PHYSIOLOGY OF THE CORONARY CIRCULATION



Three major determinants of myocardial oxy­gen consumption are contractility, heart rate, and wall tension. Myocardial wall tension is directly proportional to the pressure within the ventric­ular chamber and the radius of the ventricular chamber (Laplace relationship). The myocardial mass is a determinant of wall tension and there­fore myocardial oxygen consumption; the larger the muscle mass, the more oxygen needed.
The coronary vascular bed is able to autoregu-late, enabling myocardial oxygen and substrate delivery to equal the demand. Coronary vascular resistance is normally determined by the arter­ioles and is influenced by neural and metabolic factors. Both the sympathetic and parasympa­thetic nervous systems innervate the coronary ar­teries. Alpha receptor stimulation Causes vaso­constriction while stimulation of the beta-2 receptor as well as the vagus causes vasodilation. Metabolic factors regulate regional perfusion. Several mediators including oxygen, carbon diox­ide, and metabolites such as adenosine are prob­ably important. However, when coronary perfu­sion pressure falls to below 60 to 70 mm Hg, the vessels become maximally dilated and flow de­pends on perfusion pressure alone, since capa­bility for further autoregulation is lost. The nor­mal coronary vascular bed has a capacity to increase its blood flow four- to five-fold during maximal exercise. Hemodynamic factors that af­fect coronary perfusion include arterial pressure (especially diastolic pressure, since coronary flow occurs primarily in diastole), the time spent in diastole, and the intraventricular pressure (which exerts tension on the myocardial walls and di­minishes coronary flow).







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