Most parts of the body receive blood from branches of more than one artery, and where two or more arteries supply the same region, they usually connect. These connections, called anastomoses (a-nas -t ō -M Ō -s ē s), ′ provide alternate routes, called collateral circulation , for blood to reach a particular organ or tissue. • The myocardium contains many anastomoses that connect branches of a given coronary artery or extend between branches of different coronary arteries. They provide detours for arterial blood if a main route becomes obstructed. Thus, heart muscle may receive sufficient oxygen even if one of its coronary arteries is partially blocked. • After blood passes through the arteries of the coronary circulation, it flows into capillaries, where it delivers oxygen and nutrients to the heart muscle and collects carbon dioxide and waste, and then moves into coronary veins. Most of the deoxygenated blood from the myocardium drains into a large vascular sinus in the coronary sulcus on the posterior surface of the heart, called the coronary sinus (Figure 20.8 b). • A vascular sinus is a thin-walled vein that has no smooth muscle to alter its diameter. • The deoxygenated blood in the coronary sinus empties into the right atrium. The principal tributaries carrying blood into the coronary sinus are the following: • Great cardiac vein in the anterior interventricular sulcus, which drains the areas of the heart supplied by the left coronary artery (left and right ventricles and left atrium) • Middle cardiac vein in the posterior interventricular sulcus, which drains the areas supplied by the posterior interventricular branch of the right coronary artery (left and right ventricles) • Small cardiac vein in the coronary sulcus, which drains the right atrium and right ventricle • Anterior cardiac veins , which drain the
right ventricle and open directly into the right atrium • When blockage of a coronary artery deprives the heart muscle of oxygen, reperfusion (re -per-FY Ū -zhun), ′ the reestablishment of blood flow, may damage the tissue further. • This surprising effect is due to the formation of oxygen free radicals from the reintroduced oxygen. • free radicals are molecules that have an unpaired electron (see Figure 2.3 b). These unstable, highly reactive molecules cause chain reactions that lead to cellular damage and death. To counter the effects of oxygen free radicals, body cells produce enzymes that convert free radicals to less reactive substances. Two such enzymes are superoxide dismutase (dis-M Ū -t ā
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- Fall '15
- infarction, Myocardial ischemia