The kidneys are a pair bean-shaped, fist-sized organs that are located below the rib cage near the middle of the back. In adults they filter the about 200 quarts (190 L) of blood every day to remove waste products that result from the normal activities of tissues in the body. These wastes circulate in the blood. and if not removed they would damage the body. The kidneys also play a crucial role in regulating the amount of water and chemicals (electrolytes) in the body such as sodium, potassium and phosphorous.

Inside the kidneys are about one million tiny units called nephrons. Inside each nephron is a very thin blood vessel called a capillary that twists around a very thin tube called a tubule. This combination of capillary and tubule inside the nephron is called a glomerulous and it is here that the blood is filtered. Water, electrolytes, and waste products (but not red blood cells) can pass across the capillary wall and into the tubule. The kidney then regulates how much water and which other substances can pass back into the blood in the capillary to keep the body in balance. Waste products, excess water, and excess electrolytes remain in the tubule and eventually leave the body as urine.

The kidney also release three regulatory chemicals—erythropoietin, renin, and calcitriol—that affect other functions in the body. Erythropoietin stimulates the bone marrow to produce new red blood cells. Renin helps regulate blood pressure, and calcitriol is a form of vitamin D and is important in maintaining bones and the level of calcium in the body.

Because the kidney has many functions, there are many types of kidney disease. Congenital kidney diseases are disorders that are present at birth. Polycystic kidney disease (PKD) is a rare disorder in which children inherit defective genes from both parents that cause cysts full of fluid to develop in the kidneys and replace the blood filtering units. As a result, the kidneys cannot adequately remove wastes from the body. There are two other types of PDK. One is inherited, but does not appear until adulthood, and the other develops as a result of long-term kidney damage. In total about half a million people in the United States have some form of PKD. Hereditary disease and birth defects are the most common causes of kidney disease in children up to age 14.

Acute kidney diseases are problems that develop suddenly. Many acute kidney diseases can be cured, but some may cause permanent damage. Common acute kidney diseases include kidney infection, hemolytic uremic syndrome, nephrotic syndrome in children, and damage caused by injury to the kidney or poisoning. Hemolytic uremic syndrome is a rare disease that usually affects children under age ten and is caused by eating food contaminated with bacteria. The bacteria release a poison that damages the kidney and causes acute kidney failure. Most children who develop this disease recover and their kidney function returns to normal.

Chronic kidney disease is disease that is slow to develop and usually does not show any symptoms until kidney damage is permanent. The National Kidney and Urologic Disease Information Clearinghouse, a federal agency, estimates that about 4.5% of people over age 20 have chronic kidney disease as indicated by tests that measure kidney function. The most common cause of chronic kidney disease in the United States is diabetes. It accounts for between 33% and 40% of all new cases of chronic kidney disease in the United States. In diabetes, the body cannot break down glucose (sugar). This extra glucose in the blood damages the nephrons so that they no longer filter blood effectively.

High or uncontrolled blood pressure (hypertension) is the second leading cause of chronic kidney disease. It accounts for between 27% and 30% of all new cases of chronic kidney disease. High blood pressure damages the capillaries in the nephron, so that they can no longer work with the tubules to filter the blood. Glomerulonephtitis is a term for several different chronic kidney diseases where damage to the nephrons causes protein or red blood cells pass into the urine. Kidney cancer is uncommon, accounting for only 2% of cancer cases.

Over-the-counter analgesics (pain medications) such as aspirin, acetaminophen (Tylenol), ibuprofen (Advil), naxopren sodium (Aleve), and similar medications that can be bought without a prescription may make kidney disease worse in individuals who already have kidney damage or cause kidney damage in healthy individuals who take these medications daily for several years. The chance of damage is increased when these pain medications are taken in combination with each other or with caffeine or codeine (Some painkilling tablets are a combination of pain medications and caffeine or codeine). Individuals who take these painkillers regularly or who have been told they have kidney damage should discuss the risk of these medications with their physician.

Chronic kidney disease can lead to end-stage renal disease (ESRD) in which the there is almost total failure of the kidneys. If renal function is reduced to only 25% of normal, serious illness results. When this drops to 10–15% of normal, death occurs unless the individual receives dialysis or a kidney transplant. In 2002, there were over 100,300 new cases of ESRD, 44% of which were caused by diabetes. Treatment of ESRD in the United States cost about $25.2 billion in 2002.

Causes of kidney disease are many and varied. Leading causes are diabetes, high blood pressure, inherited disease, and infection. Acute kidney disease is often marked by a lack of urination and increased fluid build up in the body. Chronic kidney disease is often called a “silent” killer, because no obvious symptoms develop until the kidneys are permanently damaged. The National Kidney Foundation estimated in 2005 that 20 million Americans had undetected moderate chronic kidney disease. Chronic kidney disease most often results from other diseases such as diabetes or hypertension.

Simple blood and urine tests can indicated kidney disease, but more extensive testing may be needed to determine the exact nature of the disease. A blood test that measures serum creatinine, a waste product, can give an indication how well the kidneys are working. Although normal levels of creatinine vary (an average range is 0.6–1.2 mg/dL), a higher than expected level in the blood may indicate kidney damage. A blood urea nitrogen (BUN) blood test measures waste products circulating in the blood. Normal levels range from 7%ndash;20 mg/dL. The less well the kidney is working, the higher the BUN.

A 24-hour urine collection test will accurately measure how much urine the kidneys are producing in a day. A urinalysis can determine if protein or red blood cells are leaking into the urine indicating abnormal kidney function. A creatinine clearance test compares the amount of creatinine in a 24-hour urine sample with the amount of creatinine in the blood to determine how much blood the kidneys are filtering each minute.

Based on the results of blood and urine tests, other tests such as a CT scan, MRI, or kidney biopsy may be ordered.

Most treatment for kidney disease involves treating the underlying cause of the disease, such as controlling high blood pressure or diabetes. Diuretic medication (“water pills”) may be given to help relieve fluid accumulation. Antibiotics are used to treat kidney infections. Other drugs may be given to treat specific kidney diseases.

Diet and lifestyle changes are an important part of controlling kidney disease. Obesity increases blood pressure, so losing weight can help limit kidney damage, as can stopping smoking. Reducing sodium (salt) in the diet also helps control blood pressure. In certain kinds of kidney disease, potassium is removed in abnormally large quantities by the kidneys and excreted in urine. Eating more foods such as bananas, dried beans and peas, nuts, and potatoes that are high in potassium or taking a potassium supplement pill help reverse this effect. When protein is found in the urine, some physicians recommend reducing the amount of protein (mainly found in meat) in the diet.

When kidneys fail completely in ESRD, there are only two alternatives: dialysis or kidney transplant. There are two types of dialysis. Peritoneal dialysis uses a membrane in the individual's abdomen to filter waste products. The most common kind of peritoneal dialysis is continuous ambulatory peritoneal dialysis (CAPD), in which the individual is hooked up to a bag of dialysis fluid that he carries with him, allowing continuous dialysis. The fluid is changed four times a day. In another form of peritoneal dialysis, the abdomen is filled with dialysis fluid. Wastes filter into the fluid for several hours often while the individual is asleep, the then the fluid is drained from the body. Peritoneal dialysis can be done at home without the need for a health care professional.

In hemodialysis, the individual must go to a dialysis center about three times a week. His blood is sent through a machine that filters out the waste and then returns the cleansed blood to his body. The process takes three to four hours and is done by a health care professional.

Kidney transplants can come from either a living donor or a deceased donor. Donors are matched with recipients based on blood type and must take drugs to prevent their immune system from rejecting the kidney after transplantation. The United Network for Organ Sharing (UNOS) coordinates matching donor kidneys with appropriate recipients. As of 2005, over 100 clinical trials were enrolling patients with various types of kidney disease. Information on current clinical trials can be found at <http://www.clinicaltrials.gov>.

Alternative treatments tend to focus on removing excess water from the body, but have limited effect in serious disease. Asparagus (Asparagus officinalis) birch tea(Betula species), goldenrod infusion(Solidago species), horsetail Equisetum arvense), and stinging nettle (Urtica dioica) all are used to stimulate urine production.

Many individuals recover normal kidney function after developing acute kidney disease, although in some cases, such as poisoning and injury, kidney damage may be permanent. Chronic kidney disease tends to get progressively worse as the individual ages. More than 15,000 kidney transplants are done each year, and there is a often long waiting list for donated kidneys. As of 2001 (last year for which statistics were available), 80.6% of individuals receiving a transplant from a deceased donor survived for at least 5 years, and 90.4% of individuals receiving a kidney donated from a living donor survived for at least 5 years.

Maintaining a healthy body weight, getting regular exercise, and not smoking all promote kidney health. Controlling underlying diseases such as diabetes and high blood pressure are important in preventing chronic kidney diseases.