908 



PHYSIOLOGY 



TIME-RELATIONS 



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

 in the accompanying diagram (Fig. 404). 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 occupying 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 from the veins through the auricles into the 

 ventricles, so that the heart is gradually increasing in size. The systole of the 



Diastole. 



01 02 03 04 05 06 07 08 09 10 sees. 



JHeart Sounds 



dup Lubb dup 



FIG. 404. Diagram of events constituting a cardiac cycle. 



auricle then occurs and lasts about 0-1 sec. This is followed by the ventricu- 

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

 valves on both sides of the heart. 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 pres- 

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

 isometrically 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 intraventricular 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 then suddenly relaxes, the period of 

 relaxation occupying about 0-5 sec. ; the ' plateau ' of the endocardiac pres- 

 sure curve on the average lasts about 0-18 sec., and according to the condition 

 of the heart and the peripheral resistance may present a gradual ascent or 

 descent. Directly relaxation commences the ventricular pressure falls 

 below the aortic pressure or the pulmonary pressure and the semilunar valves 

 close, giving rise to the second sound. Systole is now at an end and the dias- 

 tolic period of filling recommences. The first sound is synchronous with 

 commencement of the ventricular contraction, and the same event is signalled 

 by the occurrence of the apex beat. 



