952 



PHYSIOLOGY 



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

 by the ventricular systole, the immediate effect of which is to close the 

 auriculo- ventricular valves on both sides of the heart. The whole ventricu- 

 lar 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 

 isometrically on its contents without any flow of blood occurring, lasts 



Diastole. 



01 02 03 04 05 



or, 



07 08 



lOsecs.. 



JHeart Sounds 



dup Lubb dup 



FIG. 414. Diagram of events constituting a cardiac cycle. 



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 then suddenly relaxes, 

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

 endocardiac pressure curve on the average lasts about 0-18 sec., and accord- 

 < > 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 diastolic period of filling recommences. The first sound is 

 synchronous with the commencement of the .ventricular contraction, and 

 the same event is signalled by the occurrence of the apex beat. 



Although the pulse frequently may ui'<lergo considerable variations 

 according to the condition of the individual, being hi.uher during activity 

 or under conditions of mental exritement, the greater part of the difference 

 in duration of the cardiac ctcle thereby induced falls upon the diastolic. 

 period. Thus to take wide limits the pulse rate may vary between :-J 



