256 RESPIRATION 



take up water from the plasma, which accordingly has a more concen- 

 trated supply of food-substances to offer to the tissues than the plasma 

 of arterial blood itself. Some writers see in this interchange an auto- 

 matic arrangement by which oxidation is favoured. Whatever may be 

 thought of this view and objections to it are not wanting the current 

 theory, that the corpuscles are simply passive carriers of oxygen, and 

 exercise no further influence on the plasma, breaks down in face of the 

 facts. We must admit that an active and many-sided commerce exists 

 between them and the liquid in which they float. 



The nitrogen of the blood is simply absorbed.* 



The Tension of the Blood-Gases. If the gases of the blood existed 

 in simple solution, their tension or partial pressure could be deduced 

 from the amount dissolved and the coefficient of absorption. We 

 have seen that they are mainly combined, and it is characteristic 

 of dissociable compounds of this kind that the relation between the 

 partial pressure of the gas in contact with the liquid and the quantity 

 of gas taken up is much more complicated than in the case of pure 

 physical absorption. It is therefore necessary to determine the 

 tension directly. 



This can be done by means of arrangements called aerotonometers. 

 There are various forms of aerotonometer, but the object of all is to 

 bring the blood into contact with an atmosphere or gaseous mixture 

 into which the gases of the blood can diffuse and from which gases can 

 enter the blood. When the composition of the mixture has ceased to 

 change, the gases in it are under the same partial pressure as the corre- 

 sponding gases in the blood, and all that is necessary in order to arrive 

 at the tension of the blood-gases is to determine the final composition 

 of the gaseous mixture by analysis. The speed with which equilibrium 

 is attained depends essentially upon the magnitude of the surface of 

 contact between the blood and the gas mixture in proportion to the 

 volume of the gas space. In the earlier observations with the aerotono- 

 meter it was found to be very difficult to get complete equilibrium, and 

 therefore the gas space was filled at the beginning with a mixture whose 

 gases had partial pressures as nearly equal as possible to those expected 

 in the blood. In one form of the apparatus the blood is made to pass 

 directly from the vessel to glass tubes, which it traverses at the same 

 time, the stream being divided between them; it then passes out again. 

 The tubes are warmed by means of a water-jacket to the body-tem- 

 perature. Some of them are filled with gaseous mixtures having a 

 greater, and the others with mixtures having a smaller, partial pressure, 

 say of /carbon dioxide, than is expected to be found in the blood. As 

 the latter runs in a thin sheet over the walls of the tubes, it loses carbon 

 dioxide to some of them and takes up carbon dioxide from others. 

 From the alteration in the proportion of the carbon dioxide in the tubes, 

 the partial pressure of that gas in the blood is calculated that is, the 

 partial pressure which would be necessary in the tubes in order that 

 the blood might pass through them without losing or gaining carbon 

 dioxide (p. 247). 



Bohr's aerotonometer, constructed and worked much in the same 

 way as a stromuhr (p. 121), permits the blood after passing through the 

 gas space to return to the circulation. A stream of blood can thus be 



* But according to Buckmaster and Gardner, the volume of nitrogen in 

 blood does not follow the ordinary physical laws of absorption with varying 

 nitrogen pressures in the alveolar air. 



