CHAPTEE XX 



PHYSIOLOGY OF THE HEART 



The Cardiac Cycle. 



THE series of changes that occur in the heart constitutes the cardiac 

 cycle. This must be distinguished from the course of the circulation. 

 The term cycle indicates that if one observes the heart at any 

 particular moment, the heart from that moment onwards undergoes 

 certain changes until it once more assumes the same condition that 

 it had at the moment when the observation commenced, when the 

 cycle is again repeated, and so on. This series of changes consists of 

 alternate contraction and relaxation. Contraction is known as 

 systole, and relaxation as diastole. 



The contraction of the two auricles takes place simultaneously, 

 and constitutes the auricular systole ; this is followed by the simul- 

 taneous contraction of the two ventricles, ventricular systole, and 

 that by a period during which the whole of the heart is in a state of 

 relaxation or diastole; then the cycle again commences with the 

 auricular systole. 



Taking 72 as the average number of heart beats per minute, each 

 cycle will occupy ^V of a minute, or a little more than 0'8 of a 

 second. This may be approximately distributed in the following 

 way : 



Auricular systole . about O'l + Auricular diastole . 0'7 = 0'8 

 Ventricular systole . ,, 0*3 + Ventricular diastole . 0'5 0*8 

 Total systole . . ,, 0'4 + Joint diastole . . 0*4 = '8 



If the speed of the heart is quickened, the time occupied by 

 each cycle is diminished, but the diminution affects chiefly the 

 diastole. These different parts of the cycle must next be studied in 

 detail. 



The Auricular Diastole. During this time, the blood from the 

 large veins is flowing into the auricles, the pressure in the veins 

 though very low being greater than that in the empty auricles. The 

 blood expands the auricles, and during the last part of the auricular 



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