The capillary walls here are quite thin, and the blood pressure within the capillaries is high. The result is that water, along with any substances that may be dissolved in it—typically salts, glucose or sugar, amino acids, and the waste products urea and uric acid—are pushed out through the thin capillary walls, where they are collected in Bowman’s capsule. Larger particles in the blood, such as red blood cells and protein molecules, are too bulky to pass through the capillary walls and they remain in the bloodstream. The blood, which is now filtered, leaves the glomerulus through another arteriole, which branches into the meshlike network of blood vessels around the renal tubule. The blood then exits the kidney through the renal vein. Approximately 180 liters (about 50 gallons) of blood moves through the two kidneys every day.

Urine production begins with the substances that the blood leaves behind during its passage through the kidney—the water, salts, and other substances collected from the glomerulus in Bowman’s capsule. This liquid, called glomerular filtrate, moves from Bowman’s capsule through the renal tubule. As the filtrate flows through the renal tubule, the network of blood vessels surrounding the tubule reabsorbs much of the water, salt, and virtually all of the nutrients, especially glucose and amino acids, that were removed in the glomerulus. This important process, called tubular reabsorption, enables the body to selectively keep the substances it needs while ridding itself of wastes. Eventually, about 99 percent of the water, salt, and other nutrients is reabsorbed.

At the same time that the kidney reabsorbs valuable nutrients from the glomerular filtrate, it carries out an opposing task, called tubular secretion. In this process, unwanted substances from the capillaries surrounding the nephron are added to the glomerular filtrate. These substances include various charged particles called ions, including ammonium, hydrogen, and potassium ions.

Together, glomerular filtration, tubular reabsorption, and tubular secretion produce urine, which flows into collecting ducts, which guide it into the microtubules of the pyramids. The urine is then stored in the renal cavity and eventually drained into the ureters, which are long, narrow tubes leading to the bladder. From the roughly 180 liters (about 50 gallons) of blood that the kidneys filter each day, about 1.5 liters (1.3 qt) of urine are produced.

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.

Kidneys are paired organs, each sharing equally the work of removing wastes and excess water from the blood. Remarkably, a single kidney can do the job of both if one kidney is lost through injury or disease. It sometimes occurs, although rarely, that a person is born with only one kidney. Such people are able to lead normal lives.

In a kidney transplant, the donated kidney may come from a close living relative of the patient or from a person who has recently died. The donor kidney is removed by clamping and cutting the renal vein and artery (1). The diseased kidneys in the patient may be left in place, or one or both may be removed if they cause persistent infection or high blood pressure (2). The donor kidney is placed in the pelvis region of the recipient and the organ’s renal vein and artery are attached (3). Both the donor and the recipient can survive in good health with only one functioning kidney to filter and regulate the composition of blood.

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