284 



TEXT-BOOK OF PHYSIOLOGY. 



in the auricle and ventricle as indicated by pressure changes in the jugular 

 vein and carotid artery respectively it has become possible to construct a 

 diagram of the cardiac cycle of the human heart, to designate on the ventricu- 

 lar curve the time of the opening and closing of the valves, as well as the 

 time relations of the entire series of events. A scheme of this character is 

 shown in Fig. 133, based on that constructed by Fredericq. 



AURICULAR D/ A STOLE 



'oswe ofSonlluna? 



valves 



nifio of 

 Semi-lunar valves. 



Opening of 



" valves 



.Closure of 

 urlculo vmtricula 

 valve. 



Closure of 

 u.ricalo -ventricular 

 valve. 



\l\TPJCULAKSYSTOLE\ \\ttNJR I CUTAR DIA 



Seconds 



J .2 .3 .2 .S .6 .7 .8 .1 .2 .3 



FIG. 133. A SCHEMATIC REPRESENTATION OF THE EVENTS OF A CARDIAC CYCLE. 



Though the foregoing numerical values are given for the duration of the 

 auricular and ventricular systoles and of the common pause, it must be 

 borne in mind that they are true only for the heart beating approximately 

 70 times per minute. If the number of beats increases, not only does the 

 entire cycle diminish in duration, but its different subdivisions, auricular 

 systole, ventricular systole, and diastole also diminish in duration, though 

 in unequal degrees. Thus it has been determined that with each increase 

 of 10 beats, the ventricular systole shortens by about 0.02 second and the 

 ventricular diastole by about o.io second. The opposite holds true if the 

 number of beats decreases below 70 per minute. 



The Relation of the Cardiogram to the Events of the Cardiac Cycle. 

 A comparison of a typical cardiogram, such as is seen in Figs. 123 and 

 134, with the curve of intra-ventricular pressure, shows that they correspond 

 in essential features. The slight elevation (a) on the cardiogram represents 

 the contraction of the auricle, which completing the filling of the ventricle 

 causes it to press more vigorously against the chest wall ; b-c represents the 

 contraction of the ventricles, at which moment the apex is suddenly and 

 forcibly driven against the chest wall; c-d represents the systolic plateau, 

 the time during which the ventricle is discharging blood into the aorta ; d-e 

 represents the relaxation of the ventricle, while e-f represents the time of the 

 diastole during which the heart cavities are enlarging with the incoming of 



