What is the narrowest type of blood vessel called?

The narrowest type of blood vessel is called a capillary. Capillaries are the smallest and most numerous type of blood vessel, and they form a network that connects arterioles to venules. Capillaries are only about 5-10 micrometers in diameter, which is about the size of a red blood cell. This allows red blood cells to pass through capillaries in a single file. Capillaries are lined with a single layer of endothelial cells, which are very thin and porous. This allows substances to easily diffuse between the blood and the surrounding tissues.