58 



THE HEART 



simultaneously into the auricle, ventricle and aorta are 

 shown diagrammatically in Fig. 11. 



Systole begins with a sUght rise of pressure (at 1) in the 

 auricle due to contraction of this chamber. Immediately 

 afterwards the ventricular pressure rises, slowly at first, 

 then more rapidly. As it rises, there occurs (at 2) a second 



Fig. 11. — Changes in pressure during a complete heart-beat in the left 

 auricle, left ventricle, and aorta (modified from Piper). 



rise in the auricular pressure brought about by the sudden 

 closure of the auriculo- ventricular or mitral valve. At 3, 

 the ventricular pressure is sufficient to force open the 

 aortic valve. As the blood flows into the aorta the ven- 

 tricular pressure describes a rounded summit known as 

 the systohc plateau. This terminates in a sharp fall of 

 pressure, at the middle of which the aortic valve closes, 

 the point of closure being marked by the secondary rise 



