116 APPLIED PHYSIOLOGY 



About 7 per cent, of the total work is also expended in 

 overcoming resistance in the heart itself. Assuming 

 that the volume of blood expelled from each ventricle is 

 approximately 60 c.c., the total work of a cardiac 

 cycle amounts to 0'2 kgm. (or If foot-pounds). At a 

 rate of seventy beats per minute this means 252 litres 

 of blood distributed to the organs every hour, and 

 implies a work of 815 kgm., or 20,000 per day (about 

 140,000 foot-pounds). In normal circumstances an in- 

 crease in the rate of the heart does not indicate an 

 increase in its work, for the output per systole is pro- 

 portionately lessened. The peculiarity of the heart is, 

 not that it does a great amount of work, but that it does 

 it incessantly. This cannot be due altogether to any 

 peculiarity of its muscle, for the diaphragm also works 

 continuously, and does 376 kgm. per hour. Prob- 

 ably all the voluntary muscles could do as much work 

 as the heart were it only possible for the digestive 

 organs to keep them supplied with a sufficiency of 

 potential energy in the form of food.* 



Great as the normal energy of the heart is, its reserve 

 power is greater still. Even moderate muscular work 

 demands a fourfold output of cardiac energy, and the 

 total reserve power has been estimated at thirteen times 

 the normal output during rest. In conditions of disease 

 the work of the heart is enormously augmented, largely 

 from increased resistance in driving the blood through 

 its own chambers (e.g., in mitral stenosis). In disease, 

 too, all such factors as number and duration of systoles 

 are of importance. 



* Lewy op. cit. 



