THE MECHANISM OF THE HEART PUMP 



1023 



Similar results have been obtained by Einthoven, who has allowed the 

 variations in the current passing through the microphone to be recorded directly 

 by means of a very delicate capillary or string electrometer. 



TIME-RELATIONS 



The time-relations of the various events of the cardiac cycle are 

 indicated in the accompanying diagram (Fig. 403). In man the heart 

 beats on the average about seventy-two times in the minute, so that 

 each cardiac cycle i.e. systole plus diastole can be regarded as occu- 

 pying 0'8 sec. During five-tenths of a second the ventricles are relaxed ; 

 during the first four-tenths of this period, which corresponds to the 

 diastole of the heart as a whole, blood is flowing in a steady stream 



Diastole. 



Is 



8 



oj 



a a 



'o 



01 02 03 04 05 01! 07 08 09 10 sees. 



Heart Sounds 



Lnbb ---dnp 



FIG. 403. Diagram of events constituting a cardiac cycle. 



from the veins through the auricles into the ventricles, so that the 

 heart is gradually increasing in size. The systole of the auricle then 

 occurs and lasts about O'l sec. This is followed by the ventri- 

 cular systole, the immediate effect of which is to close the auriculo- 

 ventricular valves on both sides of the heart. The point of closure 

 of the valves cannot be fixed with certainty, but, as has already 

 been shown, must occur within an instant of the time of the commence- 

 ment of the ventricular systole. The whole ventricular contraction 

 lasts 0'3 sec. ; during the first period of this the ventricle is getting 

 up pressure, the pressure rapidly rising until it equals the aortic 

 pressure. This period, during which the ventricle is simply contracting 

 on its contents without any flow of blood occurring, lasts between '02 

 and '04 sec, and is, of course, longer the higher the pressure in 

 the aorta. Directly the intra ventricular pressure rises above this 

 point the aortic valves open and blood is driven into the aorta. The 

 outflow of blood continues throughout the whole of the ventricular 

 systole, and may be taken as lasting about 0*2 sec. The ventricle 



