CH. XX.] WORK OF THE HEART 245 



being discharged into the aorta, measured in cms. of blood, and h the mean aortic 

 pressure over the same time, then : 



v* = 2g(H-h). 



Or H=h+ lTg 



(86-03) a 

 ~ + 2 x 981 

 = h + 3-77 cms. of blood. 



That is, the mean intraventricular pressure during the time the semilunar valves are 

 open is only 377 cms. of blood or 0'28 cms. of mercury higher than the mean aortic 

 pressure during the same time. We may take the mean aortic pressure during the 

 duration of systole as approximately 12 cms. of mercury or 156 cms. of blood, if we 

 take the density of mercury as being 13 times that of the blood. 



Now if Ep represents the total potential energy created by the heart per beat, 

 then, 



Ep = VgdH. 



A part of this energy, EK, is converted into kinetic energy since velocity is 

 imparted to the blood. This amount is given by the formula : 



From these two formulae 



Ep = 110 x g x 1-05 x (156 + 3*77) ergs 

 = 110 x 1-05 x (159-77) grm. cms. 

 = 18453 '4 grm. cms. 



Again Ep_ _ VgdH 



EK~ ' 



H 



~ H-h 



159-77 

 = 3-77 

 = 40 approximately. 



That is, fV of the total energy of the heart's beat is used in imparting velocity to the 

 blood. 



When the blood reaches the aorta its velocity is gradually checked, i. e. , some of 

 the kinetic energy imparted to it by the heart is reconverted into energy of pressure. 

 The remaining kinetic energy is given by the equation : 



EK I = \ mvf 

 _ Vdv? 



^T 

 = 22*275 grm. cms. 



Hence, the kinetic energy of the blood in the aorta is only approximately ^^ of the 

 total energy imparted to the blood by the heart. 



The Output of the Heart. The first estimations of the work of the heart, made 

 by Volkmann and Vierordt, gave numbers nearly double those stated in the preced- 

 ing paragraph. Recent research has shown that their estimate of the output of the 

 heart was excessive. Direct measurements of the heart's output have been made 

 by Stolnikow and Tigerstedt. The former cut off by ligature the whole of the 

 systemic circulation in the dog, and then measured the amount of blood passing 

 through the simplified circulation which consisted only of the pulmonary and coron- 

 ary vessels by means of a graduated cylinder interposed on the course of the vessels 

 (see fig. 254). Tigerstedt made his observations by means of a Stromuhr (see next 

 chapter) inserted into the aorta. Severe operative measures of this kind, however, 

 interfere with the circulation a good deal. 



