368 CIRCULATION 



deriving their force, just as any other muscular structure does, 

 from the potential energy of materials brought to them by the 

 blood and liberated in their protoplasm. 



Work done by the Heart 



If we take average figures for the human left heart as follows : 



Q = 60 c.c, 



/? = 100 mm. Hg pressure in aorta 



= 0-1 X 13-6 grams (1 c.c. of Hg weighs 13-6 gm.), 



the expression QB may be evaluated as 



60 X 0-1 m. X 13-6 = 81-6 gram-metres. 



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 

 0-4 metre per second, the velocity pressure will have a value 



mz;2 60 X (0-4)2 



~ir~ = ^~i^ TTTT' = 0'5 gram-metre. 



2g 2 X 9-8 ® 



This quantity is so small compared with the former during rest 

 that for all practical purposes the work of the heart may be taken 

 as proportional 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 X 0-02 X 13-6 = 16-1, say 16 gram-metres per beat. The 

 average heart beats 70 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. When 

 the output is increased to 20 litres per minute, as it may easily be 

 during exercise, as is shown in Table LVIII., the velocity factor 

 becomes about 10 per cent, of the total work of the heart and must 

 be taken into account. The following table (LVII.), taken from 

 Lovatt Evans, indicates the variation in the magnitude of the 

 velocity factor with the output of the heart. 



