Regulation of Myocardial Blood Flow

Abstract

The microcirculation of the heart is continuously challenged to provide as acequate supply of exygen and nutrients to the working myocardium. One aspect of this continuous change in perfusion requirements is that the demands for oxygen and energetic substrate vary considerably, even during normal activity of the heart. Thus, the tone in microvessels, which prominently controls the delivery of oxygen and nutrients must be constantly poised to match delivery with demand. To accomolish this important duty, the coronary microcirculation relies on a vanety of control mechanisms, which seem to have dominant sites of action in different microvascular locations for example, the smallest coronary arteriotes are exquisitely sensitive to metabolic factors, whereas unstream, afterioles show striking vasoactive reactions to myogenic stimuli. Even farther upstream, small arteries exhibit potent dilation to flow through the transduction of shear stress. Neurohumoral factors seem to modulate resistance of larger arterioles and small arteries, but the smallest arterioles are fairly resistant to these extrinsic influences. On the basis of these and other functional attributes that vary within the coronary microcirculation, we would like to introduce the idea of a “coronary vascular microdomain.” We also propose a scheme in which the coordination of the vasoactive responses among the different vascular microdomains occurs through adjustments in tone of flow dependent and myogenic elements. The advantage of multifactorial regulation of different colonary vascular microdomains is that it provides a buffer in the event that a pathophysiologic disturbance primarily affects one regulatory mechanism. Thus despite potential dysfunction of a control mechanism, the combined actions of others would enable adequate delivery of oxygen and nutrients to the myocardium.