CIRCULATION OF BLOOD 67 



back from the arteries, as it accumulates in the pockets of 

 the semi-lunar valves, pressing their surfaces together and 

 thus closing them. The pressure in the ventricle after 

 diastole becomes less than that in the auricle, so that now 

 the blood flows from the auricle into the ventricle, after 

 having opened the auriculo-ventricular valves. 



(lj) In tJic auricles. The contraction of the auricle serves 

 chiefly to regulate the flow in the large veins. During the 

 ventricular systole when no blood is allowed to pass from 

 the auricle into the ventricle, it flows from the veins into the 

 dilating auricle. When, during ventricular pause, it is 

 streaming into the ventricle, the auricle decreases in size 

 proportionately to its decrease in contents. 



4. The pressure in the heart. 



For finding the pressure in animals, a long canula is pushed 

 either through one of the large cervical vessels into the right 

 auricle or ventricle, or through the carotid into the left ventricle. 

 The canula is connected with an instrument for measuring the 

 pressure (mercury or spring manometer). The extent of the 

 pressure is indicated by the number of millimeters of mercury 

 which it stands above the atmospheric pressure. 



During the systole the pressure in the ventricles increases 

 rapidly at first and then more slowly. In the left ventricle 

 it reaches a height of 2OO, in the right ventricle, 60 mm Hg. 

 During diastole the pressure sinks rapidly and may become 

 negative, but before the next systole occurs it rises a little 

 because of the incoming blood. 



The period of preparation lasts from the beginning of the 

 ventricular systole (or the closing of the auriculo-ventricular 

 valves) to the opening of the semi-lunar valves; it amounts to 

 0.05-0.1 second. It can be estimated in animals by registering 

 simultaneously the pressure in the left ventricle and in the aorta; 

 the semi-lunar valves open the moment the ventricular pressure 

 becomes greater than the aortic. In man, the length of this 

 period has been found by comparing the cardiac impulse and the 

 pulse curve. 



The period of discharge is the period from the opening to the 

 closing of the semi-lunar valves; during this time the ventricular 

 pressure is higher than that of the aorta; length of period 

 0.18-0.20 second. 



