What are the tiny blood vessels that connect arteries and veins?

The tiny blood vessels that connect arteries and veins are capillaries.

Capillaries are the smallest blood vessels in the circulatory system. Arteries branch into narrower arterioles, which feed capillary beds; capillaries then drain into venules, which join to form veins. Their extremely thin walls allow oxygen, carbon dioxide, water, nutrients and waste products to move between blood and surrounding tissues.

Most capillaries are only about 5–10 micrometres in diameter and have walls made primarily of a single layer of endothelial cells. Their hairlike size explains the word’s origin: “capillary” comes from the Latin capillaris, meaning “of hair.”

The basic artery-to-capillary-to-vein route is a useful rule, though portal systems are a notable exception. In the liver, for example, blood passes through one capillary network in the digestive organs and then another in the liver before returning to the heart. Italian physician Marcello Malpighi is credited with identifying capillaries microscopically in the 17th century.