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.

Glomerulonephritis, another common kidney disease, is characterized by inflammation of some of the kidney's glomeruli. This condition may occur when the body’s immune system is impaired. Antibodies and other substances form large particles in the bloodstream that become trapped in the glomeruli. This causes inflammation and prevents the glomeruli from working properly. Symptoms may include blood in the urine, swelling of body tissues, and the presence of protein in the urine, as determined by laboratory tests. Glomerulonephritis often clears up without treatment. When treatment is necessary, it may include a special diet, immunosuppressant drugs, or plasmapheresis, a procedure that removes the portion of the blood that contains antibodies.

Other common kidney disorders include kidney stones, which are small, crystallized substances, such as calcium, that form in the kidney or other parts of the urinary tract. Smaller kidney stones can pass out of the body on their own, although this can be painful. Larger stones may require surgery, or they may be broken into smaller pieces with sound waves in a procedure called ultrasonic lithotripsy.

The kidneys may be harmed whenever injury or disease affects the rest of the body. For example, diabetes mellitus (a disease caused by a malfunctioning pancreas that produces little or no insulin) can result in impaired blood flow through the kidneys. The bacteria that cause tuberculosis can travel from the lungs and infect the kidneys. Injured muscles can release large amounts of protein into the bloodstream, blocking the nephrons. Drug use, including long-term use of some prescription medications as well as illegal drugs, can also cause kidney damage. Certain birth defects may cause the kidneys to have abnormal shapes or to function improperly.

Treatment of severe kidney disease may include kidney dialysis, a procedure in which blood is circulated through a machine that removes wastes and excess fluid from the bloodstream. Some patients use dialysis for a short time, while their kidneys recover from injury or disease. Others must use dialysis for their entire lives or until they undergo a kidney transplant. Kidney transplants are the most common of all transplant operations and have excellent success rates. Unfortunately, there are not enough kidneys available for the people who need them. More than 38,000 people in the United States alone wait for a kidney transplant each year, and fewer than 12,000 of them receive this life-sustaining organ.