RESPIRATION 67 



oxygen. Bohr found that at body temperature 2.2 cc. of oxygen 

 (measured at 0° and 760 mm.) go into simple solution in 100 cc. 

 of blood when the partial pressure of oxygen is one atmosphere/^ 

 and this is about 8 per cent less than dissolves in water. In the 

 alveolar air the partial pressure of oxygen is only about 13 per 

 cent of an atmosphere, and in the mixed arterial blood about 1 1 

 per cent, or 84 mm., of mercury. Hence the amount of free oxygen 

 dissolved in the 100 cc. of the arterial blood of a man is only about 

 0.24 cc. (measured at o°C. and 760 mm. pressure) whereas 

 about 17.4 cc. are present in combination with haemoglobin, as 

 will be shown below. It is evident, however, that the amount in 

 free solution is of great importance ; it depends upon the partial 

 pressure of oxygen in the atmosphere with which the blood is in 

 equilibrium; and, as already pointed out, Paul Bert found that 

 the physiological action of oxygen and of any other gas depends 

 upon its partial pressure in this atmosphere. 



From the standpoint of physical chemistry the "partial pres- 

 sure" of a gas in solution is simply the vapor pressure of the dis- 

 solved gas, i.e., its tendency to pass out of the solvent at any free 

 surface, or the gas pressure which will just balance this tendency 

 so that the amount of gas in solution neither increases nor de- 

 creases. But the vapor pressure of a substance in solution, or of the 

 solvent itself, varies directly, as I showed in a recent paper,^^ 

 with the diffusion pressure of the substance in solution. Hence 

 vapor pressure is a direct index of diffusion pressure; and this is 

 the reason why the partial pressure of a gas in solution is of so 

 great importance. It is owing to differences in diffusion pressure 

 that water or substances dissolved in it tend, independently of 

 active "secretory" processes, to pass in one direction or another 

 in the living body or outside it. For instance, when water passes 

 through a semi-permeable membrane into a solution of sugar or 

 salt, this is because the diffusion pressure of the pure water is 

 greater than that of the diluted water in the sugar or salt solution. 

 Van't Hoff's brilliant discovery that there is a connection between 

 the fundamental "gas laws" and the phenomena of osmotic pres- 

 sure was unfortunately marred by his failure to interpret either 

 the connection or the experimental facts correctly. As a conse- 

 quence, osmotic pressure and diffusion pressure were either com- 

 pletely misinterpreted or confused with one another. There seems 

 now to be no doubt that it is the diffusion pressures, and not the 



"Bohr, Nagel's Handbuch der Physiol., I, p. 62, 1905. 

 Haldane, Bio-Chemical Journal, XII, p. 464, 1918. 



