Capillaries enable exchange of substances between blood and tissue due to their what feature?

Capillaries enable exchange of substances between blood and tissue due to their what feature?

• A. Fragile walls
• B. Thick tunica media
• C. Valves to prevent backflow
• D. Large diameter

Answer: (A)

Capillaries enable exchange because their walls are extremely thin and highly permeable. The wall is essentially a single layer of endothelial cells with a thin basement membrane, which creates a very short diffusion path for gases, nutrients, and wastes. The extensive, highly branched network of capillaries provides a large surface area, so substances can diffuse efficiently between blood and tissue. Their small diameter also helps by slowing blood flow just enough to increase contact time for exchange. The other features described wouldn’t support exchange: a thick tunica media would hinder diffusion, valves prevent backflow mainly in veins, and a large diameter would reduce surface area-to-volume ratio and decrease exchange efficiency.

Capillaries enable exchange because their walls are extremely thin and highly permeable. The wall is essentially a single layer of endothelial cells with a thin basement membrane, which creates a very short diffusion path for gases, nutrients, and wastes. The extensive, highly branched network of capillaries provides a large surface area, so substances can diffuse efficiently between blood and tissue. Their small diameter also helps by slowing blood flow just enough to increase contact time for exchange. The other features described wouldn’t support exchange: a thick tunica media would hinder diffusion, valves prevent backflow mainly in veins, and a large diameter would reduce surface area-to-volume ratio and decrease exchange efficiency.