What happens during ultrafiltration when positive pressure is applied to the blood side of the dialyzer?

During ultrafiltration in the context of dialysis, applying positive pressure to the blood side of the dialyzer results in the movement of water from the blood into the dialysate, creating a concentration gradient that facilitates the removal of excess fluid. This is primarily aimed at decreasing fluid overload in patients with kidney failure.

When pressure is applied, it forces plasma fluid — which includes water and small solutes — through the semi-permeable membrane of the dialyzer. This process allows not just water but also small solutes (like urea and creatinine) to pass to the dialysate side, but it is predominantly the plasma fluid that is concerned in this scenario. While red blood cells and larger molecules are not able to pass through the membrane due to their size, the application of positive pressure effectively aids in the removal of excess plasma fluid, thus making the choice highlighting this process correct.

In the context of the other options, while electrolytes can also be removed during this process, the primary action of ultrafiltration is about fluid movement rather than specific ion displacement. Endotoxins, similarly, would not primarily be removed via ultrafiltration as they vary in size and may not be effectively filtered out through this process compared to