Our bodies produce several kinds of wastes, including sweat, carbon dioxide gas, feces (also known as stool), and urine (pee). These wastes exit the body in different ways: Sweat is released through pores (tiny holes) in the skin. Water vapor and carbon dioxide are exhaled (breathed out) from the lungs. And undigested food materials are formed into feces in the intestines and excreted from the body as solid waste in bowel movements.

Urine, which is produced by the kidneys, contains the by-products of our body's metabolism — salts, toxins, and water — that end up in our blood. The kidneys and urinary tract (which includes the ureters, bladder, and urethra) filter and eliminate these waste substances from our blood. Without the kidneys, waste products and other toxins would soon build up in the blood to dangerous levels.

In addition to eliminating wastes, the kidneys and urinary tract also regulate many important body functions. For example, the kidneys monitor and maintain the body's balance of water, ensuring that our tissues receive enough water to function properly and be healthy.

When you're asked to give a urine sample during a doctor's visit, the results reveal how well your two kidneys are working. For example, blood, protein, or white blood cells in the urine may indicate injury, infection, or inflammation of the kidneys, and glucose in the urine may be an indication of diabetes.

What Do the Kidneys and Urinary Tract Do?

Although the two kidneys work together to perform many vital functions, people can live a normal, healthy life with just one kidney. In fact, some people are born with just one of these bean-shaped organs. If one kidney is removed, the remaining one will enlarge within a few months to take over the role of filtering blood on its own.

Every minute, more than 1 quart (about 1 liter) of blood passes through the kidneys, adding up to about 425 gallons (1,609 liters) of blood each day. About a quarter of our blood is in our kidneys at any one time, and the kidneys cleanse all of the blood in the body about every 50 minutes.

In addition to filtering blood, producing urine, and ensuring that body tissues receive enough water, the kidneys also regulate blood pressure and the level of vital salts in the blood. By regulating salt levels through production of an enzyme called renin (as well as other substances), the kidneys ensure that blood pressure is regulated.

The kidneys also secrete a hormone called erythropoietin (pronounced: eh-rith-ro-po-uh-ten), which stimulates and controls the body's red blood cell production (red blood cells carry oxygen throughout the body). In addition, the kidneys help regulate the acid-base balance (or the pH) of the blood and body fluids, which is necessary for the body to function normally.

Where Are They and How Do They Work?

The kidneys are located just under the rib cage in the back, one on each side. The right kidney is located below the liver, so it's a little lower than the left one. Each adult kidney is about 5 inches (127 millimeters) long, 3 inches (76 millimeters) wide, and 1 inch (25 millimeters) thick. Each has an outer layer called the cortex, which contains the filtering units. The center part of the kidney, the medulla (pronounced: muh-duh-luh) has 10 to 15 fan-shaped structures called pyramids. These drain urine into cup-shaped tubes called calyxes (pronounced: kay-luh-seez). A layer of fat surrounds the kidneys to cushion and help hold them in place.

Here's how the kidneys filter blood: Blood travels to each kidney through the renal artery, which enters the kidney at the hilus (pronounced: hy-luss), the indentation in the kidney that gives it its bean shape. As it enters the cortex, the artery branches out; each branch envelops the nephrons (pronounced: neh-fronz) — 1 million tiny filtering units in each kidney that remove the harmful substances from the blood.

Each of the nephrons contains a filter called the glomerulus (pronounced: gluh-mer-yuh-lus), which contains a network of tiny blood vessels known as capillaries. The fluid filtered from the blood by the glomerulus then travels down a tiny tube-like structure called a tubule (pronounced: tu-byool) that adjusts the level of salts, water, and wastes that are excreted in the urine.

Filtered blood leaves the kidney through the renal vein and flows back to the heart.

The continuous blood supply entering and leaving the kidneys gives the kidneys their dark red color. While the blood is in the kidneys, water and some of the other blood components (such as acids, glucose, and other nutrients) are reabsorbed back into the bloodstream. Left behind is urine. Urine is a concentrated solution of waste material containing water, urea (pronounced: yoo-ree-uh, a waste product that forms when proteins are broken down), salts, amino acids, byproducts of bile from the liver, ammonia, and any substances that cannot be reabsorbed into the blood. Urine also contains urochrome (pronounced: yur-uh-krome), a pigmented blood product that gives urine its yellowish color.

The renal pelvis, located near the hilus, collects the urine flowing from the calyxes. From the renal pelvis, urine is transported out of the kidneys through the ureters (pronounced: yur-uh-ters), tubes that carry the urine out of each kidney to be stored in the urinary bladder — a muscular collection sac in the lower abdomen.

The bladder expands as it fills and can hold about half a liter (2 cups) of urine at any given time (an average adult produces about 1½ liters, or 6 cups, of urine per day). An adult needs to excrete at least one third of this amount to adequately clear waste products from the body. Producing too much or not enough urine may be a sign of illness.

When the bladder is full, nerve endings in its wall send impulses to the brain. When a person is ready to urinate, the bladder walls contract and the sphincter (pronounced: sfink-ter, a ring-like muscle that guards the exit from the bladder to the urethra) relaxes. The urine is ejected from the bladder and out of the body through the urethra (pronounced: yoo-ree-thruh), another tube-like structure. The male urethra ends at the tip of the penis; the female urethra ends just above the vaginal opening.

Things That Can Go Wrong With the Kidneys and Urinary Tract

Like other systems in the body, the entire urinary tract is subject to diseases and disorders. In kids and teens, the more common kidney and urinary tract problems include:

Congenital problems of the urinary tract. As a fetus develops in the womb, any part of the urinary tract can grow to an abnormal size or in an abnormal shape or position. One of the more common congenital abnormalities (meaning abnormalities that exist at birth) is duplication of the ureters, in which a kidney has two ureters instead of one. This defect occurs in about one out of every 125 births and can cause the kidney to develop problems with infection and scarring over time.

Another congenital problem is horseshoe kidney, where the two kidneys are fused (connected) into one arched kidney that usually functions normally, but is more prone to develop problems later in life. This condition is found in 1 out of every 500 births.

Glomerulonephritis is an inflammation of the glomeruli, the parts of the filtering units (nephrons) of the kidney that contain a network of capillaries (tiny blood vessels). The most common form of this condition is post-streptococcal glomerulonephritis, which usually occurs in young children.

Hypertension (high blood pressure) can result when the kidneys are impaired by disease. The kidneys control blood pressure by regulating the amount of salt in the body and by producing the enzyme renin that, along with other substances, controls the constriction of muscle cells in the walls of the blood vessels.

Kidney (renal) failure can be acute (which means sudden) or chronic (occurring over time and usually long lasting or permanent). In either form of kidney failure, the kidneys slow down or stop filtering blood effectively, causing waste products and toxic substances to build up in the blood.

Acute kidney failure may be due to bacterial infection, injury, shock, heart failure, poisoning, or drug overdose. Treatment includes correcting the problem that led to the failure and sometimes requires surgery or dialysis (which involves using a machine or other artificial device to remove the excess salts and water and other wastes from the body when the kidneys are unable to perform this function).

Chronic kidney failure involves a deterioration of kidney function over time. In kids and teens, it can result from acute kidney failure that fails to improve, birth defects of the kidney, chronic kidney diseases, or chronic severe high blood pressure. If diagnosed early, chronic kidney failure can be treated, but usually not reversed, and may require a kidney transplant at some point in the future.

Kidney stones (or nephrolithiasis) result from the buildup of crystallized salts and minerals such as calcium in the urinary tract. Stones (also called calculi) can also form after an infection. If kidney stones are large enough to block the kidney or ureter, they can cause severe abdominal pain. But the stones usually pass through the urinary tract on their own. In some cases, they may need to be removed surgically.

Nephritis is any inflammation of the kidney. It can be caused by infection, an autoimmune disease (such as lupus), or it may be idiopathic (which means the exact cause may not be known or understood). Nephritis is generally detected by high levels of protein and blood in the urine.

Urinary tract infection (UTI) is infection of a part of or throughout the urinary tract, usually caused by bacteria. UTIs are most commonly caused by intestinal bacteria, such as E. coli, that are normally found in feces. These bacteria can cause infections anywhere in the urinary tract, including the kidneys. Most UTIs occur in the lower urinary tract, especially in the bladder and urethra. Teen girls are more likely to develop UTIs than boys; this may be because girls have shorter urethras than boys.

Vesicoureteral reflux (VUR) is a condition in which urine abnormally flows backward (or refluxes) from the bladder into the ureters. It may even reach the kidneys, where infection and scarring can occur over time. VUR occurs in 1% of children and tends to run in families. It's often detected after a young infant or child has a first UTI.

Reviewed by: Steven Dowshen, MD
Date reviewed: May 2010