en españolLos riñones y el aparato urinario

Kidneys & Urinary Tract Basics

Our bodies produce several kinds of wastes, including sweat, carbon dioxide gas, feces (stool or poop), and urine (pee). These wastes leave the body in different ways. Sweat is released through pores in the skin. Water vapor and carbon dioxide are exhaled 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 byproducts of metabolism — salts, toxins, and water — that end up in the blood. The kidneys and urinary tract (which includes the kidneys, ureters, bladder, and urethra) filter and eliminate these waste substances from our blood. Without the kidneys, waste products and toxins would soon build up in the blood to dangerous levels.

Besides 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 work properly and be healthy.

When doctors take a urine sample, the results reveal how well the kidneys are working. For example, blood, protein, or white blood cells in pee may indicate injury, inflammation, or infection of the kidneys, and glucose in the urine may be an indication of diabetes.

What They 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 goes to the kidneys. About one fifth of the blood pumped from the heart goes to the kidneys at any one time.

Besides filtering blood, producing urine, and making 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 the hormone erythropoietin, which stimulates and controls red blood cell production (red blood cells carry oxygen throughout the body). Plus, 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.

How They Work

The kidneys are located just under the ribcage in the back, one on each side. The right kidney is below the liver, so it's a little lower than the left one. Each adult kidney is about the size of a fist. Each has an outer layer called the cortex, which contains the filtering units.

The center part of the kidney, the medulla, has 10 to 15 fan-shaped structures called pyramids. These drain urine into cup-shaped tubes called calyxes. 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, the indentation in middle of the kidney that gives it its bean shape. As it enters the cortex, the artery branches to envelope the nephrons — 1 million tiny filtering units in each kidney that remove the harmful substances from the blood.

Each of the nephrons contain a filter called the glomerulus, 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, which 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 (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, a pigmented breakdown product of blood that gives pee 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, 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 2 cups (half a liter) of urine at any given time (an average adult produces about 6 cups, or 1½ liters, of urine per day). An adult needs to produce and excrete at least one third of this amount in order 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 (a ring-like muscle that guards the exit from the bladder to the urethra) relaxes. The pee is ejected from the bladder and out of the body through the urethra, another tube-like structure. The male urethra ends at the tip of the penis; the female urethra ends just above the vaginal opening.

Kidneys and Urinary Tract Problems

Like other systems in the body, the entire urinary tract is subject to diseases and disorders.

In kids, the more common 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 common congenital abnormality (an abnormality that exists at birth) is duplication of the ureters, in which a kidney has two ureters coming from it instead of one. This defect happens in about 1 out of every 125 births and can cause the kidney to develop problems with repeated infections and scarring over time.

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

Glomerulonephritis. This 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 is post-streptococcal glomerulonephritis, which usually affects young children after a case of strep throat. Most kids with this type of nephritis recover fully, but a few can have permanent kidney damage that eventually requires dialysis or a kidney transplant.

High blood pressure (hypertension). High blood pressure can happen if the kidneys are harmed by disease. The kidneys control blood pressure by regulating the amount of salt in the body. They also produce the enzyme renin that, along with other substances, controls the constriction of muscle cells in the walls of the blood vessels, which affects a person's blood pressure.

Kidney (renal) failure. This can be acute (sudden) or chronic (happening 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 many things, including 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. Dialysis 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 children, it can result from acute kidney failure that fails to improve, birth defects of the kidney, chronic kidney diseases, repeated kidney infections, or chronic severe high blood pressure. If diagnosed early, chronic kidney failure in children can be treated but usually not reversed. The child may require a kidney transplant at some point in the future.

Kidney stones (nephrolithiasis). Kidney stones (also called calculi) are due to a buildup of crystallized salts and minerals such as calcium in the urinary tract. They also can form after an infection. Kidney stones that are large enough to block the kidney or ureter can cause severe abdominal pain. The stones usually pass through the urinary tract on their own, but some need to be removed surgically.

Nephritis is any inflammation of the kidney. This can be caused by infection, medicines, or an autoimmune disease (such as lupus), but sometimes the exact cause isn't known. Nephritis is usually detected when protein and blood are found in the urine.

Nephrotic syndrome. This type of kidney disease leads to loss of protein in the urine and swelling of the face (often the eyes) or body (often around the genitals). It's most common in children younger than 6 years old and affects more boys than girls. It's often treated with steroids.  

Urinary tract infections (UTIs). Most UTIs are 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 happen in the lower urinary tract (the bladder and urethra). UTIs affect both boys and girls, but in school-age children, girls are more likely to develop them than boys. This might be because girls have shorter urethras than boys.

Vesicoureteral reflux (VUR). In this condition, pee abnormally flows backward (or refluxes) from the bladder into the ureters. It may even reach the kidneys, where infection and scarring can happen over time. VUR occurs in 1% of children and tends to run in families. It's often detected after a young child has a first urinary tract infection. Most kids outgrow mild forms of VUR, but some can develop permanent kidney damage and kidney failure later in life.

Wilms' tumor. The most common kidney cancer in children, it's most often diagnosed in kids between 2 and 5 years of age, and affects boys and girls equally.