Capillaries link the arterioles and venules and are involved in the delivery of blood to the cells themselves. Therefore capillaries permit the leakage of plasma through fenestrations.

The ability of blood to flow through these capillaries is closely controlled by arteriolar tone. They do not have smooth muscle themselves. Hence exercise can stimulate greater opening up of the capillary beds.

The diameter of a capillary is 5 - 10 microns. Erythrocyte diameter is 6 - 8 microns.