PRESSURE CHANGES IN THE HEART DURING ITS ACTIVITY 171 



sion). It is evident that the semilunar valves must open the moment the 

 pressure in the aorta is exceeded by that in the ventricle. So smoothly is this 

 point passed that it is not marked on the pressure curve by any special 

 fluctuation. 



The period of rising tension can be determined on an animal by recording 

 the pressure in the aorta and in the left ventricle at the same time. It is 



L_ 



J-. 



FIG. 58. Pressure curves from the left ventricle (1) and the aorta (2), of the horse, after 



Chauveau and Marey. 



found (cf. Fig. 58) that the pressure curve of the ventricle rises from the 

 abscissa in the horse about 0.1 second, in the dog 0.03 second earlier than 

 does the pressure curve of the aorta. The period shows but slight variation 

 with varying blood pressure or with different rates of heart beat, which means 

 that the heart possesses in a very high degree the ability to meet almost with- 

 out loss of time very different demands upon its powers. 



In man the period of rising tension has been obtained by comparison of 

 the simultaneous . apex and pulse curves, and is given by different authors at 

 from 0.05 to 0.1 second. 



Closure of the semilunar valves must take place whenever the pressure 

 in the aorta exceeds by ever so little that in the left ventricle. By simul- 

 taneous registration of pressure in the aorta and in the left ventricle it has 

 been found that the moment of equal pressure follows shortly after the begin- 

 ning of the steep descent in the ventricle, but this instant is not shown in 

 any special manner on the tracing. 



So long as the pressure in the ventricle is higher than that in the aorta 

 or pulmonary arteries, the blood is being driven out of the heart. The dura- 

 tion of the period of ejection depends on the aortic pressure at the beginning 

 of systole, or upon the pulse frequency only to a very slight extent : it amounts 

 to about 0.18-0.20 second in the dog. 



At the close of systole the heart chambers gradually fill with blood and 

 as a consequence the intracardial pressure gradually rises slightly. 

 13 



