The Contractile Mechanism of an Elastic Sac

Abstract

The pumping action of a contracting elastic spheroid sac has been analyzed and compared with some of the mechanical properties of the heart. As in the heart, the sac shows a rising pressure with increasing volume until at a critical value the pressure begins to fall despite further volume input. The model experiments and an analysis of mechanical forces show that peak pressure to occur at approximately 3 times the unstressed volume of the sac. Ejection patterns produced by contraction of a sac and ejection of its contained fluid into an aortic analogue, together with run-off against a peripheral resistance, show many similarities to data obtained on animals and in man. Pulse pressure and volume/time curves for model elastic sacs and for the heart respond similarly to variations in filling volume, aortic compliance, and peripheral resistance. Several aspects of the dynamics of the cardiovascular system which are demonstrated in the model, such as mobilization of tension, compensation, the generation of the pressure pulse and the dicrotic notch, are discussed on the basis of data obtained. Compensatory mechanisms of the sac (decreased unstressed volume, increased wall thickness, or increased elastic modulus) indicated in the experimental data and in an analysis of forces have direct parallels in the parameters that modify the strength of the heart.