Eventually, the smallest arteries, vessels called arterioles, further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form venules… Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit ( Figure 20.2 ).

Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels.

pulmonary veins — two sets of paired vessels, one pair on each side, that are formed from the small venules leading away from the pulmonary capillaries that flow into the left atrium. pulmonary circuit — system of blood vessels that provide gas exchange via a network of arteries, veins, and capillaries that run from the heart, through the body, and back to the lungs. artery — blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel. arteriole — (also, resistance vessel) very small artery that leads to a capillary.

Be able to compare and contrast the three tunics that make up the walls of most blood vessels; distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function; describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains.