1180 PHYSIOLOGY 



straight line. The relation between the amount absorbed is not 

 altered by the presence of other gases at the same time. The pressure 

 of the whole atmosphere is 760 mm. Since the atmosphere consists 

 roughly of four parts of nitrogen with one part of oxygen, the atmo- 

 spheric pressure is due as to one -fifth to the oxygen and as to 

 four-fifths to the nitrogen. If we shake up water at C. with the 

 atmospheric air at the ordinary pressure, 100 c.c. of water will 

 absorb 4*89 c.c. X ^, and of nitrogen 2'39 x f . We may there- 

 fore extend our statement as to the solubility of gases in fluids and 

 say that the amount of gas dissolved in a fluid is proportional to the 

 partial pressure of the gas. 



When water is shaken up with a gas until it will take up no more, 

 i.e. until it is saturated for that pressure, a state of equilibrium exists 

 between the gas dissolved in the fluid and the gas in contact with the 

 fluid. In this state of equilibrium the number of molecules of the gas 

 entering the fluid is exactly equal to the number of molecules of the 

 gas leaving the fluid. If we remove the liquid after saturation, say, 

 at one atmosphere to a vessel where it is in contact with gas at a 

 pressure of half an atmosphere, the liquid will give off gas until the 

 amount left in solution is diminished to one-half. The gas dissolved 

 in a liquid thus has a pressure or tension which tends to make it escape 

 from the liquid. The only way in which we can measure this tension 

 is by rinding what pressure of gas is in exact equilibrium with the 

 liquid. Thus if we take some water containing carbon dioxide in 

 solution, divide it into two parts, and shake up one part with a gaseous 

 mixture containing 4 per cent, of carbon dioxide and the other part 

 with a mixture containing 5 per cent, of carbon dioxide, and find that 

 the solution loses gas to the former and takes up carbon dioxide from 

 the latter, we may conclude that the tension of carbon dioxide in the 

 original fluid was something between 4 and 5 per cent, of an atmosphere. 

 It is by some such means that the tensions of gases in the blood are 

 measured, the instruments for this purpose receiving the name of 

 aerotonometers . 



The solvent power of water for gases is diminished if the water 

 contains other solid substances in solution. Blood -plasma or blood - 

 corpuscles will therefore have a smaller solvent power for gases than 

 has pure water. It has been shown by Bohr that the depression of 

 solubility caused by the presence of proteins or salts in solution is 

 the same for all gases. The absorption coefficient of blood-plasma 

 for gases is reduced to 97 -5 per cent, of pure water, and of blood 

 to 92 per cent., that of the blood -corpuscles being as low as 81 per 

 cent. We may thus reckon the absorption coefficient of blood- 

 plasma, blood, and blood -corpuscles, for oxygen, nitrogen, and 

 carbon dioxide. 



