how does dialysis work

Chronic kidney failure, sometimes referred to as chronic renal failure, describes the loss of kidney function over a period of time. There are five stages, each corresponding with a decreasing level of kidney function, with the last stage called end-stage renal disease. In order to make up for the compromised renal system due to kidney damage, dialysis treatment is often required for chronic kidney failure patients. So, how does dialysis work to manage the function of healthy kidneys?

What do the kidneys do?

The renal system includes the kidneys, which function to remove waste substances and extra fluid, salts, vitamins, and minerals from the blood. The kidneys also help control blood pressure by releasing hormones involved in its regulation. The kidneys filter blood through their working units called nephrons – of which each kidney has about one million. From blood vessels, the blood flows into the nephrons and is filtered to allow only certain small molecules to pass into the tubule of the nephron. This blocks essential components in the blood from being potentially filtered out. In the nephron’s tubule, certain substances will be reabsorbed into the blood vessel if the body needs them, while waste and extra fluid and substances form urine to be excreted. Without the filtering capacity of the kidneys, excess substances and waste stay in the blood at unhealthy levels, which can lead to the impairment of other organs’ functions.

What is dialysis?

Dialysis treatment is the preferred method to remove the accumulated waste substances and fluid that a chronic kidney failure patient’s non-functional kidneys cannot. While it does perform the function of kidneys to some extent, it cannot completely match the performance of healthy kidneys. In essence, it only helps manage the damage of the compromised renal system. In general, dialysis works to move accumulated substances in the blood across a semi-permeable membrane into a dialysis fluid which contains a lower concentration of substances – a process called diffusion. This allows the accumulated waste in the blood to be filtered out of blood vessels into the dialysis fluid which is removed from the body. There are two types of dialysis used to filter patients’ blood: hemodialysis and peritoneal dialysis.

How does hemodialysis work?

Hemodialysis treatment involves a dialysis machine which pumps a patient’s blood to and from a hollow tube called a dialyzer – an artificial kidney – that works to externally filter a patient’s blood to remove accumulated waste and excess fluid. Prior to commencing hemodialysis, minor surgery is performed to create a vascular access, where two needles will be inserted, one for pumping blood to the dialyzer and one for returning filtered blood back to the blood vessel. One vascular access option, the fistula – an artery surgically connected to a vein – widens the vein allowing for easier needle insertion and quicker blood flow. The fistula is considered the best vascular access option for most patients. Grafts entail surgically placing a tube to connect an artery and vein. If neither the fistula nor graft is possible, a venous catheter can be surgically inserted, typically into a vein in the neck, however, this is considered temporary vascular access. After recovering from the minor surgery to create vascular access, dialysis treatments can start.

Dialysis begins with a needle-inserted tube drawing blood from the patient’s arm which is pumped into the dialyzer. The dialyzer has two sections – one for blood and one for dialysis solution – separated by a semi-permeable membrane. The dialysis machine forces dialysis solution in the opposite direction of the blood’s flow which pulls accumulated waste from the blood across the semi-permeable membrane into the dialysis solution. In order to remove the excess fluid in the patient’s blood, the dialyzer maintains a higher pressure in the blood section which forces the extra fluid across the membrane into the dialysis solution section. The dialysis machine works to discard the solution containing the filtered out waste and fluid through a tube attached to the dialyzer. The filtered blood returns to the patient’s body through a separate tube that is inserted via a needle in the patient’s arm. Hemodialysis is done in dialysis centers multiple times a week for several hours each visit.

It is possible to do home hemodialysis, which requires the patient and a partner to complete hemodialysis training. The dialysis machine and a short-term supply are provided to the patient to allow them to complete their treatments in their own homes, increasing the flexibility around their hemodialysis schedule.

How does peritoneal dialysis work?

Peritoneal dialysis uses a different mechanism of action than hemodialysis. Rather than a dialyzer acting as an external filter, a catheter is surgically inserted to allow for the use of the natural peritoneal membrane – located in the abdomen – as the body’s filter. Attached to the catheter, a fluid bag fills the peritoneal cavity with dialysis solution which forces waste to diffuse from the blood into the peritoneal fluid across the membrane. The waste-containing peritoneal fluid is then moved into a separate waste bag through the catheter which is then discarded. Minor surgery or laparoscopic procedure is required to insert the catheter into the peritoneal cavity, however, once inserted, peritoneal dialysis can provide patients with more autonomy and flexibility than hemodialysis. The peritoneal dialysis catheter is typically inserted slightly below the umbilicus – the belly button. There are a variety of potential exit locations for the catheter, most of which are in the abdomen or the chest. After successful insertion of the catheter, patients are trained by dialysis center health practitioners on how to use their catheter for dialysis at home.

It is possible for peritoneal dialysis to be automated by attaching the catheter to a machine which fills the peritoneal cavity with the dialysis solution and then drains it, rather than manually hooking up the fluid and waste bags to one’s catheter.

Associated complications of dialysis

Moderate complications associated with dialysis may be experienced such as hypotension and muscle cramps likely due to the high volume of fluid being drained from the body. In terms of more severe complications, dialysis treatment is thought to increase the body’s production of free radicals – highly reactive molecules. A body can experience oxidative stress when there are more free radicals than antioxidants, which stabilize free radicals, resulting in damage to a variety of tissue and cells. Another risk of the various dialysis treatments is infection at the vascular access.

References:

Buffington, M., Sequeira, A., Sachdeva, B., & Abreo, K. (2012). Peritoneal dialysis catheter placement techniques. The Open Urology & Nephrology Journal, 5(1), 4-11.

Dalal, R., Bruss, Z. S., & Sehdev, J. S. (2019). Physiology, renal blood flow and filtration. In StatPearls [Internet]. StatPearls Publishing.

Ogobuiro, I., & Tuma, F. (2019). Physiology, renal. In StatPearls [Internet]. StatPearls Publishing.

National Institute of Diabetes and Digestive and Kidney Diseases. (2018). Hemodialysis. Retrieved from https://www.niddk.nih.gov/health-information/kidney-disease/kidney-failure/hemodialysis

Punj, S., Enaam, A., Marquez, A., Atkinson, A. J., Jr, & Batlle, D. (2020). A survey on dialysis-related muscle cramping and a hypothesis of angiotensin II on its pathophysiology. Kidney International Reports5(6), 924–926. https://doi.org/10.1016/j.ekir.2020.03.003

Sachdeva, B., Zulfiqar, H., & Aeddula, N. R. (2019). Peritoneal dialysis. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK532979/

Vadakedath, S., & Kandi, V. (2017). Dialysis: A review of the mechanisms underlying complications in the management of chronic renal failure. Cureus9(8), e1603. https://doi.org/10.7759/cureus.1603

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