Normally, the human body has two kidneys, oval-shaped organs located at the back on the right and left side and partly covered by the lower ribs. In an average adult these organs are approximately four to six inches long and weigh a little over five ounces each.   Each kidney is comprised of about one million units called nephrons, with each nephron consisting of a tube-like structure called a tubule. The closed end of the tubule forms a cup-shaped structure called a glomerular capsule, surrounding a network of tiny blood vessels called the glomerulus, which filters the blood. Filtrate drains into the tubule where its concentration is altered to form urine which flows through ureters to the bladder and is stored there until being eliminated from the body through the urethra.

The kidneys daily produce a total of about 1.5 to 2.5 quarts of urine consisting of waste products and excess water. Kidneys adjust the body's balance of various chemicals (sodium, potassium, calcium, phosphorous and others) and monitor the blood's acidity. Certain hormones are also produced in the kidneys. These hormones help regulate blood pressure, stimulate red blood cell production and promote strong bones. 

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Diseases of the kidney and urinary tract are a major health concern in the United States, affecting more than 20 million Americans. Over 90,000 of these people will die this year as a result of these diseases.

Impaired kidney function can occur suddenly or take place gradually over months or years. The condition may be temporary and reversible, or permanent. It can be minor or life-threatening.

In addition to damage as a result of physical trauma as with an accident, common causes of failure include diabetes, uncontrolled hypertension (high blood pressure) and an inflammation of the kidneys called glomerulonephritis. Other causes of kidney failure are kidney stones, reduced blood flow to the kidneys, diseases such as systemic lupus erythematosus and certain drugs. Inherited kidney diseases, most commonly polycystic kidney disease, can also cause kidney failure.

Although a single kidney is generally capable of providing adequate function, when both kidneys fail, the body suffers serious consequences. There is an accumulation of certain toxic substances (urea and creatinine) and an increase in the volume of water in the body. This excess water results in swelling of tissues and high blood pressure and prevents normal activity of other organs in the body such as the heart and lungs. In addition, normal acidity and chemical balance cannot be maintained without adequate kidney function. For example, if the body cannot rid itself of excess phosphate, calcium levels will drop resulting in bone disease.

If total kidney function drops below 10 percent of normal capacity and the impairment is irreversible and permanent, the condition is known as end-stage renal disease (ESRD).

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A kidney transplant often will give an ESRD patient the most nearly normal life. This is a surgical procedure where a single healthy kidney is placed within the patient's body. Best results are seen from a donor who is a living relative whose tissues closely match those of the recipient. However, transplants from cadaver donors (individuals who have died) also are frequently very successful. There are currently nearly 30,000 Americans waiting for kidney transplants; the wait could be anywhere from a few weeks to two years or more. Unfortunately, because of a shortage of suitable donors, less than half that number will receive transplants.

Even with a successful transplant operation, 10% of transplant patients experience rejection of the donated kidney within the first year and there is a 25% rejection rate in three years. To prevent rejection, strong medication will be required for the rest of the patient's life - medication which often has serious side effects.

More than 50% of patients with end-stage kidney disease do not meet the requirements for a successful kidney transplant. This includes most people in the elderly population, those with multiple illnesses, heart disease, history of chronic infection, cancer or decreased immune function. For these patients, kidney function may be replaced by one of two methods of treatment - either peritoneal dialysis or hemodialysis. 

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Both peritoneal dialysis and hemodialysis are based on the same principles. The first principle, osmosis, is the tendency of water to move from a low concentration of particulates to a higher concentration. And the second is diffusion - the tendency of particles to distribute themselves evenly throughout a solution. Together, osmosis and diffusion comprise dialysis.

Blood cells are too large to pass across the semipermeable membrane used in dialysis. However water, naturally occurring body chemicals, and certain impurities will be transported, depending on the composition of the solution on the side of the membrane opposite the blood. This solution is known as dialysate.

For some ESRD patients, the treatment of choice is peritoneal dialysis, which uses as the dialyzing membrane the lining of the person's abdomen. This lining is called the peritoneal membrane (thus the name peritoneal dialysis). In peritoneal dialysis, dialysate enters the body through a special tube inserted into the abdomen. Excess water, wastes and chemicals pass from very small blood vessels, across the peritoneal membrane and into the dialysate. After several hours, the dialysate is drained from the abdomen. The patient's abdominal cavity is then filled with fresh dialysate and the cleaning process is repeated. This process may be repeated intermittently or continuously.

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The treatment for a majority of dialysis patients is hemodialysis (also called kidney dialysis). In this procedure, two needles are inserted into a blood vessel (usually in the patient's arm). Each needle is attached to a thin length of tubing. One tube carries blood to a machine containing a dialyzer, which is a unit comprised of many very fine hollow fibers. These fibers are made of a semi-permeable membrane. As blood flows through the fibers, dialysate flows around them, removing impurities and excess water and adjusting the chemical balance of the blood. After being cleansed and adjusted, the blood returns to the patient's body through the second tube.

Less than five percent of a patient's blood is outside the body at any time. Treatment lasts three to six hours and is usually performed three times a week. Because patients are connected to a machine, movement is limited during treatment, however they are usually positioned in a comfortable chair and may read, write, watch television or sleep.

Hemodialysis can be performed at home, however most patients are dialyzed in a hospital or in one of several thousand dialysis clinics currently in operation in the United States.

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