What happens during the dialysis procedure?
The two processes that occur during a hemodialysis session are diffusion and ultrafiltration.
Diffusion refers to a process by which small and middle molecules move, based on concentration gradients, between the blood and dialysate compartments of a dialyzer via a semipermeable membrane.
Molecules can move through the semipermeable membrane by both diffusion and ultrafiltration. Small molecules move across the semipermeable membrane from an area of higher concentration (usually the blood) to an area of lower concentration (usually the dialysate).
The overall effect is to remove small molecules that are likely to be toxins in high concentrations (such as potassium, phosphorus, and urea) while repleting those small molecules that are likely to be deficient (such as calcium or bicarbonate).
Larger molecules do not move as readily across the membrane, and molecules that are bound to protein are unlikely to be removed by dialysis. In addition, fluid can be removed during the dialysis procedure via the process of ultrafiltration