Hemodialysis cleans your blood by pumping it through a filter outside your body, where waste products and excess fluid cross a thin membrane into a cleaning solution called dialysate. A typical session runs about four hours and happens three times per week at a dialysis center, though home and alternative schedules exist. The process relies on two simple physical principles: diffusion and convection, both happening simultaneously inside a device called a dialyzer.
Diffusion and Convection: The Two Cleaning Mechanisms
The core of hemodialysis is diffusion. Your blood, loaded with waste like urea and excess potassium, flows on one side of a thin membrane. On the other side flows dialysate, a fluid specifically designed to have low concentrations of those waste products. Because particles naturally move from areas of high concentration to low concentration, toxins drift out of your blood and into the dialysate. Meanwhile, substances your body needs, like bicarbonate, move in the opposite direction from the dialysate into your blood, helping correct the acid buildup that failing kidneys can’t handle.
The second mechanism is convection. The machine applies pressure to push water out of the blood and across the membrane. As that water moves, it drags dissolved waste particles along with it. This is especially useful for removing larger molecules that don’t cross the membrane efficiently through diffusion alone. Convection also handles the critical job of removing excess fluid that accumulates between sessions, since kidneys that aren’t working well can’t produce enough urine to keep fluid levels balanced.
Inside the Dialyzer
The dialyzer, sometimes called the artificial kidney, is a plastic cylinder roughly the size of a large water bottle. Inside it are thousands of hollow fibers, each one a tiny tube made from a synthetic polymer. Polysulfone and its chemical relatives are the most commonly used membrane materials worldwide. Each fiber has an inner “skin” layer less than 50 nanometers thick (far thinner than a human hair) that acts as the actual filter. The rest of the fiber wall is a spongy, more porous structure that supports the skin without adding resistance.
Your blood flows through the inside of these hollow fibers while dialysate flows around the outside in the opposite direction. This counter-current flow keeps the concentration difference between blood and dialysate as large as possible along the entire length of the fibers, making diffusion more efficient. The membrane’s pores are sized to let small and medium waste molecules pass through while blocking blood cells and large proteins, keeping them safely in your bloodstream.
What’s in the Dialysate
Dialysate isn’t just water. It’s a carefully mixed solution of electrolytes and buffers designed to pull the right substances out of your blood while putting others back in. It contains sodium, a small amount of potassium (generally kept above 2 milliequivalents per liter to avoid dangerous drops), calcium at roughly 1.25 millimoles per liter, and bicarbonate typically at or below 35 millimoles per liter to correct acid levels.
The composition can be adjusted for each patient. Someone who retains a lot of potassium between sessions might need a dialysate with a slightly different potassium level than someone whose levels stay closer to normal. Sodium concentration is often individualized as well, balanced against how much fluid needs to be removed during the session.
How Blood Gets to the Machine
Hemodialysis requires moving large volumes of blood through the dialyzer quickly. In the United States, blood flow rates above 400 milliliters per minute are standard for most patients. To sustain that kind of flow, you need a reliable access point in a blood vessel, and there are three options.
An arteriovenous fistula is considered the best choice. A surgeon connects an artery directly to a vein, usually in the forearm, which causes the vein to enlarge and develop thicker walls over time. Fistulas last longer and have fewer problems with infection and clotting than the alternatives. An arteriovenous graft is the second choice: a synthetic tube bridges the artery and vein when a fistula isn’t possible, often because the patient’s veins are too small or damaged. A catheter, a flexible tube inserted into a large vein in the neck or chest, is generally used as temporary access while a fistula or graft is being prepared, though it sometimes becomes permanent. Catheters carry a higher risk of infection.
What Happens During a Session
When you arrive at the dialysis center, a nurse or technician checks your weight, blood pressure, and temperature. The weight measurement matters because the difference between your current weight and your target “dry weight” tells the machine how much fluid to remove. Two needles are placed in your fistula or graft (or the catheter is connected), and the machine begins drawing blood through one needle, routing it through the dialyzer, and returning the cleaned blood through the other.
Blood and dialysate flow in opposite directions through the dialyzer at carefully controlled rates. Dialysate typically flows at around 500 to 530 milliliters per minute. The machine monitors pressures, flow rates, and the composition of the dialysate continuously throughout the session. Most sessions last about four hours, three times per week, though your care team may adjust this based on how much waste and fluid your body accumulates between treatments.
Preventing Clotting in the Circuit
When blood contacts the artificial surfaces of the tubing and dialyzer membrane, it triggers the body’s clotting response. Without intervention, blood would clot inside the circuit within minutes, making treatment impossible. Early attempts at hemodialysis in the 1920s failed precisely because of this problem. It wasn’t until heparin, a blood-thinning medication, was introduced in the 1940s that hemodialysis became practical.
Today, a dose of heparin is given at the start of each session, followed by a continuous low dose throughout treatment. The infusion is typically stopped 30 to 60 minutes before the session ends so that your blood’s clotting ability returns to normal by the time the needles come out, reducing bleeding at the access site.
Measuring Whether It’s Working
Dialysis adequacy is tracked with a value called Kt/V, which represents how thoroughly the treatment cleared a key waste product (urea) from your blood relative to your body size. For the standard three-times-per-week schedule, the target is a Kt/V of 1.4 per session, with a minimum acceptable value of 1.2. If the number falls below that minimum, your care team will adjust treatment, perhaps by increasing session length, raising blood flow rate, or switching to a higher-efficiency dialyzer.
For people on alternative schedules, such as more frequent shorter sessions or long overnight treatments, adequacy is measured on a weekly basis with a different target. The decision about when to start dialysis in the first place is not based on a single lab number. Current guidelines emphasize that no specific kidney function threshold should automatically trigger the start of treatment. Instead, the decision weighs symptoms, nutritional status, fluid control, and lab trends together.
Low Blood Pressure During Treatment
The most common complication during hemodialysis is a drop in blood pressure, which affects anywhere from 8 to 40 percent of patients depending on how it’s defined. It happens when fluid is pulled out of the bloodstream faster than the body can shift fluid from tissues back into the blood vessels to compensate. The heart, blood vessels, and nervous system all play a role in compensating for this fluid loss, and when any of those systems aren’t working optimally, blood pressure falls.
People with heart failure are especially vulnerable because their hearts can’t increase output to compensate for the rapid volume change. Eating during treatment can also contribute, because digestion diverts blood flow to the gut at a time when the body is already struggling to maintain pressure elsewhere. Symptoms of a blood pressure drop include dizziness, nausea, muscle cramps, and sometimes blurred vision. The dialysis team can respond by slowing the rate of fluid removal, adjusting the dialysate temperature (cooler dialysate helps maintain blood vessel tone), or giving a small amount of fluid back.
How It Differs From Healthy Kidneys
Healthy kidneys filter blood continuously, 24 hours a day. Hemodialysis compresses that work into roughly 12 hours per week, which means waste products and fluid build up between sessions. This is why diet and fluid restrictions remain important even with regular treatment. Hemodialysis also doesn’t replicate the hormonal functions of the kidneys, like producing the hormone that stimulates red blood cell production or activating vitamin D. Those functions require separate medications.
Despite these limitations, hemodialysis sustains life for hundreds of thousands of people whose kidneys can no longer do the job. The technology continues to improve in membrane efficiency, monitoring precision, and the ability to tailor each session to an individual patient’s needs.