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4.3 Cardiovascular physiology: regulation of coronary circulation
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Published:July 2018
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Abstract
This chapter reviews coronary circulatory structural and functional aspects with a focus on human physiology and on pathophysiology relating to ischaemic heart disease. During augmented myocardial oxygen demand (e.g. during physical exercise), oxygen extraction is enhanced only marginally due to high extraction at rest. Thus, the increased demand is met by augmented coronary blood flow. The structural design of the coronary artery tree is matched to myocardial perfusion at rest, and can be derived from an economic construction principle ubiquitously present in biology: the physiological or optimality principle of minimum work. Myocardial perfusion at rest amounts to 1 mL/min/g, the level of which is maintained over a broad range of coronary perfusion pressures between 60 and 140 mmHg (coronary autoregulation). Coronary flow at rest under different coronary pressure challenges is held constant by microcirculatory resistance adaptation. Likewise, coronary flow in response to augmented myocardial metabolic challenges is increased four- to fivefold (coronary flow reserve) by a reduction in microcirculatory resistance. Active regulation of coronary microcirculatory resistance results from a balance between vasodilator and vasoconstrictor stimuli exerted by metabolic signals from the myocardium, the vascular endothelium, and neurohumoral influences. An atherosclerotic coronary stenosis is a vascular resistance in series to the downstream microcirculatory resistance inducing an epicardial coronary pressure drop. At rest, normal coronary flow is maintained due to compensatory microvascular dilatation. Hence, the capacity for further microcirculatory dilatation under increased oxygen demand is limited, that is, coronary flow reserve is reduced.
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