494 RESPIRATION 



from the following calculation of Loewy.^ The average thickness of 

 the membrane separating the alveolar air from the blood, amounts to 

 0.004 mm. In accordance with the diffusion rate of carbon dioxid 

 and nitrous oxid through the lung of a frog, the mammalian lung must 

 yield under a difference of pressure of 35 mm. Hg about 67 c.c. of oxy- 

 gen for each square centimeter of alveolar wall. The total absorp- 

 tion, therefore, amounts to 6083 c.c, a value much in excess of the 

 actual oxygen requirements of our body in quiet breathing. The lat- 

 ter is only about 250-300 c.c. It must be evident, therefore, that the 

 difference in the partial pressure of the oxygen could safely be much 

 reduced, and that a considerable portion of the total respiratory sur- 

 face could be rendered functionally useless, before a serious disturbance 

 in the normal supply of this gas would result. In the same way, it has 

 been established that the tension of the carbon dioxid in the blood 

 could be materially decreased without causing a fatal reduction in its 

 flow into the alveoli; in fact, as the speed of diffusion of this gas through 

 a moist membrane is twenty-five times greater than that of oxygen, 

 a difference in pressure of only 0.3 mm. Hg would suffice to yield the 

 250 c.c. of CO2 normally expired per minute. 



The chemical theory necessitates the assumption that the cells 

 forming the alveolar lining, actively participate in the transfer of 

 the gases. This end is accomplished with the help of inherent proc- 

 esses which are very similar to those occurring in the cells of the 

 secretory glands. Hence, we find here a condition analogous to that 

 existing in the walls of the air-bladder of the fishes. Inasmuch as the 

 contents of this organ consist at times of as much as 85 per cent, of 

 oxygen, the partial pressure of this gas must amount to 90 atmospheres, 

 while that of the oxygen in the surrounding water scarcely exceeds J^ 

 of an atmosphere (Biot). It must be obvious, therefore, that the air- 

 bladder of these animals is filled by a specific secretory activity of the 

 lining cells which is controlled by a special nervous mechanism. ^ 



The first attempt to show that the interchange of the gases in the lungs is not 

 one of simple diffusion was made by Bohr^ in 1890, but these results, indicating 

 that the oxygen tension of the blood frequently exceeds that of the alveolar air, 

 have been seriously criticised by Krogh, as well as by Haldane and Douglass. It 

 seems that certain errors in the manipulation of the aerotonometer and accidental 

 variations in the temperature have rendered these early determinations valueless. 

 In 1907 Bohr endeavored to substantiate his early contentions regarding the 

 secretory activity of the lung by the following experiment : If one lung is permitted 

 to obtain pure air and the other air containing 8.8 per cent, by volume of CO2, 

 the latter continues to give off CO 2 in spite of the fact that the tension of the CO2 

 in the venous blood of the right side of the heart equals that of an atmosphere 

 containing only 5 per cent, of this gas by volume. 



This entire subject has recently been reinvestigated by Krogh,** whose micro- 

 aerotonometric tests have shown that the pressure of the CO2 in the arterial 



1 Handb. der Biochemie, iv, 1908. 



2 Bohr, Jour, of Physiol., xv, 1893, 494. 



3 Skand. Archiv fiir Physiol., ii, 1890, 231. 

 * Ibid., xxii, 1910, 274. 



