Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of the blood vessels that surround the pituitary gland. This network of vessels infiltrate the tissue, supplying it with blood. Gases and nutrients are exchanged between the blood and surrounding tissue via the permeable walls of capillaries. The pituitary (hypophysis) is the master endocrine (hormone) gland, which hangs just below the centre of the brain of all vertebrates. The cast was made by injecting resing into the blood vessels, followed by chemical digestion of the surrounding tissue.