THE CHEMISTRY OF RESPIRATION 495 



capillaries and in the alveolar air is equal, and that the oxygen tension of the 

 latter is always slightly above that of the blood. In addition, attention has been 

 called to the fact that the pulmonary epithelium lacks all the essential char- 

 acteristics of a secreting membrane. In the mammals, for example, this lining 

 is composed, on the one hand, of small granular cells which are located in the 

 interstitial spaces between the capillaries and, on the other, of extremely thin 

 non-nucleated cells which are situated directly in the capillary wall- Besides, 

 this epithelial covering seems to be entirely lacking in birds, so that the surfaces 

 of the capillaries lie in direct contact with the air. Peculiarly enough, these 

 animals possess a very intense metabolism and must therefore be in a position to 

 interchange the gases with the greatest possible ease. In this connection, atten- 

 tion should also be called to the fact that the function of the pulmonary epithelium 

 cannot be deduced by analogy from that of the limiting membrane of the swim- 

 bladder, because the cells composing the latter are augmented by other cells which 

 form the so-called ''red glands" and exhibit true secretory properties. This same 

 statement could not justly be made regarding the lining cells of the alveoli. As 

 another point against the secretory theory might be mentioned the fact that the 

 respiratory activity may be altered at any time by increasing or decreasing the 

 CO 2 content of the inspired air or of that of the blood traversing the respiratory 

 center. Obviously, the assertion might be made that if the lining cells of the alveoli 

 were actually in possession of a secretory power, they should be able to resist 

 outside influences of this kind and should be under the direct control of the nervous 

 system. 



Douglass and Haldane'^ have recently attempted to solve this problem in an 

 indirect way by the use of carbon monoxid. It will be remembered that this gas 

 combines with the hemoglobin of the blood to form the more stable monoxid 

 hemoglobin. Thus, if blood is exposed to a mixture of O2 and CO, a certain 

 portion of each gas eventually unites with the hemoglobin, but inasmuch as the 

 latter possesses a much greater avidity for CO than for O2, a much larger amount 

 of CO enters into this combination. Assuming that the same conditions prevail 

 in the body during the inhalation of CO, these authors permitted an individual 

 to breathe a certain quantity of this gas until the blood became fully charged 

 with it. The percentage saturation of the Hb by the CO was then determined. 

 This value may justly be regarded as indicating the O2 content of the blood, be- 

 cause the amount of this gas which must be inhaled simultaneously with the CO 

 in order to produce the saturation just ascertained, is open to direct calculation. 

 These tests which were supplemented by inhalations of varying quantities of 

 oxygen, showed that the pressure of the oxygen in the arterial blood remains 

 below that of the air in the alveoli until the saturation of the hemoglobin with 

 carbon monoxid surpasses 30 per cent. Beginning at this point, the oxygen 

 tension decreases and is finally reversed. This observation led Haldane to con- 

 clude that the epithelial cells of the alveoli play an active part in the interchange 

 of the gases. Thus, it is stated that these Hning cells gather the oxygen under a 

 tension of 15 per cent, and force it to the other side of the membrane until its ten- 

 sion in the blood greatly exceeds that in the alveoli. 



Several objections may be raised against these experiments which render the 

 conclusions derived from them practically worthless. In the first place, it should 

 be noted that Haldane has employed the colorimetric method of estimating the 

 degree of saturation of the Hb by the CO, a method which has not as yet been 

 proved to be absolutely reliable. Secondly, it cannot rightly be assumed that the 

 avidity of the O2 and Hb remains the same throughout the course of these experi- 

 ments, and that the conditions under which these gases unite are the same in vivo 

 as in vitro. For these reasons, as well as others, Haldane has modified his previous 

 contention somewhat, and now seems to believe that the interchange of the gases 

 is accomplished under normal conditions by ordinary diffusion. Under abnormal 



1 Jour, of Physiol., xliv, 1912, 305. 



