32. The heart-lung preparation (Starling’s laws)

Heart-lung preparation

Starling made the so-called heart-lung preparation, where the heart and lungs are disconnected from the systemic circulation and the systemic circulation is replaced with artificial tubes. The heart will pump blood through these tubes and back into the right atrium. The lungs are ventilated artificially. The resistance of the artificial tube system could be altered, which simulates changes in the peripheral vascular resistance in vivo.

The heart-lung preparation allows investigation of how the heart function changes when the vascular resistance changes and when the venous return changes, which allowed Starling to describe the Frank-Starling mechanism.

He observed that when the venous return increased the heart contracted more forcefully, which is the basis of the Frank-Starling mechanism.

Frank-Starling mechanism

The Frank-Starling mechanism describes how a rise in venous return (or preload) also raises the stroke volume. In other words, when more blood flows into the heart more blood will flow out. The Frank-Starling mechanism allows the heart to keep the cardiac output equal to the venous return.

When an extra amount of blood flows into the ventricles the ventricular myocardial fibres are stretch to a greater length. When muscle fibres are stretched, they will contract with larger force, and so they will eject more blood.

The amount of venous return is not regulated by the heart itself but rather by the peripheral circulation. For example, during exercise veins of the leg will vasoconstrict, which “squeezes” extra blood out from the veins back to the heart. The extra venous return will cause an increase in cardiac output and therefore more blood to the muscles.