THE MECHANISM OF THE HEART PUMP 



899 



of curve obtained from the left ventricle. Very often, as in A, the curve 

 is not unlike a single muscle twitch or the curve of contraction of the frog's 

 heart muscle. Nearly always however it is possible to see on the upstroke 

 one or two elevations, the most noticeable being the elevation marked 

 Sj. This can be shown to correspond to the opening of the aortic valves. 

 This is still better marked in B, where the heart was beating very forcibly 

 and rapidly under the influence of adrenalin, and is 'also very evident in c. 

 In some cases, as in A, the rise of pressure occurs distinctly more slowly 

 after Sj. than before. In B there is a further rapid rise of pressure after 

 Sj before the curve begins to slope away, and at the change of velocity of 

 rise, there is a second wave at the point marked S 2 . This is also marked in c. 

 The slope of the curve after S x or S 2 varies considerably according to the 



Aorta 



Ventricle 

 Auricle 



345 



FIG. 399. 



67 



amount of blood the heart is sending out. In c the intraventricular 

 pressure curve runs almost horizontal for a time and this part of the curve is 

 known as the systolic plateau, but, as is evident from A, a plateau in the 

 strict sense of the term is not always present. At the end of the ' plateau ' 

 the pressure rapidly falls, and the period where the lines thin out, i.e. the 

 point at which the fall is occurring most rapidly, corresponds to the closure 

 of the aortic valves. 



The average course of the changes of pressure in the heart during each 

 beat is shown diagrammatically in Fig. 399 (Piper). The cardiac cycle 

 begins with the contraction of the auricle at 1 which may or may not give 

 rise to a slight rise of pressure in the ventricles. As the auricular contraction 

 dies away, the ventricular contraction begins at 2. This causes a very rapid 

 rise of pressure. Almost immediately after the beginning of the rise, or 



