Dialysis is a medical procedure that serves as a substitute for failing kidneys, performing their functions to maintain a person’s health. This treatment filters waste products and excess fluid from the blood, a process that is otherwise managed by healthy kidneys. It is a life-sustaining therapy for individuals experiencing kidney failure.
Kidney Failure and the Need for Dialysis
Healthy kidneys perform several functions, including filtering waste products and excess fluids from the blood, maintaining a balance of electrolytes like potassium and sodium, and producing hormones that help regulate blood pressure and red blood cell production. When kidneys fail, a condition known as end-stage renal disease (ESRD), they lose their ability to filter these substances effectively, often when only 10% to 15% of their normal function remains. This leads to a buildup of waste products and fluids in the body, causing symptoms such as nausea, vomiting, fatigue, swelling, and changes in urination frequency. Dialysis then becomes a necessary intervention to remove these substances and excess fluid, preventing severe complications like heart failure, abnormal brain function, or dangerous electrolyte imbalances.
How Dialysis Mimics Kidney Function
Dialysis operates on scientific principles to purify the blood. The core mechanisms involve diffusion, osmosis, and ultrafiltration, which work together to remove waste and excess fluid. Diffusion is the movement of solutes from an area of higher concentration to an area of lower concentration across a semipermeable membrane. In dialysis, waste products in the patient’s blood move into a special fluid called dialysate.
Osmosis involves the movement of water across a semipermeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). This principle helps remove excess fluid from the blood into the dialysate. Ultrafiltration, on the other hand, is the removal of excess fluid through the membrane by applying additional pressure, either positive pressure on the blood side or negative pressure on the dialysate side. These processes are carried out within a “dialyzer,” which contains the semipermeable membrane that separates the blood from the dialysate, allowing the exchange of substances.
Hemodialysis
Hemodialysis is a method of dialysis that involves circulating the patient’s blood outside the body through an artificial kidney. To achieve this, vascular access is surgically created, typically in the arm. The most common and preferred type of access is an arteriovenous (AV) fistula, which connects an artery directly to a vein, causing the vein to enlarge and strengthen over several months to withstand repeated needle insertions. If a fistula is not feasible, an AV graft, using a synthetic tube to connect an artery and vein, or a temporary venous catheter in the neck, chest, or leg may be used.
During a hemodialysis session, blood is drawn from the vascular access through one needle, pumped through the dialyzer where waste products and excess fluid are filtered out, and then returned to the body through another needle. The dialyzer contains a semipermeable membrane that separates the blood from the dialysate. Hemodialysis treatments last about four hours and are performed three times a week, often at a dialysis center.
Peritoneal Dialysis
Peritoneal dialysis utilizes the patient’s own peritoneal membrane, the lining of the abdomen, as a filter for waste products and excess fluid. A catheter is surgically inserted into the abdominal cavity, providing access for the dialysis fluid. Through this catheter, a sterile dialysate solution is introduced into the peritoneal cavity.
The dialysate remains in the abdominal cavity for a prescribed “dwell time,” typically several hours, during which waste products and excess water from the blood vessels within the peritoneum move across the membrane into the dialysate, primarily through osmosis driven by glucose in the dialysate. After the dwell time, the spent dialysate is drained from the abdomen through the catheter into a collection bag. This entire process of filling, dwelling, and draining is known as an “exchange.”
There are two main types of peritoneal dialysis: Continuous Ambulatory Peritoneal Dialysis (CAPD) and Automated Peritoneal Dialysis (APD). With CAPD, exchanges are performed manually, usually three to five times daily, each lasting about 30 minutes. APD uses an automated machine called a “cycler” to perform multiple exchanges typically overnight while the patient sleeps.