Diastole

DIASTOLE (from Greek diastole - expansion), the relaxation of the heart chambers occurring after systole; during diastole, the heart fills with blood. Atrial diastole occurs while the ventricle is still in systole; this is followed by a general pause of the ventricles and atria, during which blood flows from the atria into the ventricles until the pressure in them equalizes, after which atrial systole occurs while ventricular diastole is still continuing (the presystolic stage). The duration of diastole varies within significant limits (0.05 - 2.0 seconds), and it is precisely at the expense of diastole that the heart rate varies. The flow of blood into the heart during diastole occurs partly due to the residual motive force imparted to the blood by ventricular systole, partly due to the aspiration of blood into the heart owing to negative intrathoracic pressure, and partly, finally, due to the suction action of the heart itself (elasticity) and by virtue of the expanding action exerted on the heart by the blood flow through the coronary vessels. Authors who consider diastole an active process assume that the aspiration action of the heart depends not only on the mechanical elasticity of its walls, but also on the active contraction (during diastole) of special muscle bundles. The question of the activity or passivity of diastole has not yet been finally resolved; however, the wide limits of the heart's adaptability regarding diastolic filling to the needs of the organism are considered by many to be a weighty argument in favor of the active nature of diastole; the same argument is served by the fact that during experimental filling of the pericardial cavity with fluid that hinders diastole, stimulation of the vagus nerve still produces diastolic relaxation. In connection with this question, comparative physiological data are of interest: in most animals possessing a heart, the role of diastole is passive and can be reduced to vis a tergo, but in those animals in which the blood on its return path to the heart first enters the pericardial space and is already absorbed from there into the heart (Crustaceae, Tracheata), special diastolic apparatuses are needed. The circumstance that the pressure in the ventricles during diastole never drops below zero indicates that one cannot speak of complete diastolic relaxation. The slight tension of the musculature remaining even by the end of diastole is, in the opinion of some authors, analogous to the tone of striated musculature, while by others it is considered a "residual contracture" remaining after systole. At the expense of variations in the magnitude of this contracture, the degree of diastolic relaxation can change. (The diastolic action of digitalis is attributed to its effect on this contracture.)