332 CHANGES IN THE BLOOD. [BOOK IL 



The relations of Oxygen in the Blood. 



When a liquid such as water is exposed to an atmosphere con- 

 taining a gas such as oxygen, some of the oxygen will be dis- 

 solved in the water, that is to say will be absorbed from the 

 atmosphere. The quantity which is so absorbed will depend on 

 the quantity of oxygen which is in the atmosphere above ; that is 

 to say on the pressure of the oxygen ; the greater the pressure of 

 the oxygen, the larger the amount which will be absorbed. If on 

 the other hand water, already containing a good deal of oxygen dis- 

 solved in it, be exposed to an atmosphere containing little or no 

 oxygen, the oxygen will escape from the water into the atmosphere. 

 The oxygen in fact which is dissolved in the water is in a state of 

 tension, the degree of tension depending on the quantity dissolved; 

 and when water containing oxygen dissolved in it is exposed to any 

 atmosphere, the result, that is whether the oxygen escapes from the 

 water into the atmosphere, or passes from the atmosphere into the 

 water, depends on whether the tension of the oxygen in the water is 

 greater or less than the pressure of the oxygen in the atmosphere. 

 Hence when water is exposed to oxygen, the oxygen either escapes 

 or is absorbed until equilibrium is established between the pressure 

 of the oxygen in the atmosphere above and the tension of the 

 oxygen in the water below. This result is, as far as mere absorption 

 and escape are concerned, quite independent of what other gases 

 are present in the water or in the atmosphere. Suppose a half- 

 litre of water were lying at the bottom of a two-litre flask, and that 

 the atmosphere in the flask above the water was one-third oxygen; 

 it would make no difference, as far as the absorption of oxygen by 

 the water was concerned, whether the remaining two-thirds of the 

 atmosphere was carbonic acid, or nitrogen, or hydrogen, or whether 

 the space above the water was a vacuum filled to one-third with 

 pure oxygen. Hence it is said that the absorption of any gas 

 depends on the partial pressure of that gas in the atmosphere to 

 which the liquid is exposed. This is true not only of oxygen and 

 water, but of all gases and liquicU which do not enter into chemical 

 combination with each other. Different liquids will of course 

 absorb different gases with differing readiness; but, with the same 

 gas and the same liquid, the amount absorbed will depend directly 

 on the partial pressure of the gas. It should be added that the 

 process is much influenced by temperature. Hence, to state the 

 matter generally, the absorption of any gas by any liquid, will 

 depend on the nature of the gas, the nature of the liquid, the pres- 

 sure of the gas, and the temperature at which both stand. 



Now it might be supposed, and indeed was once supposed, that 

 the oxygen in the blood was simply dissolved by the blood. If this 

 were so, then the amount of oxygen present in any given quantity 

 of blood exposed to any given atmosphere, ought to rise and fall 



