510 RESPIRATION AND MUSCULAR WORK. [BOOK n. 



Hence the capacity for arduous muscular labour is deter- 

 mined not by the respiratory mechanism alone, nor by the 

 vascular system alone, but by both, and especially by both 

 working together in harmony and concert. The increased 

 ventilation would be idle unless it were accompanied by a 

 quicker circulation, and the quicker circulation would simi- 

 larly be of comparatively little use unless accompanied by 

 increased ventilation. To a bystander the working of the 

 respiratory pump is much more obvious than that of the vascular 

 system, and indeed the subject himself is much more directly 

 conscious of changes in the former than of changes in the lat- 

 ter. Hence when the organism ceases to be able to meet the 

 demands which the labour is making upon it, the subject is 

 said to be " out of breath," though in a large number of cases 

 the failure lies much more at the door of the vascular than of 

 the respiratory system. And, as a rule, it may perhaps be 

 said that when two men differ in their capacity for strenuous 

 work, such as running a race, the difference, though it is often 

 familiarly spoken of as one of " wind " or power of breathing, 

 is in reality not a difference in ventilating capacity but a dif- 

 ference in the power of the heart to keep up to and work in 

 harmony with the increased respiratory movements. 



Thus there are two main factors in respiration, the respira- 

 tory mechanism proper, and the circulation, the one bringing 

 the air to the blood, and the other the blood to the air. We 

 may remind the reader that there is also a third factor, and that 

 one of great moment, the amount of haemoglobin, that is, the 

 number of red corpuscles, in the blood. The amount of oxygen 

 taken up from the lungs depends not only on the strokes of the 

 respiratory and the vascular pumps but also on the richness of 

 the blood in red corpuscles. A body which from loss of blood 

 or from disease is anaemic is thrown out of breath by very slight 

 exertion, not so much because the respiratory or the vascular 

 pump is weak, but because, through lack of oxygen-carriers, 

 with their best efforts the combined pumps can only deliver to 

 the tissues, including the medulla, an inadequate supply of 

 oxygen. And fat persons, whose store of haemoglobin in pro- 

 portion to their body weight is always below par, are proverbi- 

 ally "scant of breath." 



