Dialysis is a treatment procedure used for patients in the end stage of kidney failure. This process cleans the blood and removes waste products and excess fluids from the body just as normal kidneys do. In case of acute kidney failure, dialysis is required for a short period, but end stage renal disease requires the patient to undergo dialysis on a regular basis for a longer time. The only other treatment for kidney failure is kidney transplant.
Dialysis is of 2 types: hemodialysis and peritoneal dialysis. In hemodialysis, blood is passed through a dialyser, also known as an artificial kidney, whereas in peritoneal dialysis, the blood is cleansed within the body.
Before hemodialysis is begun, the patient must undergo a minor vascular surgery in the arm, which creates an arterio-venous fistula (AV Fistula). The AV Fistula creates an access for the needles needed to connect the blood circulation to the dialysis machine.
The dialysis machine works by filtering a patient’s blood and removing waste products and excess fluids. The patient’s blood is passed through the AV Fistula into the dialysis machine through plastic tubing. The dialysis machine contains a large canister called the dialyzer, containing thousands of small fibers that form a semi-permeable membrane that filters out waste products. Fresh dialysis solution is pumped around the filters while the patient’s blood runs through it, separated by the thin membrane.
The dialyzer works on the principle of diffusion, therefore the waste products and excess fluids pass through the fibers in the membrane into the dialysis solution. The dialysis solution, also called the dialysate, helps to carry away the waste products and excess fluid from the blood and is then discarded. The newly filtered blood is passed through a bubble trap to ensure there are no air bubbles in the blood, and the blood is then returned to the patient’s body through the vein of the AV Fistula.
As aforementioned, the dialysis machine works on the principle of diffusion. The dialyzing fluid contains less solutes than the blood. Therefore, the excess fluid, waste products and electrolytes will pass from the blood to the dialysate due to the concentration gradient. The blood and dialyzing fluid flow constantly through the dialyzer, and increasing the blood flow and the flow of the dialyzing fluid will optimize the diffusion process even further.
The dialysate has the same electrolytes and non-electrolytes as blood, but in a lower concentration than the blood of a patient with kidney disease. This blood is called the uremic blood. However, the dialysate contains no phosphate, urea, urate, sulfate or creatinine to create a lower gradient of these electrolytes in order for them to pass from the blood into dialysate, and cleanse the blood. This, however, is not the case for electrolytes and non-electrolytes such as magnesium, bicarbonate, lactate and glucose which may be at the same levels as uremic blood or even higher since the intention is not to remove it.
The dialysis machine contains less than 500 milliliters of blood at any given time, and the blood flow back into the patient’s body may be continuous or intermittent. A small amount of heparin is added to the dialysis machine to prevent the blood from clotting and this also helps the blood flow.
Modern day dialysis machines are very efficient, and remove about 100ml to 200ml of urea from the blood per minute. The healthy kidney however, removes 70ml of urea per minute, but a healthy kidney works continuously while dialysis is performed only 2 to 3 times a week. Furthermore, the regulation of water, electrolyte and non-electrolyte levels with a dialysis machine is not able to match the kidney’s ability as this is in a constant state of flux to maintain the optimum levels and therefore homeostasis.