Question

Draw the ‘length-tension relationship’ curve for left ventricular function i.e. Stroke Volume (SV) versus End-Diastolic Volume (EDV). Use this to explain the Frank Starling mechanism and explain how the autonomic nervous system can affect this curve.

physiology

Answer 1

Frank-Sterling law:

Ventricular function curve relates: Ventricular performance, in terms of stroke volume, cardiac output, as dependent variable to ventricular pre-load, in measure of ventricular filling pressure, end diastolic volume, end diastolic diameter, length, end diastolic wall stress, and cross section area. This, dependent relationship of ventricular performance on ventricular preload, is the basis of Frank-Sterling law.

Frank-Sterling law states that: The energy of contraction is the function of the length of the muscle fiber. Frank-Sterling law, more precisely, the Frank mechanisms and Sterling laws, describes the ability of the heart to change the force of contraction ad thus, the stroke volume, in response to the venous return.

Thus, Stroke volume = End diastolic volume – End systolic volume.

Thus, the stroke volume icreases, with increase in volume of blood in the ventricles.

The phenomenon is described by, Frank-Sterling curves (in term of inotropic states).

The ventricular filling (measured as end diastolic volume) and the preload, increases with increase in the venous return.

It was described by the length- tension relationship of the sarcomeres of myocardium.

At rest, as per the ascending limb of the curve, increase in stretch of the cardiac muscle fiber, increases the tension, till an optimum length is reached. The cardiac muscle usually will not function, beyond this point, as described by the descending limb of the curve.

Effect of Autonomic nervous sytem on the curve:

The sinoatrial node (SA node) or pacemaker mainly determines the heart rate of an individual. This in turn is related to the conductance of ions like Potassium, Sodium, and Calcium. Or neurohormones like epinephrine, norepinephrine, acetylcholine, mediated by sympathetic or parasympathetic stimulations of vagal nerve. Autonomic nervous system includes the sympathetic or parasympathetic nerves stimulation. Resting condition is governed by the parasympathetic system. Sympathetic stimulation increases the contractibility of cardiac muscles, this increases the stroke volume. As a result the curve is shifted towards the left.