642 PHYSIOLOGY OF RESPIRATION. 



Normal air volume breathed per minute = 7,325 to 9,000 c.c. 



Air with 10 to 11.5 per 



cent, oxygen " " " " 8,166 to 9,428 " 



Air with 8 to 10 per 



cent, oxygea " = 9,093 to 12,810 " 



A reduction of one-half of the oxygen in the air breathed had little 

 effect upon the respirations. From our present standpoint, how- 

 ever, the important thing i not the amount of oxygen in the air, 

 but the amount in the blood. Paul Bert's experiments* upon 

 living animals indicate that when the oxygen of the air is reduced 

 by a half the amount of oxygen in the blood is diminished by about 

 one-third. Assuming this to be correct, it is evident that a very 

 considerable reduction may be made in the oxygen of the blood 

 without noticeably affecting the respirations. A similar conclusion 

 may be drawn from Haldane's experiments f with carbon monoxid. 

 He found upon breathing mixtures of this gas that no distinct effects 

 were observable until the blood was about one-third saturated with 

 the gas that is, had lost one-third of its oxygen. Zuntz's ex- 

 periments, in which the C0 2 in the air breathed was increased, while 

 the oxygen remained normal, gave quite different results, as follows : 



Normal air volume breathed per minute, 7,433 c.c. 



Air of 20.2 per cent. O, 0.95 per 



cent. CO 2 " " " " 9.060 " 



Air of 18.06 per cent. O, 2.97 per 



cent. CO 2 " ." " " 11,326 " 



Air of 18.42 per cent. O, 11.5 per 



cent. CO 2 " " 32,464 " 



These and similar results show that small differences in the 

 amount of the carbon dioxid in the blood have a distinct effect 

 upon the activity of the respiratory center. Under normal con- 

 ditions the respiratory center receives blood containing 19 to 20 

 volumes per cent, of oxygen, while the venous blood flowing away 

 from the center still holds 10 to 12 per cent. Considering the 

 small effect of lowering this oxygen supply by one-third, it is 

 difficult to believe that normally the amount of oxygen is so 

 deficient for the normal metabolism as to set up a constant 

 stimulus. The trend of recent work favors rather the view that 

 the normal stimulus to the respiratory center is the carbon dioxid. 

 When this substance is present above a certain amount or tension 

 it acts as a stimulus and gives rise to the moderate movements of 

 normal inspiration. If the tension of the carbon dioxid is increased 

 its stimulating action becomes stronger and leads to the production 

 of a condition of hyperpnea and dyspnea. On the other hand, 

 if for any reason, such as active ventilation of the lungs, the tension 



*Bert, "La pression barometrique," 1878, 691. 

 fHaldane, "Journal of Physiology," 18, 442, 1895. 



