Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 257 20.2 Arteries Arteries are contractile and elastic with relatively thick, muscular walls. Elasticity permits passive changes in vessel diameter in response to changes in blood pressure. The contractility of the arterial walls enables them to change in diameter actively. Arterial smooth muscles contract when stimulated, thereby constricting the artery. This process is called vasoconstriction . The opposite process, vasodilation , results in relaxation of the smooth muscles and increases the diameter of the lumen. Vasoconstriction and vasodilation affect the afterload on the heart, peripheral blood pressure, and capillary blood flow. When a vessel is damaged and excessive bleeding results, contractility is vitally important for the vascular phase of hemostasis to reduce blood loss. In transporting from the heart to peripheral capillaries, blood passes through elastic arteries, muscular arteries, and arterioles. Elast c Arteries Elastic arteries, or conducting arteries, are large vessels with diameters up to 2.5 cm (1 in.). Large volumes of blood are carried away from the heart through these vessels. The pulmonary trunk and aorta, as well as their major arterial branches (the pulmonary, common carotid, subclavian, and common iliac arteries), are elastic arteries. Pressure changes during the cardiac cycle could potentially damage a thin vessel; subsequently, the walls of elastic arteries are extremely resilient and can tolerate changes. During ventricular systole, pressures rise rapidly, and the elastic arteries expand as the stroke volume is ejected. Ventricular diastole exhibits a drop in blood pressure within the arterial system, and the elastic fibers recoil to their original dimensions. Vessel expansion cushions the sudden rise in pressure during ventricular systole, and their recoil slows the drop in pressure during ventricular diastole. This feature is important because blood pressure is the driving force behind blood flow: The greater the pressure oscillations, the greater the changes in blood flow. The arterial system elasticity dampens the lows and highs of pressure that accompany the heartbeat. By the time blood gets to the arterioles, the pressure oscillations have disappeared, and blood flow is continuous. Muscular Arteries Muscular arteries, also known as medium-sized arteries or distribution arteries, distribute blood to skeletal muscles and internal organs. Muscular arteries populate most of the arterial system. These arteries are characterized by a thick tunica media that contains more smooth muscle cells than does the tunica media of elastic arteries. Arterioles Arterioles, with an internal diameter of 30 µm or less, are considerably smaller than muscular arteries. The change in diameter of these vessels is most responsive to local conditions or stimuli from sympathetic or endocrine sources. The amount of force needed to push blood around the cardiovascular system is affected by changes in the diameter: a restricted vessel requires more pressure to push blood through than a dilated one. The force opposing blood flow is called resistance (R) ; arterioles are called resistance vessels. Occasionally, local arterial pressure exceeds the capacity of the elastic components of the tunics, producing an aneurysm , or bulge in the weakened wall of an artery. of 368