In hemodialysis, diffusion moves solutes from

• A. From lower concentration to higher concentration
• B. Against a concentration gradient
• C. From higher concentration to lower concentration
• D. By filtration

Answer: (C)

Diffusion is the movement of solutes down their concentration gradient through a semipermeable membrane. In a hemodialysis setup, the dialyzer has blood on one side and dialysate on the other. The dialysate is prepared with low levels of waste solutes, while blood contains higher concentrations of toxins like urea and creatinine. Because of this gradient, those wastes move from the blood (higher concentration) into the dialysate (lower concentration) across the membrane. This is a passive process that doesn’t require energy. It differs from moving substances from low to high concentration (which would be active transport) and from moving water or solutes by pressure alone (filtration).