Dialysis filters waste and excess fluid from your blood when your kidneys can no longer do the job. There are two main types, hemodialysis and peritoneal dialysis, and they work in fundamentally different ways. Hemodialysis pumps blood through an external machine, while peritoneal dialysis uses the lining of your abdomen as a natural filter. Here’s what actually happens during each one.
How Hemodialysis Works
Hemodialysis routes your blood through a machine that houses a filter called a dialyzer. Blood leaves your body through a needle or catheter, travels through tubing into the dialyzer, gets cleaned, and returns to your body through a second line. The entire circuit is continuous: blood is leaving, being filtered, and coming back all at the same time.
Inside the dialyzer, your blood flows on one side of a semi-permeable membrane while a cleaning fluid called dialysate flows on the other side, moving in the opposite direction. Waste products like urea and excess potassium are at high concentrations in your blood and low concentrations in the dialysate, so they naturally move across the membrane through diffusion. Small molecules cross easily. The membrane’s pores are sized to let waste through while blocking larger proteins your body needs, like albumin.
Excess fluid is removed separately through a pressure-based process. The machine creates a pressure difference across the membrane that pushes water out of your blood and into the dialysate. Your care team programs exactly how much fluid to remove each session based on how much weight you’ve gained from fluid between treatments. The dialysate itself contains sodium, potassium, calcium, magnesium, chloride, bicarbonate, and glucose in carefully balanced concentrations designed to mimic healthy blood plasma. This balance prevents too much of any one electrolyte from being pulled out.
Getting Connected: Vascular Access
Before hemodialysis can start, you need a reliable way to move blood in and out of your body quickly enough for the machine to work. There are three options.
An arteriovenous fistula (AVF) is the preferred long-term option. A surgeon connects an artery directly to a vein, usually in the forearm. The increased blood flow causes the vein to enlarge and strengthen over time, a process called maturation that takes about six weeks on average. Ideally, a fistula is created three to six months before you’ll need it. About 25% of fistulas never mature adequately, in which case another approach is needed.
An arteriovenous graft uses a synthetic tube to bridge an artery and vein. Grafts mature faster, typically ready for use within about two weeks once the surrounding tissue incorporates the material. They’re often used when a fistula isn’t feasible.
A central venous catheter is a tube placed into a large vein, usually in the neck or chest. It can be used right away, which makes it the go-to option when dialysis needs to start urgently. Tunneled catheters are designed for longer-term use, while nontunneled versions are temporary, typically removed before a patient leaves the hospital because of higher infection and dislodgement risks.
What a Hemodialysis Session Looks Like
A standard in-center session lasts about four hours and happens three times per week, typically on a Monday/Wednesday/Friday or Tuesday/Thursday/Saturday schedule. You sit in a reclining chair while a technician or nurse connects you to the machine through your access site. Throughout the session, staff monitor your blood pressure, heart rate, temperature, and oxygen levels. The machine itself tracks flow rates and pressures in the tubing, sounding alarms if anything falls outside normal range.
Home hemodialysis offers more flexibility. Depending on the approach, sessions can range from two to ten hours and happen anywhere from three to seven times per week. Some people do shorter daily sessions of two to four hours, five to seven days a week. Others run longer treatments overnight while they sleep, three to six nights per week. More frequent sessions more closely replicate what healthy kidneys do continuously.
How Peritoneal Dialysis Works
Peritoneal dialysis skips the external machine entirely. Instead, it uses the peritoneum, the thin membrane lining your abdominal cavity, as the filter. A permanent catheter is surgically placed into your abdomen, and dialysis solution flows through it into your belly. The solution sits there for a set period called a dwell, during which wastes and extra fluid pass from the tiny blood vessels in the peritoneum into the solution. Then you drain the used solution out and fill with a fresh bag. This complete cycle, fill, dwell, drain, is called an exchange.
There are two main approaches. Continuous ambulatory peritoneal dialysis (CAPD) is done manually. You perform three to five exchanges spread throughout the day, each one taking about 30 to 40 minutes for the fill and drain portions. Between exchanges, the solution sits in your abdomen while you go about your normal activities. No machine is involved.
Automated peritoneal dialysis (APD) uses a small bedside machine called a cycler that performs multiple exchanges overnight while you sleep. The cycler handles the filling, timing, and draining automatically. This frees up your daytime hours and reduces the number of times you physically connect and disconnect, which many people find more convenient for work or school. Performing exchanges while lying down at night also puts less pressure on the abdomen compared to the upright position used during manual daytime exchanges.
Common Side Effects During Treatment
The most common complication of hemodialysis is a drop in blood pressure during the session, known as intradialytic hypotension. It affects roughly 10% to 12% of outpatient treatments. It can happen for several reasons: fluid being removed faster than the body can compensate, or sometimes within the first 30 minutes as blood circulates through the external tubing before significant fluid removal has even started. Symptoms include dizziness, nausea, and muscle cramps. Staff can respond by slowing the fluid removal rate, adjusting the dialysate temperature, or repositioning you.
Muscle cramps are another frequent complaint, often linked to rapid shifts in sodium and fluid levels. When the dialysate’s sodium concentration is much lower than your blood’s, osmotic substances get pulled out quickly and the resulting pressure changes trigger cramping. Electrolyte shifts, particularly in potassium, need to be managed carefully. Removing potassium too aggressively can increase the risk of abnormal heart rhythms during or after treatment.
Peritoneal dialysis has a different side-effect profile. Infection at the catheter site or within the abdomen (peritonitis) is the primary concern. Keeping the catheter and connection points sterile during exchanges is the most important thing you can do to prevent it. Some people experience bloating or a feeling of fullness from the fluid sitting in their abdomen.
Hemodialysis vs. Peritoneal Dialysis
- Location: Hemodialysis is most commonly done at a dialysis center, though home setups exist. Peritoneal dialysis is almost always done at home.
- Schedule: In-center hemodialysis locks you into three fixed sessions per week. Peritoneal dialysis happens daily but can be done overnight with a cycler, leaving days free.
- Independence: Peritoneal dialysis gives you more control over your schedule and doesn’t require traveling to a center. Hemodialysis at a center means trained staff handle the technical details.
- Diet and fluid restrictions: Hemodialysis patients typically follow stricter dietary limits between sessions because waste builds up over the gaps between treatments. Peritoneal dialysis, done daily, keeps waste levels more stable.
- Access maintenance: Hemodialysis requires ongoing care of a fistula, graft, or catheter in a blood vessel. Peritoneal dialysis requires care of an abdominal catheter.
The choice between the two depends on your medical situation, lifestyle, and personal preferences. Both accomplish the same core goal: clearing waste and fluid your kidneys can no longer handle.