Anatomy & Physiology

Anatomy & Physiology Study Guide The arteries of the pulmonary circuit differ from those of the systemic circuit in that they carry deoxygenated blood (the only arteries to do so). As the pulmonary trunk curves over the superior border of the heart, it gives rise to the left and right pulmonary arteries. These large arteries enter the lungs before branching into smaller and smaller arteries. The smallest branches, the pulmonary arterioles, provide blood to capillary networks that surround alveoli. The walls of these small air pockets are thin enough for gas to be exchanged between the capillary blood and inspired air. As it leaves the alveolar capillaries, oxygenated blood enters venules that unite to form larger vessels carrying blood toward the pulmonary veins. These four veins, two from each lung, complete the pulmonary circuit by emptying into the left atrium. The systemic circuit transports oxygenated blood from the left ventricle to tissues and organs other than the pulmonary exchange surface and returns deoxygenated blood to the right atrium. The systemic circuit supplies the capillary beds in all parts of the body not serviced by the pulmonary circuit. The systemic circuit begins in the left ventricle and ends at the right atrium and, at any moment, contains about 84 percent of total blood volume. 20.11 Systemic Arteries By convention, several large arteries are called trunks; examples are the pulmonary, brachiocephalic, thyrocervical, and celiac trunks. Because most of the major arteries are paired with one artery of each pair on either side of the body, the terms right and left will appear in figures only when the arteries on both sides are labeled. The Ascending Aorta The ascending aorta begins at the aortic valve of the left ventricle. The left and right coronary arteries begin in the aortic sinus at the base of the ascending aorta, superior to the aortic valve. The Aortic Arch The aortic arch curves like the handle of a cane across the superior surface of the heart, connecting the ascending aorta with the descending aorta. Three elastic arteries originate along the aortic arch and deliver blood to the head, neck, shoulders, and upper limbs: (1) the brachiocephalic trunk, (2) the left common carotid artery, and (3) the left subclavian artery. The brachiocephalic trunk, also called the innominate artery, ascends for a short distance before branching to form the right subclavian artery and the right common carotid artery. The Subclavian Arteries The subclavian arteries generate blood to the arms, chest wall, shoulders, back, and CNS. Three primary branches rise before a subclavian artery exits the thoracic cavity: (1) the internal thoracic artery, supplying the pericardium and anterior wall of the chest; (2) the vertebral artery, providing blood to the brain and spinal cord; and (3) the thyrocervical trunk, providing blood to muscles and various tissues of the neck, shoulder, and upper back. After leaving the thoracic cavity and passing across the superior border of the first rib, the subclavian is called the axillary artery. This artery crosses the axilla to enter the arm, where it gives rise to humeral circumflex arteries, which supply structures near the head of the humerus. Distally, it becomes the brachial artery, which provides blood for the rest of the upper limb. Achieve Page 269 of 368 ©2018