THE MECHANISM OF THE HEART PUMP 901 



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 

 cava3, changes of pressure witMn 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. 398 A ? p. 898, 

 and in this figure the relations of the different elevations in the auricular 

 tracing to the intraventricular 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 

 auricular systole. 



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

 ning, of the ventricular systole. This is caused by the sharp closure of the 

 mitral valve. 



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

 secondary 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 ' of 

 Chauveau and Marey. At this point the ventricle is entirely relaxed and 

 the auriculo- ventricular valves open, so allowing the blood 'to flow freely from 

 the auricle into the ventricle. This negative wave is not always well 

 marked, and is represented only by a series of small undulations in 

 Fig. 398 A. 



