190 Chapter XII 



even on their own ground the supporters of the diffusion theory had 

 gone far to prepare their own downfall. 



Those who opposed the secretory had thrown upon their antagon- 

 ists the whole onus of proving that there was a difference of oxygen 

 pressure between the arterial blood and the alveolar air. They said 

 in effect, " Unless you can show that the oxygen pressure in the 

 arterial blood is greater than that in the alveolar air, diffusion will 

 suffice to explain the passage of a gas from one to the other." Their 

 own aerotonometer results showed no measurable margin between the 

 two tensions ; they were therefore complacent enough. It was at 

 this point that Bohr replied, " Diffusion from one side of the alveolar 

 epithelium to the other must involve some difference of pressure ; 

 this may be immeasurably small, or it may not : the physical theory 

 at least presents the advantage that it offers the opportunity of 

 calculating the difference of pressure necessary to produce the flow 

 of gas." 



He attacked the matter as follows. The first operation which 

 confronts a molecule of oxygen, which would pass from the alveolar 

 air through the epithelium, is the physical one of passing from the 

 gas into the fluid surface covering the lung. He therefore proceeded 

 to investigate the laws which governed the passage of gases into 

 fluid surfaces with the following result. 



The argument was as follows : 



() To calculate #, the number of cubic centimetres of gas which enters a surface 

 in a minute, let s be the area of the surface, p the pressure of the gas, and y the 

 invasion coefficient which is defined as the amount of gas which enters 1 sq. cm. of 

 the surface in one minute at the atmospheric pressure. 



syp 



Q= -*-=- 



760 



(6) To calculate &, the quantity of gas which leaves the surface of a fluid charged 

 with the gas in a minute, where s the area of the surface, the quantity of gas dissolved 

 in 1 c.c. of the fluid, and /3 the evasion coefficient which is denned as the quantity 

 of gas that leaves 1 sq. cm. of the surface in one minute when 1 c.c. of the fluid holds 

 1 c.c. of the as. 







(c) Consider the special case of a fluid which is in equilibrium with a gas, the 

 pressure of the gas being 760 mm. Since the equilibrium exists, the number of 

 molecules of the gas which enter and leave the surface must be equal, and there- 

 fore 



</ = &, 

 in other words 



