The Ultrastructure of the Heart in Systole and Diastole

The relations between ultrastructure and function have been examined in the canine left ventricle under known hemodynamic conditions. Left ventricles of normal dogs were fixed acutely by rapid perfusions of the coronary arteries in either diastole or systole, and also following acute ventricular distension, or potentiation of contraction. Sarcomere lengths at the midwall of the left ventricle averaged 2.07 µ at end diastole and 1.81 µ at end systole, changes that are adequate to explain the degree of normal ventricular emptying. Following marked ventricular emptying induced by postextrasystolic potentiation, sarcomeres shortened to an average of 1.60 µ; with acute ventricular distension resulting from over-transfusion, average sarcomere length in diastole increased to 2.25 µ. In each instance, sarcomere lengths correlated with the changes in sarcomere length predicted from changes in the dimensions of a thick-walled sphere. The importance of dispersion of sarcomere length within a given layer of the ventricular wall was noted and its potential significance discussed.