Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in muscle tissue. This network of vessels infiltrates the tissue, supplying it with blood. Gases and nutrients are exchanged between the blood and surrounding tissue through the permeable walls of capillaries, the smallest blood vessels. Muscle tissues contract to provide mechanical movement and support for body structures against gravity. They also provide tension and pressure in the vascular and digestive systems. The cast was made by injecting resin into the blood vessels. The surrounding tissues were then chemically digested. Magnification: x45 at 6x7cm size.