How does a kidney dialysis machine work?

A dialysis machine mimics some of the functions of a human kidney. A primary job of kidneys is to remove urea and certain salts from blood so they can exit the body in urine.

A dialysis machine removes blood from the patient and runs it through tubes made of a semi-porous membrane. On the outside of the tubes is a sterile solution made of water, sugars and other components. Certain substances including red blood cells, white blood cells and other important blood components cannot fit through the pores in the membranes because those components are too large. The urea and salt, however, flow through membranes easily and are removed through the sterile solution.

During dialysis, a patient's blood is passed through an external filter (a hemodialyzer) that performs some of the functions of the kidney: removing wastes, excess fluids, and salts from the blood and correcting levels of specific chemicals. Patients on dialysis have their blood filtered several times a week; to make this process easier, doctors create a window to the bloodstream that allows blood to be removed and returned quickly, efficiently, and safely. The window is usually in the arm and completely beneath the skin.

Doctors use one of two procedures to create a dialysis access, a fistula or a graft. In both cases, an artery is connected to a vein to increase the blood flow through the vein. Over time the vein enlarges, carrying more blood and making it easier for dialysis technicians and nurses to gain access to the bloodstream.

To create a fistula, the surgeon joins an artery to a large, nearby vein. Over the following weeks and months, the vein increases in size. If the vein is blocked or too small to use, doctors will create a graft, an artery-to-vein connection using a synthetic tube instead of the patient's own blood vessels. Because fistulas are constructed using the patient's own blood vessels, they are generally more durable and resistant to infection than grafts.

Continue Learning about Dialysis

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.