Capillaries are the smallest type of blood vessel and have walls about how thick?

• A. One cell layer thick
• B. Two cell layers thick
• C. Three cell layers thick
• D. Four cell layers thick

Answer: (A)

Capillary walls are extremely thin to maximize exchange between blood and surrounding tissues. They are made of a single layer of endothelial cells that line the lumen, sitting on a thin basement membrane. This one-cell-thick barrier provides the shortest possible diffusion path for gases, nutrients, and wastes, enabling efficient transfer as blood moves through the microcirculation. Larger vessels, like arteries and veins, have thicker walls with multiple layers of smooth muscle and connective tissue, which is why capillaries are the only vessels with walls about one cell layer thick.