What is ultrafiltration in the context of hemodialysis?

Ultrafiltration in the context of hemodialysis refers to the process where excess fluid is removed from the blood during treatment. This is achieved by applying pressure to the blood as it passes through a semipermeable membrane in the dialysis machine. The membrane allows water and small solutes to pass through while retaining larger molecules and cells, effectively pushing the fluid out of the blood. This mechanism helps manage fluid balance in patients with kidney failure, who cannot adequately remove excess fluid on their own.

The other options do not accurately describe ultrafiltration. The separation of solutes in fluids pertains to diffusion rather than ultrafiltration. Increasing solute concentration does not accurately reflect the goals of ultrafiltration; instead, it is focused on fluid removal. Nutrient absorption is a different physiological process and is not relevant to the ultrafiltration concept in hemodialysis. Therefore, the definition of ultrafiltration closely aligns with the process of fluid being pushed through a semipermeable membrane, highlighting its primary function in hemodialysis treatments.