You pour the boiling water over your teabag. Sure enough, the water begins to darken, and the aroma of fresh-brewed tea fills the room. You know how it works: the essence of the tea moves through the teabag into the water. Eventually the water inside and outside the teabag will each have the same concentration of tea.
Dialysis—both hemodialysis (HD) and peritoneal dialysis (PD)—is based on the same simple principle as making tea.
Both types of dialysis use a semi-permeable membrane, like a teabag, to filter toxins out of the body. For HD, multiple membranes fill the inside of a long cylinder called a dialyzer, which plays the most important role in the dialysis treatment. The tubes, gauges, and pumps on the machine serve only to safely channel the blood to and from the dialyzer.
In PD, the membrane is a person’s peritoneum—the thin membrane that encases the organs in the abdomen. In both PD and HD, the blood and the dialysate are separated by the membrane. The two fluids never mix.
Dialysate consists of pure water mixed with minerals and electrolytes—called solutes. It contains lower amounts of solutes than are normally found in the body. This causes waste products in the blood, such as urea and creatinine, to move through the membrane into the dialysate, just as tea is drawn into water. Over time, the solutes in both the blood and the dialysate come into balance. This process is called diffusion.
In HD, the blood and the dialysate move through the dialyzer in opposite directions, forcing the extra water in the blood to be pushed through the semi-permeable membrane into the dialysate. This process is called ultrafiltration. Removing excess fluid too quickly can cause a drop in blood pressure. Keeping your fluid intake under control between treatments can help minimize this common occurrence. A drop in blood pressure is less likely with PD because excess fluid moves into the PD dialysate more gradually by osmosis.
It’s important to know your dialysis prescription. For HD, this includes the specific amounts of potassium and bicarbonate solution that are added to your dialysate, the size of your dialyzer, and the frequency and hours of your treatments. Your doctor prescribes these on the basis of your laboratory values and adjusts them as necessary. Patients on PD are usually more aware of their dialysis prescription because they learn to do all aspects of their treatment themselves.
Normal kidneys perform diffusion and ultrafiltration continuously—24 hours a day, 7 days a week. Dialysis can never fully replicate the work of a normal kidney. Though dialysate removes some impurities from the blood, it’s not a perfect system. So what are our options?
Obviously, if we increase the number of hours we dialyze, we will more effectively replicate a normal kidney. Many people choose PD for this reason, since the dialysate inside the peritoneum works around the clock to remove wastes. The saturated dialysate is changed often so that dialysis takes place continuously.
Others have chosen home HD. This allows them to do longer or more frequent treatments than are possible in-center. Another option is to do home HD overnight during sleep (called nocturnal HD). Some dialysis centers now offer this option. You can take control of your health by choosing the treatment option that’s best for you.
Of course, a transplant will give you the best kidney function! Getting more dialysis will help your body remain strong in preparation for a new kidney.
No matter which type of dialysis you choose, remember the principle behind it. It’s as simple as making a cup of tea.
About the Author
Shari Gilford has lived with kidney disease for over 30 years, having experienced several dialysis modalities–CAPD, CCPD, in-center hemodialysis, and nocturnal home hemodialysis. She received training as a dialysis technician, then performed her own hemodialysis treatments both in-center and at home. She received her third transplant in March of 2006. In 2010, she and her husband David traveled to north India to live at an orphanage for a year, teaching English and life skills to 32 children. She believes dialysis or a transplant should not prevent anyone from fulfilling their dreams if we have the courage to take a risk.
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