Capillary is the smallest type of blood vessel.

• A. Artery
• B. Vein
• C. Capillary
• D. Venule

Answer: (C)

Capillaries are the smallest type of blood vessels. Their walls are only a single layer of endothelial cells and their very small diameter—about 5 to 10 micrometers—lets red blood cells pass through in single file. This close contact with tissue cells makes capillaries the site where exchange happens: oxygen and nutrients move from blood into tissues, while carbon dioxide and waste move from tissues into blood. Arteries are larger, with thick muscular walls built to handle high pressure as blood travels away from the heart. Veins have thinner walls and valves to return blood to the heart, and they are larger than capillaries. Venules are small veins but still bigger than capillaries. So the statement that capillaries are the smallest blood vessels is correct.