188 THE HUMAN MOTOR 



involves an increased performance of work by the heart and the 

 respiratory muscles. In round figures the beats of the heart 

 increase from 72 to 110 per minute in the performance of a large 

 amount of work (250,000 kilogrammetres in eight hours), and the 

 volume of air breathed from 500 to 2,000 litres per hour. According 

 to the average of experiments by various authorities, the heart 

 pumps 150 grammes of blood per cycle at a pressure of about T ^ 

 of that of the surrounding atmosphere equivalent to height of 

 l-25meties. This is 0-150 X 1-25 = OT875 kgm. of work done. 

 In repose it will be : 



0-1875 x 72 = 13-75 kgm. per minute, 

 and at work (value above), 



01875 X HO = 20-625 kgm. per minute, 

 an increase of 6-875 kgm. per minute, or 



6-875 x 60 = 412-50 kgm per hour. 



As to the respiration, the pressure of the air expired varies from 

 0-06 m. to 0-30 m. of water (see 337) in repose and work respec- 

 tively, which gives : 



500 x 0-08 = 30 kilogrammetres, in one case, and 

 2,000 X 0-30 = 600 kilogrammetres 



in the other, pei hour. The increase of woik done is therefore 570 

 kilogrammetres per hour on the average. As the volume of air, 

 both inspired and expired, there is a double amount of muscular 

 work done. 



570 x 2 = 1,140 kilogrammetres. 



Hence the expendituie D rf covers an increase of work done by 

 the organs equal to : 



412 -j- 1,140 = 1,552 kilogrammetres per hour. 



As regards the circulation of the blood this work is dissipated 

 into heat by friction (see 60). In accurate calculations of 

 the work done ana the muscular yield, these 1,552 kilogrammetres 

 must be added to the work done by the muscles per hour ; the 



T 



yield, in consequence, will be higher than the quotient 



The circulatory and respiratory activity determined by the 

 working of the muscles is due to a secretion of the latter, which 

 is carried to the nervous centres controlling the heart and the 

 lungs. This causes a greater quantity of oxygen to be placed 

 at the service of the muscles because, in passing more frequently 

 through the lungs, the blood absorbs more oxygen there, which 

 latter is also renewed frequently on the surface of the lungs. 



132. An Important Observation. -In stating that the yield is 

 the quotient of the useful work by the expenditure, it has not been 



