Frank-Starling Mechanism and Short-Term Adjustment of Cardiac Flow

The Frank-Starling Law of the heart is a filling-force mechanism, a positive relationship between the distension of a ventricular chamber and its force of ejection. The functioning of the cardiovascular system is usually described by means of two intersecting curves: the cardiac and vascular functions, the former related to the contractility of the heart and the latter related to the after-load imposed to the ventricle. The crossing of these functions is the so-called operation point, and the filling-force mechanism is supposed to play a stabilizing role for the short-term variations in the working of the system. In the present study, we analyze whether the filling-force mechanism is responsible for such a stability within two different settings: one-ventricle, as in fishes, and two-ventricle hearts, as in birds and mammals. Each setting was analyzed under two scenarios: presence of the filling-force mechanism and its absence. To approach the query, we linearized the region around an arbitrary operation point and put forward a dynamical system of differential equations to describe the relationship among volumes of ventricular chambers and volumes of vascular beds in face of blood flows governed by pressure differences between adjacent compartments. Our results show that the filling-force mechanism is not necessary to give stability to an operation point. The results indicate that the role of the filling-force mechanism is related to decrease the controlling effort over the circulatory system, to smooth out perturbations and to guarantee faster transitions among operation points. Summary Statement We address the role of the Frank-Starling mechanism and show that it has no role in the stability of the circulatory system. Rather, it accounts for decreasing the controlling effort and speeding up changes in cardiac output.