She Thought Both Dialysis Types Took Blood Out. One Does Not

At a kidney education visit in May 2023, Elena opened a notebook and wrote, “Does all my blood come out?” Her specialist had raised dialysis after years of chronic kidney disease, or CKD, which means the kidneys have been damaged and cannot do all their usual work as well as they should.

She had seen pictures of hemodialysis machines, with tubes carrying blood away from the body and back again. Then she heard about peritoneal dialysis, which involves putting dialysis fluid into the abdomen through a catheter, a soft tube placed there for ongoing treatment. In her notebook, she drew a line between the two names and added: “Blood or belly?”

That question gets close to the main difference. Both treatments clean the blood, but only hemodialysis sends blood through equipment outside the body. Peritoneal dialysis leaves the blood inside its vessels and brings the cleaning fluid into the body instead.

The membrane does the sorting

Healthy kidneys continuously filter blood. They remove waste made by the body, help control how much water remains, and keep dissolved salts such as potassium within useful ranges. They also have roles that dialysis cannot fully copy, including helping regulate red blood cell production and bone health.

Dialysis takes over part of the filtering job when kidney function is no longer enough to maintain the body’s internal balance. The treatment depends on a semipermeable membrane, meaning a thin barrier that allows water and some small dissolved substances to cross while holding back blood cells and larger proteins.

This is selective movement, rather than blood passing through an open strainer. The membrane has spaces at a microscopic scale. Small waste molecules can cross it more readily than cells can, and the direction of movement depends partly on how much of a substance is present on each side.

Diffusion is the name for particles spreading from an area where they are more concentrated to an area where they are less concentrated. Food coloring dispersing through a glass of water shows the same physical tendency. During dialysis, waste is more concentrated in the blood than in the fresh dialysis fluid, so waste moves across the membrane into that fluid.

Extra water needs a related mechanism. In hemodialysis, pressure across the membrane helps remove it through ultrafiltration, which means pushing water across a membrane. In peritoneal dialysis, dissolved particles in the dialysis fluid create an osmotic pull. Osmosis means water moving across a semipermeable membrane toward the side with more dissolved particles.

Elena boxed the word “membrane” in her notebook. The treatment names sounded less mysterious once she understood that both methods rely on a barrier between blood and dialysis fluid. The major change was where that barrier sat.

Hemodialysis moves blood through an outside filter

During hemodialysis, blood travels from the body through tubing to a dialyzer, the filtering part of the machine. Inside the dialyzer are many hollow fibers made from semipermeable membrane. Blood flows inside those fibers while dialysis fluid, called dialysate, passes along the other side.

The two fluids remain separated. Blood cells and most blood proteins stay in the fibers, while certain wastes cross into the dialysate. The used dialysate leaves the machine, carrying away what crossed the membrane. The filtered blood then returns to the body through the tubing.

Blood usually leaves and returns through a dialysis access. An access may be made by connecting an artery and vein under the skin, by using a synthetic vessel, or by placing a catheter into a large vein. The access is the route for blood flow. It is not the part that performs the filtering.

Pressure in the dialyzer also moves water out of the blood. The care team sets a fluid-removal goal using clinical information such as changes in body weight, blood pressure, symptoms, and treatment response. That is why the machine tracks more than waste removal: the amount and speed of water movement matter too.

Hemodialysis does not send the body’s entire blood supply into the machine at once. Only a portion is in the tubing and dialyzer at any moment, while the rest continues circulating. Over the course of treatment, blood passes through the circuit repeatedly.

Back in her notebook, Elena crossed out a sketch showing the machine as a tank holding all her blood. She replaced it with a loop: blood out through tubing, across a membrane, then back to the body.

Peritoneal dialysis brings fluid to blood vessels in the abdomen

Peritoneal dialysis uses the peritoneum, the thin tissue lining the inside of the abdomen and covering many abdominal organs. This tissue contains a network of small blood vessels. During treatment, it becomes the semipermeable membrane between the blood in those vessels and dialysis fluid placed in the abdominal space.

Fresh dialysate enters through the abdominal catheter and remains there for a set period known as a dwell. Waste moves from blood in the small vessels, across the peritoneum, and into the dialysate. Extra water also moves into the fluid because it contains glucose or another substance that creates an osmotic pull.

The fluid is later drained through the same catheter. It now contains waste and water removed from the blood, even though the blood itself never left the blood vessels. Fresh fluid replaces it, and the exchange begins again.

Some people perform exchanges by hand during the day. Others use a machine called a cycler to move fluid in and out, often while they sleep. Those approaches change how the exchanges happen, but they use the same membrane and the same basic movement of waste and water.

This cleared up the second half of Elena’s notebook question. “Belly” did not mean that the stomach or intestines filtered blood, and dialysis fluid did not travel through the digestive tract. The work happened across the peritoneum, beside its many small blood vessels.

The treatments use the same physics at different speeds

Hemodialysis usually moves blood and dialysate through the filter at controlled rates during distinct treatment periods. Peritoneal dialysis works more gradually because dialysis fluid stays in the abdomen for repeated dwells. Both methods must be repeated because the body keeps producing waste and taking in fluid between treatments.

Neither treatment washes blood until it becomes free of every unwanted substance. Dialysis is better at removing some small molecules than larger substances or wastes that bind tightly to proteins. Treatment also shifts levels over time, while healthy kidneys work continuously.

The composition of dialysate matters. It is prepared so that selected substances move in a useful direction across the membrane, rather than allowing everything small to leave the blood without limit. The clinical team adjusts a person’s treatment plan using laboratory results, symptoms, remaining kidney function, and how the body responds.

Elena turned to a clean notebook page and made two columns. Under hemodialysis she wrote “blood goes to membrane.” Under peritoneal dialysis she wrote “fluid goes to membrane.” The destination changed.

The filtering principle did not.

What this explanation cannot decide

This piece cannot show which type of dialysis fits a particular person, whether dialysis is needed, or when it should begin. Those decisions can depend on medical findings as well as home support, work, mobility, abdominal health, blood vessel access, and what a person can manage day after day.

A nephrologist, meaning a doctor who specializes in kidney care, can explain the medical tradeoffs. A dialysis nurse or educator can describe what treatment feels like in daily life, including equipment, storage, training, and support. A social worker can discuss insurance and practical barriers. None of those conversations can be replaced by a diagram of fluid movement.

Questions people ask

Does dialysis remove all waste from the blood?

No. Dialysis lowers levels of certain wastes and removes extra water, but it does not reproduce every kidney function or keep the blood unchanged between treatments. How much it removes depends on the membrane, treatment duration, blood or fluid flow, and the characteristics of each substance.

Does peritoneal dialysis put dialysis fluid into the bloodstream?

The fluid goes into the abdominal space, not into blood vessels or the digestive tract. Waste and water cross the peritoneal membrane from small blood vessels into that fluid. The used fluid then drains out through the abdominal catheter, carrying the removed material with it.

Why might someone feel tired after dialysis?

People describe fatigue for different reasons. Fluid shifts, changes in blood pressure, anemia, kidney disease itself, sleep problems, and other health conditions may contribute. An explainer cannot identify the cause for one person; the dialysis team or nephrologist can assess symptoms in the context of treatments and laboratory results.

How do people choose between hemodialysis and peritoneal dialysis?

The choice can reflect medical needs, daily routine, home circumstances, available support, and personal preference. Some people can use either method, while a health issue may make one less suitable. At her education visit, Elena left both options open and underlined one notebook phrase: “where the membrane is.”