158 



BIOLOGICAL CHEMISTRY 



the air is low, or when the oxygen requirements of the body 

 are increased by strenuous exercise, it may be that the oxygen 

 pressure in the blood may rise above that of the alveolar air, 

 or that the amount of oxygen taken up by the blood is greater 

 than can be accounted for by the process of diffusion. 



In either of these cases we must assume that the lung 

 secretes oxygen from the alveolar air into the blood, and it 

 is important to have such a definite case as the swim bladder 

 of oxygen-secreting fish to show the possibility of oxygen 

 secretion. 



The process of diffusion can be calculated by the difference 

 in oxygen tension between the alveolar air and the pulmonary 



Fio. 33. Drawing of tracheae FIG. 34. Diagram of lung of bird, 

 from wall of intestine of 



inc^f oVirvun'ricr Virviw fVi^ air The air sac A ^y its expansion, as shown by 

 insect showing how the air the dotted UneS] draws air past the alveoli of the 



IS brought into intimate lung B. Eddy currents will mix the air in the 



contact with the tissues. alveoli. Contraction of the air sac to the heavy 



The arrangement of the trachea 1 wil1 force , air out v a S ain - e W<J ves ? els 



differs with the structure of the tissues form a network over the surface of the alveoli. 



in which they are distributed. 



blood, after taking into consideration the area of the lung 

 surface through which diffusion can take place and the length 

 of time during which it is taking place. 



The recent experiment by Barcroft and his co-workers * 

 shows that at a low oxygen pressure and during the perform- 

 ance of work, the oxygen tension in the blood, as calculated 

 from the percentage saturation of haemoglobin, is less by about 

 7 mm. than the oxygen tension of the alveolar air. In the 

 absence of direct information as to the rate at which oxygen 

 will diffuse through the lung at this difference of pressure 

 we cannot say whether this disproves any secretory activity : 

 as the pressure is less in the blood it certainly indicates that 



* J. Barcroft, A. Cooke, H. Hartridge, T. R. Parsons and W. 

 Parsons, Jourw. Physiol., 1920, vol. 53, p. 450. 



