Which vessels are the smallest and the site of exchange with tissues?

• A. Arteries
• B. Veins
• C. Capillaries
• D. Venules

Answer: (C)

Capillaries are the smallest blood vessels and the site of exchange with tissues. Their walls are just one cell thick, sometimes with tiny gaps or fenestrations, which makes them highly permeable. This thin barrier, combined with a massive collective surface area from all capillaries and the slow flow of blood through them, allows oxygen and nutrients to diffuse out of the blood into tissues and carbon dioxide and wastes to diffuse from tissues into the blood. They form the microcirculation, linking arterioles to venules so that blood can move between the arterial and venous systems while enabling exchange at the tissue level. Arteries have thick walls to cope with high pressure, veins have thinner walls and valves to return blood to the heart, and venules are small veins that collect blood from capillaries rather than serve as the main exchange surface.