Urea diffusion in kidney dialysis

Steps:

• Identify the semi-permeable membrane separating blood from dialysis fluid, allowing small waste molecules like urea to pass.
• Note that blood contains high urea concentration, while dialysis fluid has none, driving diffusion from blood to fluid.
• Observe shapes: triangles (△) appear only in blood initially and move to dialysis fluid, indicating small diffusible wastes.
• Confirm other shapes either stay in blood (large molecules) or are absent in blood (fluid components).

Why D is correct:

• Urea is a small waste molecule that diffuses across the membrane per Fick's law of diffusion, matching the triangle's movement pattern.

Why the others are wrong:

• A (○): Represents glucose, present in both sides without net movement.
• B (●): Represents proteins, too large to cross the membrane.
• C (□): Represents salts, balanced in dialysis fluid to prevent diffusion.

Final answer: D