How does a dialysis machine exchange wastes across a membrane?

Hemodialysis uses a device called a dialyzer to replace part of the kidneys’ work. Blood passes through many narrow hollow fibers. Dialysis fluid travels outside those fibers, usually in the opposite direction. The thin fiber walls form a large selectively permeable surface between the two streams, while the bulk fluids remain separate.

Small dissolved substances can cross the membrane. For a permeable waste such as urea, a higher concentration in blood than in nearby dialysis fluid produces net diffusion toward the fluid. Individual molecules move both ways, but more move down the concentration gradient. Equal concentrations produce no net diffusion for that substance.

Flowing fluid continually changes the material beside each part of the membrane. Opposite flow helps preserve useful concentration differences along the exchange surface. Direction still depends on the particular substance: something more concentrated in dialysis fluid can diffuse toward blood. Blood cells are too large to pass through the membrane.

Water removal has a separate pressure-controlled mechanism called ultrafiltration. It should not be explained as waste molecules pulling whole blood through a sieve. The machine manages circulation and fluid removal, and used dialysis fluid carries away transferred material. A dialyzer replaces some kidney functions, but it does not reproduce every regulatory and hormone-related role of living kidneys.