THE CIRCULATION 251 



Such a tracing shows that at the moment of ventricular 

 systole the pressure in the aorta is higher than that in the 

 left ventricle. 



As ventricular systole advances the intra-ventricular pres- 

 sure rises and becomes higher than the aortic. At that 

 moment the aortic valves are thrown open and a rush of 

 blood takes place into the aorta, raising the pressure and 

 expanding the artery, and causing the upstroke, and crest of 

 the pulse curve. As the ventricle empties itself the intra- 

 ventricular pressure tends somewhat to fall, and, at the same 

 time, a fall in the intra-aortic pressure also takes place. If 

 all the blood does not leave the ventricle in the first gush, 

 e.g. when the intra-aortic pressure is high as compared with 

 the force of the heart (Fig. 121, continuous line), there is a 

 residual outflow which arrests the diminution in the aortic 

 pressure, or may actually raise it, causing the predicrotic 

 wave. As this residual outflow diminishes, the aortic pres- 

 sure again falls and continues to fall until the moment of 

 ventricular diastole. At this instant the intra-ventricular 

 pressure suddenly becomes less than the intra-aortic and the 

 semilunar valves are forced downwards towards the ventricles, 

 and thus the capacity of the aorta is slightly increased and 

 the pressure falls. This fall in pressure is indicated by the 

 dicrotic notch. But the elasticity of the semilunar valves at 

 once makes them again spring up, thus increasing the pres- 

 sure in the aorta and causing the second crest, the dicrotic 

 wave. After this the pressure in the arteries steadily 

 diminishes till the mean is reached, to be again increased 

 by the next ventricular systole. 



The form of the pulse wave varies according to the 

 relationship between the arterial pressure, and the activity 

 of the heart. 



If the heart is active and strong in relation to the arterial 

 pressure, the main mass of the blood is expelled in the first 

 sudden outflow, and the residual flow is absent or slight 

 (Fig. 121, dotted line). In this case there is a sudden and 

 marked rise of the arterial pressure, followed by a steady fall 

 till the moment of ventricular diastole. The rebound of the 

 semilunar valves is here marked and causes a very prominent 

 dicrotic wave, while the predicrotic wave is absent (Fig. 120, 1). 



