THE MECHANISM OF THE HEART PUMP 1013 

 the intraventricular pressure, due to the contraction of the auricles, 

 which may last about '05 sec. This elevation, which is not always 

 present, is immediately followed by the ventricular contraction, which 

 lasts from to 2. From to 1 the ventricle is getting up pressure, 

 so that at 1 the intraventricular pressure is equal to the aortic pressure. 

 This process takes from -02 to -04 sec. Directly the intraven- 

 tricular pressure rises above this point the aortic valves open and 

 blood is driven into the aorta. The outflow of blood lasts from 1 to 2, 

 about 0-2 sec. At 2 the ventricle suddenly relaxes, the period of 

 relaxation occupying about *05 sec. The flat part of the curve is 

 often spoken of as the systolic plateau, and on an average occupies 

 about '18 sec. According to the condition of the heart and peri- 

 pheral resistance, this plateau may present a gradual ascent or descent 

 v. Fig. 416). Almost immediately after relaxation commences 



01 23 01 23 



FIG. 396. Curve ol intraventricular pressure v, compared with pressure in 

 aorta A. Each vibration of time-marker = T $ T sec. (HFRTHLE.) 



the intraventricular pressure falls below the aortic, so that the aortic 

 valves close somewhere near the upper part of the descent (at 3). 



The ventricular tracing can thus be divided into the following 

 parts : 



(1) A small elevation due to the contraction of the auricles, 

 05 sec. 



(2) A very steep ascent, about the middle of which the aortic valves 

 open. The point at which these valves open, which is about -02 to 

 04 sec. after the beginning of the rise, is not as a rule marked on the 

 pressure tracing. 



(3) A prolonged stage lasting about two- tenths of a second, during 

 which the pressure remains almost level, or rises or falls slightly, and 

 known as the plateau. 



(4) A rapid fall due to the relaxation of the ventricular muscle. 

 The beginning of this fall is attended by closure of the aortic valves, 

 signalled in some cases by a distinct notch in the curve. 



(5) A very gradual ascent, during which the relaxed ventricles are 



