The kidneys maintain the body’s internal balance by filtering the blood. They regulate fluids and electrolytes, such as sodium and potassium, which is necessary for cell function. They also excrete metabolic waste products like urea and creatinine, and maintain the body’s acid-base balance.
When chronic kidney disease progresses to its final stage, known as end-stage renal disease (ESRD) or kidney failure, the kidneys can no longer perform these functions adequately. This failure is defined by a glomerular filtration rate (GFR) typically falling below 15 mL/min/1.73 m², leading to the accumulation of toxins, fluid overload, and severe electrolyte imbalances. At this point, artificial maintenance of kidney function, primarily through dialysis or transplantation, becomes necessary for survival.
Hemodialysis: The Filtering Process
Hemodialysis (HD) is an extracorporeal treatment that cleans the blood outside the body using a specialized machine. The core component is the dialyzer, which contains a selectively permeable membrane. The patient’s blood flows through tiny capillaries inside the dialyzer, while a cleansing fluid called dialysate flows on the opposite side.
The process relies on two main principles: diffusion and ultrafiltration. Diffusion is the movement of dissolved particles, or solutes, from an area of high concentration in the blood to an area of low concentration in the dialysate. This concentration gradient allows waste products like urea to move across the semi-permeable membrane for removal.
Ultrafiltration is the mechanism for removing excess fluid, achieved by manipulating pressure. A pressure difference, the transmembrane pressure, is created between the blood and dialysate compartments. This hydrostatic pressure gradient forces plasma water out of the blood and across the membrane, carrying dissolved substances with it, a process known as solvent drag.
Hemodialysis is typically performed three times a week, with each session lasting approximately three to four hours. This schedule manages fluid and waste accumulation that occurs between treatments. Modern machines use volumetric control to precisely manage the amount of fluid removed, ensuring the patient achieves a prescribed target weight loss.
Peritoneal Dialysis: Using the Body’s Own Membrane
Peritoneal dialysis (PD) utilizes the patient’s own body as the filter, specifically the peritoneum, the thin lining of the abdominal cavity. This membrane is rich in blood vessels and acts as a semi-permeable barrier. The dialysis solution, or dialysate, is introduced into the peritoneal cavity through a surgically placed catheter.
The process of cleansing occurs within the abdomen through a cycle called an exchange, which has three phases: fill, dwell, and drain. During the fill phase, the dialysate flows into the peritoneal cavity. The solution contains dextrose or icodextrin, creating an osmotic gradient that pulls waste products and excess water from the blood across the peritoneal membrane and into the dialysate.
The dwell phase is when the dialysate remains in the abdomen, allowing the exchange of fluids and wastes. Following the dwell time, the used fluid, saturated with toxins, is removed during the drain phase. This used fluid is often referred to as effluent.
Peritoneal dialysis has two main formats: Continuous Ambulatory Peritoneal Dialysis (CAPD) and Automated Peritoneal Dialysis (APD). CAPD is a manual method requiring three to five exchanges daily, using gravity to move the fluid. APD employs a machine called a cycler to perform multiple exchanges automatically, usually overnight while the patient sleeps.
Establishing Access for Treatment
A reliable access point must be established to connect the patient to the treatment system. For hemodialysis, this involves creating a vascular access that can handle the high blood flow rates necessary for effective filtration. The preferred long-term method is the Arteriovenous Fistula (AVF), created by surgically connecting an artery directly to a vein, usually in the arm.
The AVF requires several months to mature, meaning the vein must enlarge and thicken to accommodate repeated needle sticks. If vessels are unsuitable, an Arteriovenous Graft (AVG) may be used—a prosthetic tube connecting the artery and vein under the skin. For urgent initiation, a Central Venous Catheter (CVC) is inserted into a large vein, such as in the neck or chest, providing immediate, temporary access.
For peritoneal dialysis, access is achieved with a soft, flexible peritoneal catheter surgically placed into the abdomen. The procedure is minor surgery, often done under local anesthesia, positioning the catheter tip within the peritoneal cavity. A short segment of the catheter, the exit site, remains outside the body.
Proper care of the exit site, including daily cleaning and inspection, is necessary to prevent infection. For the peritoneal catheter, surgical placement is planned in advance, allowing for a healing period before dialysis begins. In some cases, a catheter may be placed and temporarily buried under the skin until treatment is scheduled to start.
Selecting the Appropriate Maintenance Method
The choice between hemodialysis (HD) and peritoneal dialysis (PD) involves medical factors, lifestyle considerations, and patient preference. Medical suitability is important, as certain abdominal conditions or severe lung disease can make PD less appropriate. PD is often considered a first-line therapy for suitable patients as it may better preserve remaining kidney function.
Lifestyle plays a large role in the selection process. HD usually requires three weekly visits to a center, which limits flexibility. PD offers greater independence as a home-based therapy, allowing for more flexible scheduling around work or travel. PD patients must be comfortable performing the exchanges themselves or have a reliable care partner, whereas in-center HD is overseen by professionals.
The home environment must accommodate the chosen therapy. PD requires storage space for supplies and the cycler machine, if automated. While HD can also be done at home, it involves a more complex machine and preparation than PD. The continuous nature of PD can lead to fewer dietary and fluid restrictions compared to the intermittent treatments of in-center HD.