THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 307 



must be effected very shortly after the beginning of ventricular sys- 

 tole, i.e., at the moment when the intra ventricular pressure just barely 

 rises above the intra-auricular. A fraction of a second later, the 

 swiftly rising pressure opens the semilunar valves (2), and clearly, the 

 outward displacement of these flaps must occur at the moment when 

 the intraventricular pressure just barely overcomes the pressure pre- 

 vailing in the arteries. As is indicated in the accompanying figure, 

 the opening of the semilunar valves must take place late in systole at 

 the beginning of the plateau. As soon as the ventricular musculature 

 relaxes, the pressure in the ventricles falls below that prevailing in the 

 arterial channels. In consequence of this reversion of the pressures, 

 the semilunar valves are shut under the weight of the blood as it 

 endeavors to seek a place of least resistance and return into the main 



A 



Fia. 158. DIAGRAM TO ILLUSTRATE THE POSITION OP THE CAHDIAC VALVES DURING (A) 



AURICULAR SYSTOLE AND (V) VENTRICULAR SYSTOLE. 



Only one-half of the heart is represented. 



chambers (3). The closure of the semilunar valves, therefore, is effected 

 immediately after the beginning of ventricular diastole, i.e., as soon as 

 the intraventricular pressure falls below the arterial. Late in diastole, 

 the auriculovenl ricular valves again open and permit the next pre- 

 systolic filling of the ventricles (4). 



It will be seen, therefore, that the ventricles are converted into com- 

 pletely closed cavities twice in the course of each cardiac cycle, but only 

 for the briefest possible time (Fig. 160). This must be so, because the 

 semilunar valves cannot open until the main chambers have been com- 

 pletely shut off against the auricles by the closure of the mitral and 

 tricuspid valves (1 to 2). The'semilunar valves close sometime before 

 the mitral and tricuspid valves are opened by the inflowing auricular 

 blood (3 to 4). It must also be evident that the ventricles do not 

 eject their contents into the arteries as soon as their systolic movement 

 begins, but only subsequent to the moment when the intraventricular 

 pressure exceeds that prevailing in the arterial trunks. The discharging 

 period of these cavities, therefore, begins with the opening of the semi- 



