Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in connective tissue. The fine network of smaller vessels branching off from the main vessel infiltrates the tissue, supplying it with blood. Gases and nutrients are exchanged between the blood and surrounding tissues through the permeable walls of capillaries, the smallest of blood vessels. Connective tissues provide structural support and cohesion throughout the body. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissues. Magnification: x27 at 6x7cm size.