Blood vessel. Computer artwork of branching blood vessels. Arteries carry blood away from the heart. They branch (as seen here) forming arterioles that branch further into capillaries. The blood in the capillaries supplies cells with oxygen. The capillaries then rejoin to form venules and finally veins, which return to the heart. The walls of blood vessels are made of smooth muscle, which contracts to maintain blood pressure. Constriction and dilation of the blood vessel causes blood pressure to increase and decrease respectively.