Chapter 6: Cardiovascular System
Arteries can be classified as elastic or muscular. Elastic arteries lie closer to the heart and their primary function is to conduct blood to different areas of the body. This blood flows at high pressure, with marked changes in blood velocity. To accommodate this, elastic arteries have thick walls containing many, concentrically arranged, fenestrated elastin laminae. These are interspersed by smooth muscle cells and ground substance rich in proteoglycan. As elastic vessels become progressively smaller, the pulsatile nature of blood flow is more a result of elastic recoil rather than the heat beat itself.
Moving away from the heart, arteries contain progressively less elastin and become muscular arteries. This can be an abrupt or gradual transition. Muscular arteries have a distributive function, serving to take blood to specific organs. Branching and increased number of arteries means that the total volume is increased and flow velocity is reduced. Muscular arteries contain a thick wall of helically arranged smooth muscle cells, with lesser quantities of elastin.
Arterioles are less than approximately 100 µm wide in the contracted state. Regulation of blood flow to the capillary bed happens at the level of the metaarteriole. Precapillary sphincters can also limit flow to the capillaries that cannot tolerate pressures normally encountered in the arterial system.
Blood flow to the capillary bed can be yet further regulated by arteriovenous anastomoses, which allow blood to bypass regional capillaries. Anastomoses arising in close proximity, and surrounded by a fibrous capsule, are referred to as a glomus bodies. The importance of arteriovenous anastomoses is best demonstrated in erectile reproductive tissue.