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Basic Information About the Kidneys

We have two kidneys, each about the size of your fist. Each kidney contains approximately 1 million microscopic filtering and processing units called nephrons. Each nephron contains a tiny filter called a “glomerulus” (glo-MER-yoo-lus). Glomeruli are microscopic balls of capillaries, or blood vessels, which filter the blood that passes through them. The kidneys clean and process the blood; working together, the 2 million nephrons filter and process 3-4 liters of blood approximately every 5 minutes, 24 hours a day, 7 days a week.

This article addresses the following questions:


What do kidneys do?

The kidneys have several functions:

Remove potentially harmful waste products from the body. These include urea (a protein breakdown product), creatinine (a byproduct of normal muscle function), various types of acids, and a host of other substances that are toxic if allowed to accumulate.

Regulate the composition of the blood and some of our body fluids, ensuring that they contain the normal amounts of water, salt, potassium, phosphorus, acid, and many other components that are necessary for normal body functioning.

Produce substances called hormones that affect various body processes. One such hormone is erythropoietin (or “epo”), which stimulates the bone marrow to make oxygen-carrying red blood cells. Another is a special form of vitamin D, which helps to regulate the amounts of calcium and phosphorus in the body and is necessary for normal bone structure.


What are the signs and symptoms of kidney disease?

Some common symptoms and signs of kidney problems are:

Protein or blood in the urine: this may be discovered on a routine urinalysis that is done as part of an annual physical exam, a preoperative evaluation, or a school, employment, or insurance physical. In many (but not all) types of kidney disease, kidney damage results in leakiness of the tiny glomerular filters. These leaks allow protein and red blood cells, which are normally kept in the bloodstream, to spill into the urine, where they can be detected on urinalysis.

Abnormal blood tests of kidney function: a routine panel of blood tests (which is often done either as part of an annual physical, or in preparation for surgery) includes a BUN (blood urea nitrogen) and a creatinine. These two substances are normal metabolic waste products that are excreted by the kidneys. In kidney disease, their excretion is decreased, so that the amounts in the blood rise and can be detected on routine blood tests. This is often the very first indication of kidney disease.

High blood pressure: although this is a very common finding, particularly as people become older, it can also be the first sign of kidney disease. High blood pressure can also itself be a cause of kidney disease, so people who have high blood pressure for a number of years, particularly if it is difficult to control with medications, need to be repeatedly screened for evidence of kidney damage.

Edema, or swelling, especially of the feet, lower legs, and sometimes the face and hands: this is a sign of fluid buildup that can occur in several conditions, most commonly in heart failure. However, it often is the first sign of protein leakage from the kidneys.

Symptoms of a urinary tract infection: burning on urination, frequency (the sensation of needing to urinate very frequently), urgency (the sensation of needing to urinate urgently), bloody or cloudy urine. Although urinary tract infections do not usually cause permanent or progressive kidney damage if promptly treated, they can certainly cause a patient to seek medical attention.

Symptoms of advanced kidney failure: loss of appetite, nausea vomiting, fatigue, sleepiness, itching, twitching, and a metallic taste in the mouth. These are signs of far advanced chronic kidney disease (CKD). They often indicate that the person is accumulating dangerous amounts of waste products because the kidneys are not working to excrete them. Patients with these symptoms are often in need of dialysis (artificial kidney treatments).


What is chronic kidney disease, and how common is it?

Chronic kidney disease (CKD) is a condition in which the kidneys gradually lose their function over a period of months to years. CKD is not a single disease, but rather a condition caused by any of a number of diseases (such as diabetes or hypertension; see below under “common causes”). CKD is divided into stages, depending on the level of function. The level of kidney function is determined by the glomerular filtration rate (GFR). GFR is the rate at which the 2 million nephrons filter blood. For a normal-sized person, a normal GFR is greater than 90 ml/minute.

The stages of kidney disease are defined as follows:

Stage 1: Normal GFR, but with signs of kidney disease based on imaging studies and/or the presence of protein in the urine. Approximately 5.9 million Americans are included in this stage.

Stage 2: GFR 60-90 ml/min: 5.7 million.

Stage 3: GFR 30-60 ml/min: 7.6 million.

Stage 4: GFR 15-30 ml/min: 400,000.

Stage 5: GFR <15 ml/min: 400,000. At this stage, people are generally considered to be in need of (or very close to being in need of) dialysis or kidney transplantation in order to sustain life.


What are the most common causes of chronic kidney disease?

The two most common causes of CKD in the United States are diabetes and hypertension. Together, these account for approximately 70% of patients who are on dialysis in the US. Many dialysis patients have both conditions, especially if blood sugar is not well-controlled. Listed below are brief descriptions of some of the most common causes of CKD:

Diabetes: Ether type I (juvenile-onset) or type II (adult-onset) diabetes may result in kidney damage after a number of years, especially if blood sugar is not well-controlled. Usually by the time a person develops diabetic kidney disease, they have had diabetes for at least 10 years, and the diabetes has often affected their eyes, blood vessels, and nerves (especially of the feet).

Hypertension (high blood pressure): This is a common disorder and one to which African Americans are especially prone. It may be a symptom of another type of kidney disease, or it may occur on its own, especially if high blood pressure tends to run in a family. In either case, high blood pressure can cause severe, permanent kidney damage.

Chronic glomerular disease: This type of disease attacks the microscopic kidney filters (glomeruli) of the kidney, destroying and/or scarring them, so that they are not able to perform their normal function of filtering. There are many diseases that can result in damage to the glomeruli. These include: systemic lupus, cancer, vasculitis, hepatitis B, and hepatitis C. Other types of glomerular disease are IgA nephropathy (which is especially common in southeast Asia) and focal segmental glomerulosclerosis (which is especially common among young and middle-aged African Americans).

Atherosclerosis (hardening of the arteries): Deposits of cholesterol and other lipids within blood vessels can impair blood flow to parts of the kidney and result in damage, just as atherosclerosis in the coronary arteries can lead to heart attacks, and in the brain arteries can lead to strokes.

Polycystic kidney disease (PKD): This is the most common genetic kidney disease. It is inherited as an autosomal dominant disorder, meaning that either a mother or a father can pass it on, and about half of their offspring will inherit the gene that causes the disease. Babies with PKD are born with normal kidneys, but over the next 40-50 years, their kidneys are gradually replaced with fluid-filled cysts, causing them to lose their function.

Urinary tract obstruction: Anything that blocks the flow of urine from the kidneys down the ureters, into the bladder, and out of the bladder through the urethra can cause urine to back up within the kidneys and cause temporary and/or permanent damage. A common cause of obstruction is an enlarged prostate gland, which prevents urine from flowing freely out of the bladder. Other causes of obstruction include kidney stones, which can block one or both ureters, and tumors.


Learn more about the kidneys:

Kidney failure information:


For more information:

Go to the Kidney Diseases health topic.