What is the definition of "capillary"?

A capillary is a small blood vessel. The human body has capillaries throughout it. They play a crucial part in the microcirculation of the human body by diffusing gases, nutrients and waste products across their walls to surrounding tissues.

Capillaries are comprised of a single layer of flattened endothelial cells. Unlike veins and arteries, capillaries contain no muscular or adventitial layers. Their thinness allows them to efficiently exchange gases such as oxygen and carbon dioxide across the lumen to tissues and organs. Capillaries also transmit blood throughout the body. They connect arterioles to venules and enable oxygenated and de-oxygenated blood to reach all parts of the body.