[Biomechanics and physiological role of the atria in the cardiac cycle]

Tensograms of deep and superficial muscular layers of the atrium wall, the ECG, intraarterial pressure were recorded in acute experiments on dogs. Rapid filming revealed movement of labels in the atrium myocardium. Asynphasic character of the atrium muscles contractions was revealed. The deep loop-form layer starts contracting first, narrowing lumen of veins and hindering to some extent the retrograde blood flow from the atria. Within 0.02-0.03 sec the superficial transversal layer starts contracting. The combined contractions of both layers form the atrium systole. Contraction of the deep muscle layer terminates 0.02-0.04 sec earlier than the superficial one whose continuing contraction at the beginning of the atrium diastole leads to stretching of the relaxed deep layer and opening of the lumen of veins discharging into the atria.