380 THE RESPIRATION 



Taking all these facts together, a strong case appeared to be made 

 for the acidosis hypothesis and this seemed to be almost established since 

 Haldane and his collaborators were apparently able to apply it in ex- 

 plaining the results of their numerous investigations of the respiratory 

 function. It will be observed, however, that all of the evidence is cir- 

 cumstantial in nature, and that the changes observed in anoxemia may 

 be due to entirely different causes. Lowering of the alkaline reserve of 

 the blood, lowering of the tension of C0 2 in the alveolar air, and hyperp- 

 nea can undoubtedly all result from deficiency of oxygen-supply to the 

 tissues, but the sequence in which the changes occur may be exactly the 

 reverse of that which is assumed in the acidosis hypothesis; it is pos- 

 sible, namely, that the deficiency in oxygen first excites the respiratory 

 center, the increased breathing then causes a blowing oif of the free C0 2 

 from the blood, and the alkali that is thus set free is then excreted from 

 the blood in the endeavor to hold C H at a constant level. If some method 

 were available for precise measurements of C H of the arterial blood at 

 frequent intervals it would be possible to settle this question once and for 

 all, but such is not the case, and we must seek for proof for the new 

 hypothesis by indirect means.* Assuming then that the respiratory 

 center is excited by a slight degree of anoxemia, let us see how the known 

 facts fit in. The diminution in oxygen in the blood excites the respiratory 

 center and causes increased breathing. This results in a blowing off of 

 C0 2 from the blood into the alveolar air, so that there comes to be 

 relatively more C0 2 excreted than 2 absorbed, and the respiratory 

 quotient becomes raised. We have shown this very clearly in experi- 

 ments on decerebrated cats breathing in oxygen-poor atmospheres ; even 

 when the oxygen percentage was only slightly reduced, R.Q. had already 

 risen to over unity. As a result of this blowing-off of C0 2 , C H of the blood 

 must tend to fall and a condition of alkalosis, rather than of acidosis 

 results. This may explain the tendency for the breathing to return to- 

 wards the normal. It is important for practical reasons to realize that 

 when this condition becomes established increased breathing will not 

 compensate for the anoxemia, for the advantage gained by higher satura- 

 tion of the blood with oxygen is counteracted by the alkalosis in which 

 the hemoglobin holds on to the oxygen with greater avidity. When 

 such is the case, as already pointed out elsewhere (p. 401), the oxy- 

 gen is not so readily given up to the tissues. Under these con- 

 ditions neither increased breathing nor increased bloodflow can force 

 more oxygen into the tissues, so that both the respirations and the pulse 

 become less rapid after the initial acceleration. It is this prolonged 



*It may be stated, however, that CH of the blood of man after he has been for some time in 

 rarefied air is still normal as judged by determination of the dissociation curve of his blood (page 

 402) in a partial pressure of CO 2 equal to that of his alveolar air (Barcroft). This result shows 

 at least that the acid-base equilibrium is ultimately restored under the altered conditions. 



