Name the type of blood vessel with a thin (one cell layer thick) wall that allows for exchange of oxygen, nutrients, carbon dioxide and metabolic products.

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

Answer: (C)

The key idea is diffusion across vessels that are extremely thin to allow exchange between blood and surrounding tissues. Capillaries have walls that are only one cell thick, creating a very short distance for molecules to travel. This structure makes it possible for oxygen and nutrients to pass from the blood into cells and for carbon dioxide and other metabolic wastes to move from cells into the blood to be carried away. That direct exchange ability is what defines capillaries as the vessels specialized for transfer between blood and tissues. Arteries, with their thick, muscular walls, and veins, with thinner walls and features like valves, are built more for moving blood under pressure or back to the heart, not for the rapid exchange that capillaries enable. Venules are small veins that collect blood from capillaries, but they don’t specialize in the exchange that capillaries perform.