Kidney School™—a program of Medical Education Institute, Inc.

Module 10—Getting Adequate Dialysis

Your Dialysis Prescription

Just like a drug, dialysis is given with a prescription from a doctor. The prescription is designed to make sure you get enough treatment to feel well and keep doing the things you enjoy. To decide on the right prescription for you, your doctor starts with a careful look at:

  • Your weight and height
  • Your lab test results
  • Your medical condition(s)
  • Your residual kidney function (how much filtering your kidneys are still doing)

A prescription will be custom made for you, based on your unique needs.

To understand your prescription, first you need to know a little bit about how dialysis cleans the blood. Wastes in the blood are mainly removed through a natural process called diffusion.

Diffusion happens when fluids are on both sides of a semipermeable membrane—like a tea bag. Semipermeable means the membrane has tiny holes that let tiny particles through, like tea, but not larger ones, like tea leaves. Blood cells and protein are too big to pass through the membrane, so these stay in your body during treatment. In dialysis, the membrane can be one of two things:

  1. In peritoneal dialysis (PD), the membrane is the peritoneum, which lines the abdomen.
  2. In HD, the membrane is thousands of hollow, plastic fibers called a dialyzer.

On one side of the membrane is your blood. On the other side of the membrane is a fluid called dialysate, or "bath." Wastes from your blood diffuse through the membrane and into the bath. Once used, the bath is thrown away.

Mug of green tea

How does dialysis remove only the wastes? The secret is in the bath.

In nature, diffusion goes on until fluids on both sides of a membrane have the same concentration. For example, until your tea is as strong as it can get. This takes time.

The process can be speeded up by creating a gradient—making the fluid on one side of the membrane stronger (more concentrated) than the other side.

Your blood has high levels of wastes. So, your doctor will prescribe a bath with no wastes. This gradient forces wastes from your blood to move across the membrane and into the bath—where they can be thrown away.

How Does a Hollow Fiber Dialyzer Work?


A dialyzer for HD is a collection of thousands of hollow plastic fibers, each thinner than a hair. The fibers are gathered into a clear plastic cylinder. At each end of the tube, the fibers are held in place by a clay-like potting material. Blood enters the dialyzer through the red port on one end, passes through the insides of the hollow fibers, and leaves through the blue port on the other end. Dialysate enters through the blue port on the side of the dialyzer, flows around the outside of the hollow fibers, and then leaves through the red port on the side. Wastes from the blood are carried away in the used dialysate.

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