Capillaries. Coloured scanning electron micrograph (SEM) of a resin cast of the blood vessels in the upper eyelid. A fine network of small vessels (capillaries) branch off from main vessels (underneath the capillaries) and infiltrate the tissue of the eyelid, supplying it with blood. Gases and nutrients are exchanged between the blood and surrounding tissues through the permeable walls of the capillaries. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissue.