THE MECHANISM OF THE HEART PUMP 945 



ventricle remains relaxed and the pressure is approximately zero, in some 

 cases rising slightly towards the end. 



The period of outflow of blood lasts from the moment at which the 

 aortic valves open to the moment at which in the relaxing ventricle the 

 pressure falls below that in. the aorta and the aortic valves close. It there- 

 fore corresponds with the duration of that part of the curve which has been 

 called the ' plateau.' The maximum pressure attained in the left ventricle 

 naturally depends on the height of the aortic pressure and is always greater 

 than this. Under normal conditions in the dog, with an average aortic 

 pressure of 100 mm. Hg., the pressure in the ventricle may be 130 or 150 mm. 

 Hg. The difference between the average pressure in the aorta and the 

 maximum pressure attained during contraction of the ventricle will naturally 

 be greater the larger the amount of blood which is thrown out at each 

 contraction. Thus in one case in a dog of 10 kilos., with an average aortic 

 pressure of 100 mm. Hg., the maximum pressure in the left ventricle was 

 145 mm. Hg. with an output of 2040 c.c. per minute, and 115 mm. Hg. with 

 an output of 650 c.c. per minute. On the right side the maximum pressure 

 is much less, corresponding to the low resistance of the pulmonary system 

 of blood vessels. Otherwise the general course of the curves is very similar 

 on the two sides of the heart. The maximum pressure in the right ventricle 

 may be taken as varying between 25 and 35 mm. Hg. under ordinary 

 conditions. 



CHANGES OF PRESSURE IN THE AURICLES 



Owing to the absence of valves between the right auricle and the venae 

 cavse, changes of pressure within this cavity are transmitted along the veins. 

 The venous pulsation is especially marked in circumstances which give rise 

 to high venous pressure, so that the veins are not entirely emptied at any 

 part of the cardiac cycle. The most superficial observation shows that the 

 jugular vein pulsates twice for each heart beat. The exact form of the 

 pressure tracing in the auricles varies considerably according to the inflow 

 of blood and the state of filling of their cavities. A typical tracing with 

 a moderate inflow of blood is given in Fig. 408 A, p. 942, and in this figure 

 the relations of the different elevations in the auricular tracing to the intra- 

 ventricular events can be made out. A somewhat different curve is given 

 in Fig. 400, but it will be noted that the essential features of the curves are 

 identical. In every case the auricle curve presents the following features : 



(1) The first positive wave (pre-systolic wave) corresponding to the 

 turicular systole. 



(2) The second positive wave k (first systolic wave) occupying the begin- 

 iing of the ventricular systole. This is caused by the sharp closure of the 

 dtral valve. 



(3) A third positive wave (second systolic wave) which may present 

 icondary undulations. This rise of pressure is due to the gradual filling of 



the auricles while the auriculo- ventricular valves are still shut. 



(4) A negative wave which corresponds to the ' post-systolic vacuum ' 

 60 



