170 



A TEXTBOOK OF PHYSIOLOGY 



The amount of work done by the heart varies with the pulse 

 frequency per minute (Fig. 71). 



TABLE SHOWING DURATION op SYSTOLE TO DIASTOLE, ETC., WITH DIFFERENT 

 PULSE-FREQUENCIES. 



Pulse- 

 Frequency 

 per Minute. 



50 

 60 

 70 

 80 

 90 

 100 



Duration of Duration of 



Systole in Hun- Diastole in Hun- Ratio of Systole to Hours of Work 

 dredths of a dredths of a < Cardiac Cycle. per Diem. 

 Second. Second. 



In Fig. 71 pulse-frequencies per minute are indicated along the 

 abscissa. Durations of systole and of diastole are given in hundred ths 

 of a second. Their respective curves show that the systole shortens 

 by about y^ second for each increase of ten beats per minute, and 

 that the diastole shortens by about j 1 ^ second. 



FIG. 74. EXCITATION OF VAGO-SYMPATHETIC. (L. H.) 

 Note the after-effect: a staircase augmentation of the heart-beat. The stars indicate 



the^beginning and end of stimulation. 

 The time is marked in seconds. 



The downstroke represents contraction. 



The shaded and unshaded portions represent respective time of 

 work and time of rest at various pulse-frequencies. 



The rate of beat and the arterial pressure also influence the amount 

 of oxygen consumption by the heart muscle. This is well seen in the 

 diagram (Fig. 72). 



