What are channels with a very small internal diameter called?

Capillary tubes are specifically designed to have a very small internal diameter, which is essential for their function in the circulatory system. These tubes are the smallest blood vessels in the body, allowing for the exchange of gases, nutrients, and waste products between blood and tissues. Due to their small size, capillaries can effectively facilitate this exchange by maximizing the surface area relative to volume.

The narrow diameter also helps regulate blood flow and pressure within the circulatory system. The design of capillaries enables red blood cells to travel in a single file, ensuring efficient oxygen delivery to tissues. This structural characteristic differentiates capillary tubes from other types of blood vessels, such as arterioles and larger veins, which have wider diameters and serve different functions in circulation.