Arteries carry blood away from the heart in two distinct pathways: The systemic circuit. In this pathway, oxygen-rich blood is carried away from the heart and toward tissues of the body. The pulmonary circuit. In the pulmonary circuit, oxygen-depleted blood is carried away from the heart and into the lungs where it can acquire fresh oxygen and get rid of carbon dioxide. Arteries can also be divided into elastic and muscular arteries based off of the material of their tunica media or middle layer. Elastic arteries are closer to the heart where blood pressure is highest contain more elastic fibers, which allows them to both expand and contract with the surges of blood that occur when the heart beats Muscular arteries are further from the heart where blood pressure is lower contain more smooth muscle tissue and less elastic fibers Artery wall layers The walls of arteries are three distinct layers: Tunica intima. The innermost layer that’s made up of cells called endothelial cells as well as elastic fibers. Tunica media. The middle, and often the thickest layer, that’s made up of smooth muscle cells and elastic fibers that can help control the diameter of the blood vessel. Tunica externa. The outermost layer that’s made up of elastic fibers and collagen. This layer predominantly provides structure and support.
