Which structural features optimize diffusion in capillary beds?

• A. Thick endothelial walls with wide diameter
• B. Thin endothelial cells, narrow capillary diameter, large surface area, and maintained capillary density
• C. Presence of basal lamina only
• D. Very high capillary blood flow velocity

Answer: (B)

Diffusion in capillary beds works best when the exchange barrier is as thin as possible, the surface area for exchange is large, and there are many capillaries available so there are many parallel routes for molecules to move into tissues. Thin endothelial cells lower the diffusion distance that gases and solutes must cross, speeding up exchange. A narrow capillary diameter increases the surface area relative to the volume and tends to slow the blood flow enough to allow more time for diffusion to occur. Keeping capillary density high means more capillaries per tissue volume, providing more sites for diffusion overall. Put together, thin walls, abundant surface area, and ample capillary networks maximize diffusion.

Thick endothelial walls would add diffusion distance and hinder exchange. Having only the basal lamina without the endothelial layer would remove the primary diffusion route and impair exchange. Very high capillary blood flow velocity reduces the time available for diffusion, making exchange less efficient.