THE CIRCULATION OF THE BLOOD 285 



of the valves, the close of the ventricular systole may be convenientlv 

 selected. 



During the systolic plateau the blood is passing from the ventricle into the 

 aorta. Independent of the slight elevations and depressions there is an 

 absolute fall of pressure between the beginning and the end of the plateau 

 There is also a corresponding fall in the aortic pressure, corresponding to 

 these two points. The curve of the difference of pressure shows, however, 

 that the ventricular pressure is slightly higher than the aortic. This fall in 

 both ^ ventricular and aortic pressures is due to the escape of blood from the 

 arterial into and through the capillary system. At 3 (see Fig. 125), however, 

 whether completely emptied or not, the ventricle suddenly relaxes, and its 

 pressure soon falls below that in the aorta. As soon as this takes place 

 the semilunar valves must close, if regurgitation into the ventricular cavity 

 is to be prevented. A comparison of the aortic pressure curve shows a 

 slight notch, the "dicrotic notch," just preceding a slight elevation, the 

 "dicrotic" wave. This notch occurs at the moment when the semilunar 

 valves close. The corresponding point on the ventricular pressure curve 

 has been placed just where the ordinate 4 cuts the descending portion. 

 As yet, however, the pressure is higher in the ventricle than in the auricle, 

 and continues so until near the line of atmospheric pressure. At this 

 point the pressure in the auricle, due to the accumulation of blood 

 during the ventricular systole, now forces open the mitral valve and the 

 blood flows into the ventricle. The opening of the mitral valve occurs about 

 the point where the ordinate 5 cuts the curve. 



The ventricular pressure curve affords but slight, if any, indication of the 

 auricular systole. It apparently does not give rise to any noticeable increase 

 in the ventricular pressure. The slight rise in the pressure curve, which 

 just precedes the abrupt rise due to the ventricular systole, may be taken as 

 an indication of an increasing pressure due to the inflow of blood from the 

 auricle, as a result of the auricular systole. Immediately following this 

 event the ventricular systole begins and as soon as the pressure in the ven- 

 tricle exceeds that in the auricle the mitral valve closes. This is marked 

 on the curve where the ordinate cuts it, at o. With the closure of the mitral 

 valve the blood becomes imprisoned within a closed cavity, closed at one 

 orifice by the mitral valve and at the other orifice by the semilunar valves. 

 As the blood is incompressible the intra-ventricular pressure under the force 

 of the ventricular contraction rapidly rises and continues so to do until the 

 pressure in the ventricle exceeds that in the aorta, at which moment the semi- 

 lunar valves are suddenly opened and the blood discharged. A comparison 

 of the aortic curve shows that for a short time during the ventricular systole 

 the pressure is falling, but at one point the curve turns at a sharp angle and 

 rapidly rises. This is an indication that the semilunar valves are suddenly 

 thrown open and the blood begins to pass into the aorta. This event occurs at 

 a moment marked on the ventricular curve by the ordinate i. Beyond this 

 point the pressure continues to rise, for the aortic pressure must not only be 

 exceeded, but a certain velocity must be imparted to the blood. Between 

 the ordinates i and 4, the semilunar valves remain open and the blood 

 passes into the aorta. 



In accordance with the foregoing: the ventricular systole may be sub- 

 divided into two periods: 



