What are the smallest blood vessels in the human body?

The story behind the answer

Capillaries are the smallest blood vessels, about 5–10 micrometers wide—just enough for red blood cells to squeeze through single file. Their walls are only one endothelial cell thick, making them the prime site for exchanging oxygen, carbon dioxide, nutrients, and wastes with tissues. Arterioles and venules are larger vessels that feed into and drain from capillaries, and lymphatics are part of the separate lymphatic system.

Memory aid: “Capillary” comes from Latin capillus, meaning hair—picture hair-thin tubes threading through every tissue. Also, a “cap” fits tightly; capillaries “cap off” arteries and veins at the site of exchange.