Capillaries are the smallest blood vessels in the human body. These thinly-walled tubular structures measure between 5 and 10 micrometers in diameter, which is 50 times finer than a baby's hair, according to Kids Discover.
A typical adult contains approximately 10 billion capillaries that stretch for 25,000 miles if lined from end to end. These blood vessels form the juncture between the arterioles and venules, which are the smallest branches of arteries and veins, respectively.
Capillaries primarily function in the exchange of materials between the blood and tissue fluid. Based on permeability, capillaries are classified into three types: continuous, fenestrated and sinusoidal. Continuous capillaries are less absorptive than fenestrated capillaries, which allow for the diffusion of fluids and small-sized molecules. Sinusoidal capillaries are the most pervious, where large particles can easily pass through.