THE MECHANISM OF THE HEART PUMP 



1015 



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 circum- 

 stances which give rise to high venous pressure, so that the veins are 

 not entirely emptied at any part of the cardiac cycle. The most super- 

 ficial observation shows that the jugular vein pulsates twice for each 

 heart-beat. The relation of the alterations of pressure in the auricle 

 to those in the ventricle is well shown in the accompanying diagram 



Svsto/e Ventricles} Pause 



FIG. 398. Simultaneous tracings of pressures in aorta (dotted line), 

 left ventricle, and left auricle. The time of occurrence of the heart 

 sounds, and the opening and closing of the valves are also shown. 



by Fredericq (Fig. 398). 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 W or be (first systolic wave) 

 occupying the beginning of the ventricular systole. This is caused by 

 the sharp closure of the tricuspid valve. 



(3) A steep first negative wave b'd, caused by the opening of 

 the semilunar valves and the projection of the blood from the ven- 

 tricles into the large arteries. This negative pressure can be regarded 

 as due partly to the ballistic recoil of the heart as it shoots out a mass 

 of blood, partly to the lengthening of the arteries and the corresponding 

 movement of the auriculo-ventricular ring towards the diaphragm. 



