SEC. 5. THE RESPIRATORY CHANGES IN THE LUNGS. 

 The Entrance of Oxygen. 



354. We have already seen that the blood in passing 

 through the lungs takes up a certain variable quantity (from 8 to 

 12 vols. p.c.) of oxygen. We have further seen that the quantity 

 so taken up, putting aside the insignificant fraction simply absorbed, 

 enters into direct but loose combination with the haemoglobin. 

 In drawing a distinction between the oxygen simply absorbed 

 and that entering into combination with the haemoglobin, it 

 must not be understood that the latter is wholly independent 

 of pressure. On the contrary, all chemical compounds are in 

 various degrees subject to dissociation at certain pressures and 

 temperatures ; and the existence of the somewhat loose compound 

 of oxygen and haemoglobin is dependent on the partial pressure of 

 oxygen in the atmosphere to which the haemoglobin is exposed. 

 Not only will a solution of haemoglobin or a quantity of blood 

 either absorb oxygen and thus undergo association or undergo 

 dissociation and give off oxygen according as the partial pressure 

 of oxygen in the atmosphere to which it is exposed is high or low, 

 but also the amount taken up or given off will depend on the 

 degree of the partial pressure ; the haemoglobin as we have seen 

 may be partially as well as wholly reduced. The law however 

 according to which absorption or escape thus takes place is quite 

 different from that observed in the simple absorption of oxygen by 

 liquids. The association or dissociation is further especially depen- 

 dent on temperature, a high temperature favouring dissociation, so 

 that at a high temperature less oxygen is taken up than would be 

 taken up (or, as the case may be, more given off than would be 

 given off) at a lower temperature, the partial pressure of the 

 oxygen in the atmosphere remaining the same. 



Moreover in the blood we have to deal not with haemoglobin 

 in simple solution, in which the molecules are dispersed uniformly 

 through the solvent, but with the haemoglobin segregated into 



