Question

Explain in detail the Frank-Starling law of the heart. Be sure to define what it is, and describe how it operates (the mechanism). Explain why it is an important way that cardiac output is regulated. Demonstrate your understanding by describing a particular scenario when it would operate and its significance.

Answer 1

The Frank-Starling Law represents the relationship between stroke volume and end-diastolic volume - "Volume of blood ejected by the ventricle depends on the volume present in the ventricle at the end of diastole.". The main principle is the length-tension relationship in cardiac muscle fibers. This can also be observed in skeletal muscles. In other words, the greater the heart muscle is stretched during filling, the greater is the force of contraction and the greater the amount of blood pumped into the aorta. The stretching of the cardiac muscles increases the sensitivity of the myofibrils to calcium, causing a greater number of actin-myosin cross-bridges to form within the muscle fibers. This is the basis of the length-tension relationship. The degree of stretching in cardiac muscles is dependent on the venous return.

The importance of this law is that it allows the cardiac output to be synchronized with the venous return. This also ensures that the cardiac output of the right and left ventricles is equal. The significance of this law can be demonstrated during premature ventricular contraction. When a premature ventricular contraction occurs, there is early emptying of the left ventricle into the aorta. But the next ventricular contraction occurs at its regular interval. This allows more time filling of the left ventricle leading to a stronger contraction. Thus, the Frank-Starling law comes into action and brings back the left ventricular end-systolic volume to the baseline.