262 THE ANIMAL. XII. 



The nitrogen and argon are simply in solution in the blood 

 just as they would be in water ; but in the case of oxygen and 

 carbon dioxide the quantity present is much greater than can be 

 explained by simple solution. 



In the case of oxygen, for example, the amount present does 

 not vary appreciably with the pressure of the gas in the lungs, 

 whereas the amount of a gas dissolved by a liquid is directly 

 proportional to the pressure of the gas. The greater portion 

 of the oxygen (probably all but about one volume per 100 

 volumes of blood) is in the state of combination with haemo- 

 globin. The oxyhaemoglobin is, however, easily dissociated, 

 and if the quantity of dissolved oxygen be diminished from 

 any cause the combined oxygen is diminished proportionately. 

 The great bulk of the oxygen of the blood is thus to be found 

 in the red corpuscles, while only a small quantity is in the 

 plasma. 



With carbon dioxide the case is different. Of the 40 to 46 

 volumes of the gas present in 100 volumes of blood about two 

 are in the state of simple solution, the remainder in a state of 

 weak combination, partly with the haemoglobin of the red cor- 

 puscles, but mainly as bicarbonates in solution in the plasma. 



Respiration consists in bringing air into close proximity to 

 the blood stream in the lungs, separation being only maintained 

 by the thin walls of the capillaries and air cells or alveoli. 

 The air in the alveoli is not renewed by mechanical expulsion 

 due to inspiration and expiration, but by diffusion from and 

 into the bronchial tubes. 



The partial pressure of the dissolved carbon dioxide in the 

 venous blood is greater than that in the air of the alveoli ; 

 consequently the blood plasma loses carbon dioxide, thus pro- 

 ducing dissociation of the bicarbonates in solution. The 

 partial pressure of the dissolved oxygen in the venous blood is, 

 however, less than that of the alveolar air, and consequently 

 the plasma takes in oxygen. This disturbs the equilibrium 

 between the dissolved oxygen of the blood plasma and the 

 combined oxygen of the oxy haemoglobin, causing the formation 

 of more of the latter with a simultaneous diminution of the 

 former. The blood plasma is thus able to take in a further 

 quantity of oxygen from the air. In this way it is probable 



