Frank-Starling Law of the Heart

The sympathetic system also innervates vascular smooth muscle and regulates the radius of the blood vessels. All types of blood vessels except capillaries are innervated however, the most densely innervated vessels include arterioles and veins. An increase in sympathetic stimulation of vascular smooth muscle causes vasoconstriction and a decrease in stimulation causes vasodilation. Constriction of arterioles causes an increase in TPR and therefore MAP. Constriction of veins causes an increase in venous return (VR) which increases end-diastolic volume (EDV), SV (Frank-Starling law of the heart), CO, and MAP. 

In other words, the increase in cardiac output occurs by extrinsic (sympathetic stimulation) and intrinsic (increased VR and the Frank-Starling law of the heart) mechanisms. Venous return is also markedly increased by the compression of blood vessels in the working muscles. TTie increase in CO causes an increase in MAP, and the increase in MAP contributes to an increase in muscle blood flow. 

FIGURE 10.7 The Frank-Starling law of the heart 3-D of Sagawa s [1967] left-ventricle equation for a 10-kg dog CO, cardiac output MAP, mean arterial pressure MLAP, mean left arterial pressure. 

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