Question

Describe Frank-Starlings Law of the heart in words and on a graph and identify if it is an intrinsic or extrinsic control mechanism.

Answer 1

Frank–Starling law of the heart represents the relationship between stroke volume and end diastolic volume.The law states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles, before contraction (the end diastolic volume), when all other factors remain constant.

As a larger volume of blood flows into the ventricle, the blood stretches the cardiac muscle fibers, leading to an increase in the force of contraction. The Frank-Starling mechanism allows the cardiac output to be synchronized with the venous return, arterial blood supply and humoral length, without depending upon external regulation to make alterations. The physiological importance of the mechanism lies mainly in maintaining left and right ventricular output equality

Frank–Starling law of the heart is a intrinsic control mechanism

Mechanisms

Increased venous return increases the ventricular filling (end-diastolic volume) and therefore preload, which is the initial stretching of the cardiac myocytes prior to contraction. Myocyte stretching increases the sarcomere length, which causes an increase in force generation and enables the heart to eject the additional venous return, thereby increasing stroke volume.

This phenomenon can be described in mechanical terms by the length-tension and force-velocity relationships for cardiac muscle. Increasing preload increases the active tension developed by the muscle fiber and increases the velocity of fiber shortening at a given afterload and inotropic state.

One mechanism to explain how preload influences contractile force is that increasing the sarcomere length increases troponin C calcium sensitivity, which increases the rate of cross-bridge attachment and detachment, and the amount of tension developed by the muscle fiber (see Excitation-Contraction Coupling). Other mechanisms are undoubtedly involved. The effect of increased sarcomere length on the contractile proteins is termed length-dependent activation.