Dialysis does the job your kidneys can no longer handle: it filters waste products, excess fluid, and unbalanced minerals out of your blood. The process relies on a simple principle from chemistry. When blood passes along one side of a thin membrane and a specially mixed cleaning fluid sits on the other side, waste naturally moves from the “dirty” side to the “clean” side. That core idea powers both major types of dialysis, though each one uses a different setup to make it happen.
The Basic Science: Diffusion and Ultrafiltration
Two physical forces do most of the work in dialysis. The first is diffusion: dissolved particles move from an area of high concentration to an area of low concentration across a membrane with tiny pores. Your blood is loaded with waste products like urea and excess potassium. The cleaning fluid (called dialysate) is formulated with low concentrations of those substances, so waste flows naturally across the membrane and into the fluid. Smaller molecules cross faster, and the process works best when the concentration difference between the two sides is greatest.
The second force is ultrafiltration, which handles excess water. Pressure is applied to push fluid from the blood side of the membrane to the dialysate side. This is how dialysis removes the extra fluid that builds up between treatments, since failing kidneys can no longer produce enough urine to keep fluid levels balanced.
How Hemodialysis Works
Hemodialysis routes your blood out of your body, through an external filtering machine, and back again. The centerpiece of that machine is the dialyzer, sometimes called an artificial kidney. Inside it are thousands of hollow fibers, each thinner than a strand of hair, bundled together. These fibers have an asymmetric structure: a thin, selective layer on the inside where blood flows and a more open, porous structure on the outside. Blood travels through the inside of the fibers while dialysate flows in the opposite direction around the outside, maximizing the concentration difference at every point along the filter.
The membrane’s pores are sized to let small waste molecules pass through while keeping larger, useful components like blood cells and most proteins on the blood side. Modern dialyzers use synthetic polymer blends engineered to be compatible with blood, preventing the membrane from triggering clotting or immune reactions.
The dialysate itself is carefully mixed. It contains sodium at roughly 140 milliequivalents per liter (close to your blood’s normal level), potassium around 2 milliequivalents per liter (low enough to pull excess potassium out of your blood), calcium near 2.8, and bicarbonate around 35. That bicarbonate is important because it corrects the acid buildup that happens when kidneys stop regulating blood pH. In short, the dialysate is designed to coax your blood chemistry back toward normal ranges through diffusion alone.
Treatment Schedule and Duration
Most people on hemodialysis go to a dialysis center three times per week. Sessions typically last between 3 and 4 hours. According to the U.S. Renal Data System, treatment times have been trending shorter: in 2016, about a third of patients had sessions lasting 4 hours or more, but by 2024, that dropped to roughly a quarter. The largest group now dialyzes for 3.5 to just under 4 hours per session, and more than a third have sessions under 3.5 hours.
Some people do hemodialysis at home, which opens up more flexible schedules. Home treatments can be done more frequently (five or six times a week for shorter sessions) or overnight while sleeping. More frequent dialysis removes waste and fluid more gradually, which can reduce side effects.
Vascular Access: Connecting to the Machine
Hemodialysis requires a reliable way to move large volumes of blood in and out of your body. There are three options, and the choice matters more than most people realize.
- Arteriovenous fistula (AV fistula): A surgeon connects an artery directly to a vein, usually in the forearm. The vein gradually enlarges and strengthens over 1 to 4 months before it’s ready to use. This is considered the gold standard because it has the lowest infection rate, lasts the longest, and leads to fewer hospitalizations.
- Arteriovenous graft (AV graft): A synthetic tube bridges an artery and vein in the arm. It’s an alternative when veins aren’t strong enough for a fistula, and it’s ready to use sooner, though it doesn’t tend to last as long.
- Central venous catheter: A tube placed in a large vein in the upper chest. It can be used immediately, which makes it the go-to option in emergencies or while waiting for a fistula to mature. The trade-off is a significantly higher risk of infection.
If you know in advance that you’ll need dialysis, your care team will usually plan fistula surgery months ahead so it has time to mature before your first treatment.
How Peritoneal Dialysis Differs
Peritoneal dialysis skips the external machine entirely and uses a membrane you already have: the peritoneum, the thin lining of your abdominal cavity. This lining is rich in tiny blood vessels, and it can act as a natural filter.
A soft catheter is surgically placed in your abdomen. Through it, you fill your abdominal cavity with dialysate. The sugar dextrose in the fluid creates an osmotic pull that draws waste products, excess chemicals, and extra water out of the blood vessels in the peritoneal lining and into the fluid. After a set dwell time (anywhere from 30 minutes to several hours depending on the method), you drain the used fluid and replace it with a fresh batch.
There are two main approaches. Continuous ambulatory peritoneal dialysis involves manual exchanges about four times a day. Automated peritoneal dialysis uses a machine called a cycler to perform exchanges overnight while you sleep. Either way, peritoneal dialysis happens at home, which gives you more control over your schedule. If lab results show waste clearance isn’t sufficient, your care team can adjust the dextrose concentration to pull more aggressively.
Common Side Effects During Treatment
The most frequent complaints during hemodialysis are low blood pressure and muscle cramps, and they’re closely linked. Both are triggered primarily by fluid removal. When the machine pulls water out of your blood faster than your body can compensate, blood pressure drops. That drop reduces blood flow to your muscles, and for some people the combination of reduced flow, shifts in mineral concentrations, and nervous system stress response causes painful cramping.
Research suggests that cramping is more likely in people whose bodies don’t mount the right hormonal response to maintain blood flow during fluid removal. Proposed contributing factors include low magnesium, low sodium, and reduced oxygen delivery to muscle tissue during periods of low blood pressure. When cramps do happen during a session, the typical response is a small saline infusion and a pause in fluid removal, which usually brings relief quickly.
Other common side effects include fatigue after treatment (many people feel wiped out for the rest of the day), nausea, headaches, and itchy skin. These tend to improve with adjustments to the treatment prescription or diet.
How Doctors Measure Whether Dialysis Is Working
Your care team tracks a metric called the urea reduction ratio, or URR. It’s straightforward: it compares the level of urea in your blood before and after a session. A target of 67% or higher means dialysis is clearing enough waste. A more detailed calculation called Kt/V factors in your body size and treatment time; the minimum adequate value for three-times-weekly hemodialysis is 1.2. In practice, a URR of 0.67 to 0.72 corresponds to a Kt/V of 1.2 to 1.4 with over 99% accuracy, so the simpler blood test gives a reliable picture of whether your treatments are doing enough.
If numbers fall short, adjustments might include longer sessions, a higher dialysate concentration, or a faster blood flow rate through the dialyzer.
Diet and Fluid Changes on Dialysis
Dialysis replaces some of what kidneys do, but it doesn’t run 24 hours a day the way healthy kidneys do. Between treatments, waste and fluid accumulate, so what you eat and drink matters. Most hemodialysis patients work with a renal dietitian to manage potassium, phosphorus, sodium, protein, and fluid intake. The specific limits vary from person to person based on lab results, body size, residual kidney function, and the type of dialysis.
Potassium and phosphorus get the most attention because both build up quickly and cause serious problems. High potassium can affect heart rhythm, and high phosphorus pulls calcium from bones over time. Your dietitian will help you identify which foods are highest in each and set a daily target. Fluid restrictions are also individualized. The goal is to limit weight gain between hemodialysis sessions to a manageable amount so the machine doesn’t have to remove too much fluid at once, which is what triggers the blood pressure drops and cramping described above.