Which process during hemodialysis is primarily responsible for removing excess fluid from the blood?

• A. Diffusion
• B. Ultrafiltration
• C. Dialysate flow
• D. Osmosis

Answer: (B)

Fluid removal during hemodialysis happens mainly through ultrafiltration, a pressure-driven movement of water across the dialysis membrane from blood into the dialysate. The machine creates a transmembrane pressure that pushes plasma water out, and the ultrafiltration rate is set to match the patient’s fluid needs while avoiding intolerance. Diffusion clears dissolved solutes like urea and creatinine by concentration-gradient transport, but it does not primarily remove water. Osmosis would move water in response to osmotic differences, yet with the dialysate formulated to balance osmolality, it is ultrafiltration that drives the net removal of excess fluid.