282 CIRCULATION 



That is, about 80 gram-metres of work is done in overcoming the 

 resistance of the conducting tubes. This value is only approxi- 

 mate, as the work done in forcing a fluid along an elastic tube in 

 which the pressure falls steadily, say from 150 mm. Hg to 50 mm. 

 Hg is not strictly proportional to the average pressure, but would 

 need to be determined by integration. The error is, however, 

 less than 10 per cent. If the blood is expelled at a velocity of 

 0-5 metre per second, the velocity pressure will have a value 



wv 2 60x(0-5) 2 



=0-7 gram-metre. 

 2g 2 X9-8 



This quantity is so small compared with the former that for all 

 practical purposes the work of the heart may be taken as pro- 

 portional to the output multiplied by the average arterial pressure, 

 i.e. W=Q . R. 



Similarly the work of the right heart may be estimated from 

 the average pressure of the pulmonary artery (20 mm. Hg) as 

 60 xO-02 xl3-6=16-l say, 16 gram-metres per beat. The average 

 heart beats seventy times per minute and, therefore, in 24 hours 

 the work done by the heart (of a man at rest) will be about 10,000 

 kilogram-metres. 



Muscular work, of course, augments this figure not only by 

 increasing the volume of blood per beat and increasing the number 

 of beats but by raising the value of the velocity factor. During 

 a short sprint, an athlete may have a pulse rate of 180 per minute 

 with an output of 180 c.c. at each beat and an average arterial 

 pressure of 120 mm. Hg. Then for the systemic heart : 



QR=I80 xO-12 xl3-6=294 gram-metres, 

 and QR (pulmonary) =180 xO-025 x!3-6=61 gram-metres. 



If the time of outflow is considered as 3/8 of each cardiac cycle, 

 of which there are 180 per minute, then the contents of the 

 ventricle, 180 c.c., are shot into the aorta at the rate of 1440 c.c. 

 per second (say 1500 c.c.). If the cross-section of the root of the 

 aorta be taken as 625 sq. mm., then the velocity of expulsion will 



be - =2-48 metres per sec. (say 2-5 metres). Therefore : 

 wv z 180 X (2 -5) 2 



2r~Tx^ 7gra res - 



The total work on both sides of the heart will be : 



T 29 f t + ? I +L? 1 J~ 6 -^ [= 46 2 gram-metres per beat 

 Left side/ (Right side/ 



=83-2 kilogram-metres per minute. 



