378 RESPIEATION [CH. XXIV. 



amount in which it is present in a mixture. This is true for each gas 

 in a mixture, the presence of the others making no difference. 



In the atmosphere, for instance, the total barometric pressure is 

 760 mm. of mercury ; the amount of oxygen in the air is roughly one- 

 fifth, and the pressure it exercises is also one-fifth of 760 ; the nitro- 

 gen accounts for the other four-fifths. The carbonic acid is present 

 in such small quantities that the pressure it exercises is only a frac- 

 tion of a millimetre. 



In the alveolar air (which can be obtained by catheterisation) the 

 carbonic acid is present in larger and the oxygen in smaller amount ; 

 and in the intermediate air passages there is an intermediate condi- 

 tion : hence, as in the two chambers we first considered, oxygen 

 diffuses down to the air vesicles, and carbonic acid from them. These 

 movements are, however, by themselves too slow to be efficient, and are 

 assisted by the large draughts which are created in the respiratory 

 tract by the respiratory movements of the chest. 



Catheterisation of the lungs. In animals determinations of the composition of 

 the alveolar air have been made in an occluded portion of the lung by Pfliiger's lung 

 catheter. This consists of a fine elastic catheter surrounded by a tube with an 

 enlargement towards the free end. It is introduced through the dog's trachea into 

 a bronchus, and it must be small enough to allow air to pass freely to other parts 

 of the lung. The rubber enlargement is then inflated ; this shuts off a portion of 

 the lung, from which the alveolar air is then withdrawn by the inner tube. In such 

 experiments, the alveolar air was found to contain 3 '5 per cent, of carbon dioxide, 

 whereas the expired air contained 2*8 per cent. The number 3*5 is higher than 

 normal, for under the conditions of the experiment it was undiluted with any tidal 

 air. Analysis of the air so obtained gives its composition after an equilibrium has 

 been set up with the gases of the blood, which is passing through the occluded por- 

 tion of the lung. 



Gases of the Blood. From 100 volumes of blood, about 60 

 volumes of gas can be removed by the mercurial air-pump. The 

 average composition of this gas in dog's blood is: 



Arterial blood. Venous blood. 



Oxygen 20 .... 8 to 12 



Nitrogen . . . . 1 to 2 . . . . 1 to 2 



Carbonic acid ... 40 .... 46 



The nitrogen in the blood is simply dissolved from the air just as 

 water would dissolve it; it has no physiological importance. The 

 other two gases are present in much greater amount than can be 

 explained by simple solution; they are, in fact, chiefly present in 

 loose chemical combinations. Less than one volume of the oxygen 

 and about two of carbonic acid are present in simple solution in 

 the plasma. 



Oxygen in the Blood. The amount of gas dissolved in a liquid 

 varies with the pressure of the gas; double the pressure and the 

 amount of gas dissolved is doubled. The oxygen of the blood 

 does not vary directly with oxygen pressure, for the amount of 



