RESPIRATION. THE GASEOUS INTERCHANGES 425 



Tissue Respiration. Our knowledge of the use of oxygen and 

 the production of carbon dioxid by the tissues is not very com- 

 plete. The following general facts maybe stated here:. (1) The 

 tissues take up oxygen from the lymph as fast as it is brought by 

 the blood and use it in oxidative processes at the same rate; 

 careful experiments fail to show that there is any storage of oxy- 

 gen in the tissues for future use. (2) Tissue oxidations differ from 

 ordinary oxidative processes, such as occur when fuel is burned in 

 a furnace, for example, in that they are carried on through the 

 agency of enzyms known as oxidases. The chemical process of oxi- 

 dation carried on thus is not direct as in ordinary burning; it oc- 

 curs at a lower temperature, and requires a longer time; but it must 

 be remembered that the amount of heat produced by the oxidation 

 of a given weight of fuel is always the same whether the process 

 be rapid or slow. Tissue oxidations, therefore, are not necessarily 

 inefficient because they go on slowly. (3) The amount of work 

 that a man's organs do, is not dependent on the amount of oxygen 

 supplied to them, but the amount of oxygen used by him depends 

 on how much he uses his organs. It is necessary to emphasize 

 this fact because of the notion, which seems to be rather wide- 

 spread, that bodily processes are augmented by increasing the 

 supply of oxygen to them. The man who goes from his ill-ven- 

 tilated office to the open country, and feels the impulse to vigorous 

 exercise as he breathes the pure country air, is apt to attribute 

 his sensations of virility to an imagined augmentation of all his 

 bodily processes through the increased amount of oxygen breathed 

 in. The fact is that whatever augmentation of activity he may 

 experience is the result of the agreeable sensory stimulations com- 

 ing to him, which arouse his tissues to activity, either reflexly or 

 voluntarily. Increased oxygen consumption is, therefore, never 

 the cause, but always the result of augmented tissue activity. 



Respiratory Changes in Muscular Exercise. With every in- 

 crease in degree of muscular activity there is corresponding in- 

 crease in oxygen consumption and carbon dioxid production up 

 to a limit which is set by the ability of the blood to carry oxygen. 

 Since, as already noted (p. 421), the blood as it leaves the lungs is 

 virtually saturated with the gas under resting conditions, an in- 

 crease in the amount transported by it can come about only by 

 a more rapid flow of the blood or by a more complete use by the 



