How a kidney dialysis machine works

Posted on Tue, Apr. 21, 2009

How a kidney dialysis machine works

By Marshall Brain

Kidney failure is an extremely common condition in the United States, with about 100,000 people experiencing it each year. And the number is growing. Kidney failure is one of the many potential side effects of diabetes, and the number of adults diagnosed with diabetes is growing as the population becomes more obese.

Once the kidneys fail, it creates a major problem. The purpose of the kidneys is to filter toxins out of the blood. When the filter fails, the toxins rapidly accumulate. In just a couple of days the toxins reach fatal levels, and the patient dies.

If your kidneys fail, one solution available today is a large piece of equipment known as a kidney dialysis machine. It is also called an artificial kidney. A dialysis machine takes blood from the body, cleans it and then sends it back to the body, usually through needles/tubes placed in the arm. A patient on kidney dialysis would normally get three treatments per week, and it typically takes several hours to clean the blood.

What goes on inside the machine? Its job is to replicate the function of the kidneys. One of the important jobs of your kidneys, when they are working correctly, is to remove urea from the blood. That is why we call the output of the kidneys urine. The urea gives your body a way to get rid of nitrogen waste, which it needs to do when it converts protein to energy. While the kidneys are filtering out the urea, they are also filtering excess salts, various drugs flowing in the bloodstream, excess vitamins, etc. You may have noticed that your urine changes color after taking certain vitamins. It’s because the kidneys filter the excess vitamins out.

To replicate this activity, the dialysis machine needs a way to do the filtering while leaving all the red and white blood cells, along with the platelets, intact. Blood is a mixture of cells and platelets suspended in plasma. Plasma is mostly water and salt, and it is in the plasma that the harmful toxins accumulate.

By looking at the earliest dialysis machines, you can understand the purification process. The first machines, created by Willem J. Kolff, pumped blood through a sausage casing suspended in a large tub of water. The water had enough salt and other chemicals in it to act like very clean plasma. The sausage casing had tiny pores, small enough to keep the cells and platelets in but large enough to let the plasma flow in and out of the casing. The toxins in the plasma would be greatly diluted in the large tub of artificial plasma, to the point where the blood returning to the body was clean.

Modern dialysis machines are far more sophisticated, but they work on the same principle. The machine creates a chemically balanced fluid called the dialysate that looks just like plasma. The dialysate absorbs the toxins as the blood passes through the machine. The machine also removes excess water from the blood just like the kidneys would.

Given that a patient needs to be hooked up to a dialysis machine every other day for treatment, and given that the entire quantity of the patient’s blood will flow through the machine a number of times during treatment, how is the body handle all of those large needle sticks? Doctors normally create a special vein in the arm to handle the needles. This vein could either be a natural fistula formed using the patient’s own blood vessels, or it could be a plastic tube called a graft. The nurse uses this special blood vessel to get access to your blood stream.

As you might imagine, being on dialysis has some side effects. Your kidneys naturally keep chemicals in the blood at constant levels. On dialysis, there are peaks and valleys. You have to think about water intake, salt intake, vitamins, medicines and the food you eat much more closely.

What is the moral of this story? Probably this: if your kidneys are currently healthy, you want to do everything in your power to avoid kidney failure. If that means losing 50 pounds to lessen the chance of obesity-related diabetes, now might be a good time to start on the diet. Because dialysis is not a lot of fun.

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