The maintenance of adequate blood circulation is a highly complex process that depends on proper function of the heart, the integrity of the vasculature, and the maintenance of a delicate balance between the coagulation and fibrinolytic systems. Failure of blood supply to a tissue (ischemia) may be localized, due to arterial obstruction or deficient venous drainage, leading to infarction (ischemic necrosis of tissue); or generalized, due to severe decrease in cardiac output, leading to a generalized decrease in tissue perfusion (shock).
Atherosclerosis—the deposition of lipid in the intima of large or medium-sized arteries, with accompanying fibrosis—is the major cause of arterial disease in the United States (Chapter 20: The Blood Vessels). Thrombosis occurs in atherosclerotic arteries. It represents the most common cause of arterial obstruction and is the leading cause of death in the United States. Narrowing or occlusion of the coronary and cerebral arteries is responsible for myocardial infarction (heart attack) and cerebral infarction (stroke), respectively. Over 4 million Americans have clinically evident atherosclerosis; Americans suffer 1.25 million heart attacks and 500,000 strokes each year. Over 800,000 of these episodes are fatal, representing 40% of all deaths in the United States. Similar statistics apply to Western Europe; a much lower incidence is seen in developing countries.
Effect of Arterial Obstruction
The effect of arterial obstruction on a tissue is governed by the degree of reduction of blood flow to the tissue in relation to its metabolic needs. Tissue changes resulting from arterial obstruction are influenced by several factors.
Availability of Collateral Circulation
(Figure 9-1.) Collateral circulation varies between two extremes: In tissues with a rich collateral arterial supply, blood flow is not significantly decreased by occlusion of one artery (Figure 9-1A); eg, radial artery occlusion does not produce ischemia in the hand because the collateral ulnar artery circulation will compensate. In tissues with no collateral arterial supply, obstruction of the end artery supplying the tissue leads to complete cessation of blood flow and infarction (Figure 9-1B), eg, the central artery of the retina or the middle cerebral artery.
Effect of arterial obstruction on tissues. A: Loop of intestine supplied by 3 arteries. Obstruction of the major supply artery has no effect on the tissue because normal blood flow is maintained by collaterals. B: The sole artery of blood supply to the retina is the central retinal artery (which is therefore an end artery), obstruction of which causes retinal infarction. C: The posterior wall of the left ventricle is supplied by both left and right coronary arteries. Obstruction of the major supplying artery is partially compensated for by increased flow in collaterals. The exact effect depends on several other factors (see text). In the example ...