426 THE HUMAN BODY 



tissues of that brought to them. Both these methods enter as a 

 matter of fact. The familiar increase in the rate of the heart, in 

 combination with a slight increase in the amount of blood dis- 

 charged with each beat, suffices to augment the blood-flow about 

 2|-2f times. We saw above (p. 417) that ordinarily venous blood 

 contains about 60 per cent as much oxygen as arterial. In exercise 

 the amount of oxygen in venous blood is very much reduced; in 

 extreme cases none at all may remain. By this more complete 

 utilization, in connection with the more rapid flow, the total 

 oxygen carrying power may be raised about 7-7-J times. The 

 other factors, carbon dioxid transport, and lung ventilation, have 

 much wider limits, so that the bound is established, as stated 

 above, by the oxygen carrying power. 



In connection with this an interesting point arises. Repeated 

 reference has been made to the fact that ordinarily during the 

 passage of the blood through the tissue capillaries it gives up only 

 40 per cent of its oxygen. The suggestion was made (p. 422) that 

 this relatively small disbursement is due to the short stay of the 

 blood in the capillaries. During muscular activity there is a 

 much more rapid blood-flow, with a corresponding shortening of 

 the time required for the blood to pass through the capillaries, 

 yet in spite of this we find the blood giving up virtually all its 

 oxygen, instead of only 40 per cent of it. A recent discovery may 

 help us to explain this apparent paradox. It has been shown that 

 the dissociation of oxyhemoglobin is much more rapid in an en- 

 vironment rich in carbon dioxid than in one containing only small 

 amounts of this substance. One result of muscular exercise is a 

 great outpouring of carbon dioxid from the active tissues. It may 

 be supposed that in the presence of this outpouring the dissocia- 

 tion of oxyhemoglobin is so greatly accelerated that the increased 

 rate of blood-flow is more than counter-balanced. 



If, as may readily happen, the activity becomes so great that 

 the oxygen supply cannot keep pace with the needs of the muscles 

 we have the result already discussed in Chap. VII (p. 112), namely, 

 an outpouring of sodium lactate into the blood. The effect of this 

 is to make more pronounced that acid condition which, as stated 

 previously (p. 403), constitutes the real stimulus to the respiratory 

 center. . The dyspnea is, therefore, markedly increased with the 

 appearance of this substance in the blood. We would probably 



