The main function of capillaries is enabling the exchange between water, oxygen, carbon dioxide, nutrients and many other substances between blood and the surrounding tissues. The capillaries also connect the arterioles and venules to create uninterrupted blood flow through the body.
Capillaries function in a group, or a capillary bed, which is an interweaving network of capillaries. The more active a cell is the more capillaries that are needed to supply the proper blood flow and exchange of materials. These blood vessels are microscopic, but their presence is viewable at times. When a person blushes, this occurs in the capillaries and turns the cheeks red. This is partially possible because the wall of the artery is only one cell thick.