350 RESPIRATION. 



The Relations of Oxygen in the Blood. 



286. When a liquid such as water is exposed to an atmosphere contain- 

 ing a gas such as oxygen, some of the oxygen will be dissolved in the water, 

 that is to say, will be absorbed from the atmosphere. The quantity which 

 is so absorbed will depend on the pressure of the oxygen in the atmosphere 

 above ; the greater the pressure of the oxygen, the larger the amount which 

 will be absorbed. If the pressure of the whole atmosphere remain the same, 

 at 760 mm. of mercury for instance (the ordinary atmospheric pressure), the 

 pressure of the oxygen may be increased or diminished by increasing or 

 diminishing the proportion of oxygen in the atmosphere. So that with an 

 atmosphere remaining at any given pressure the quantity of oxygen absorbed 

 will depend on the quantity present in that atmosphere. If, on the other 

 hand, water, already containing a good deal of oxygen dissolved in it, be ex- 

 posed 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, like the oxygen in the atmosphere above, stands at a certain 

 pressure, the amount of pressure 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 atmo- 

 sphere or passes from the atmosphere into the water, depends on whether 

 the pressure 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 be- 

 tween the pressure of the oxygen in the atmosphere above and the pressure 

 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 was 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 ex- 

 posed. This is true not only of oxygen and water, but of all gases and 

 liquids which do not enter into chemical combination with each other. 

 Different liquids will, of course, absorb different gases with differing readi- 

 ness ; but with the same gas and the same liquid, the amount absorbed will 

 depend directly on the partial pressure of the gas in the overlying space. 

 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 pressure 

 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 steadily and regularly as the par- 

 tial pressure of oxygen in that atmosphere is increased or diminished ; the 

 absorption (or escape) of oxygen ought to follow what is known as the 

 Henry-Dalton law of pressures. But this is found not to be the case. If 

 we expose blood containing little or no oxygen to a succession of atmospheres 

 containing increasing quantities of oxygen, we find that at first there is a 

 very rapid absorption of the available oxygen, and then this somewhat sud- 

 denly ceases or becomes very small ; and if, on the other hand, we submit 



