402 RESPIRATION. 



see that the same condition has a notable effect in promoting the perspira- 

 tion; it probably has a similar influence over other secretions. On the 

 other hand, as we have seen ( 316), there are reasons for thinking that the 

 activity of the respiratory centre and so the energy of the whole respiratory 

 act is influenced by chemical changes, other than the decrease of oxygen and 

 increase of carbonic acid, brought about in the blood by the activity of the 

 skeletal muscles. 



The closeness and the intricacy of the ties which thus connect the respira- 

 tory system with almost all parts of the body may be illustrated by consider- 

 ing the effects of muscular work on the body, and the conditions which, 

 apart from the capacity of the muscles themselves and of the motor nervous 

 apparatus which puts them to work, determine the power of the body to do 

 work. During work, especially arduous work, the muscular contractions rob 

 the blood of much oxygen and load it with much carbonic acid. This 

 change in the blood would itself increase the activity of the respiratory centre 

 and the energy of the respiratory movements, and might be sufficient to 

 secure such an increase of these movements that the deficiency of oxygen and 

 increase of carbonic acid should never overstep certain limits. But, as we 

 have said, apparently other products of muscular metabolism act so potently 

 in stimulating the respiratory centre, that the respiratory movements are 

 more than sufficient to compensate the changes in the gases of the blood. 

 The efficacy of the augmented respiratory movements is much increased by 

 a concomitant increase in cardiac activity and a swifter or fuller stream of 

 blood through the lungs; indeed, unless backed up by the cardiac increase, 

 the mere increase of the pulmonary ventilation might prove inadequate. 



Hence the capacity for arduous muscular labor is determined 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 similarly be of com- 

 paratively 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 latter. 

 Hence when the organism ceases to be able to meet the demands which the 

 labor 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 vas- 

 cular 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 ventilat- 

 ing capacity but a difference 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 respiratory 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 ansemic 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, 



