702 PHYSIOLOGY OF RESPIRATION. 



of the blood. On the other hand, if the blood supplying the center 

 is more arterialized than normal, by active ventilation of the lungs, 

 for instance, the center acts more feebly or may fail to act altogether, 

 giving the condition known as apnea. These facts may be accepted 

 as completely demonstrated, but they do not go far enough. When 

 we speak of the arterial blood being more venous than normal we 

 mean that it contains less oxygen and more carbon dioxid than 

 normal arterial blood. Which of these conditions serves to stimulate 

 the center, and which may be regarded as the constant stimulus 

 throughout life? Much evidence has been collected to show that 

 the action of the respiratory center is increased when the tension 

 of the C0 2 in the blood is raised without altering that of the oxygen, 

 and that a similar result is obtained, but much less readily, if 

 the tension of oxygen is greatly diminished without any change 

 in that of the carbon dioxid, so that it must be admitted that a 

 change in either factor may act as a stimulus. Experiments, how- 

 ever, have indicated that the accumulation of the CO 2 is much the 

 more efficient stimulus of the two.* Zuntz reports the following 

 interesting experiments, in which the extent of the respiratory 

 movements was measured by the amount of air breathed in a 

 minute. In one series the amount of oxygen in the air breathed 

 was reduced. This change did not affect the quantity of carbon 

 dioxid in the blood. The following results were obtained: 



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. 



oxygen " " = 9,093 to 12,810 " 



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

 effect upon 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. Haldane 

 states that when the oxygen in the air breathed is reduced 

 from 20.9 per cent, to 14 or 15 per cent., a concentration of 

 oxygen in which an ordinary flame will be extinguished, there is no 

 perceptible disturbance in breathing. If, however, a further re- 

 duction is made suddenly to 7 or 8 per cent, there is marked panting 

 and the lack of oxygen in the blood is indicated by the blue color of 

 the face. If the diminution in oxygen is effected gradually, as 

 happens, for example, when a person rebreathes the air in a 



* See Zuntz, "Archiv f. Physiologic," 1897, 379. See also Friedlander 

 and Herter, "Zeit. f. physiol. Chemie," 2, 99, and 3, 19. 

 t Haldane, "Journal of Physiology," 18, 442, 1895. 



