The smallest type of blood vessel in the body is the capillary. Capillaries are microscopic vessels typically 5 to 10 micrometers in diameter, made up of a thin layer of endothelial cells. They connect the smallest arteries (arterioles) to the smallest veins (venules) and are the site where the exchange of oxygen, nutrients, and waste products occurs between blood and tissues. Their extremely small diameter allows red blood cells to pass through in single file, facilitating efficient exchange at the cellular level.