SEC. 4. THE RESPIRATORY CHANGES IN THE LUNGS. 



285. The Entrance of Oxygen. 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 enter- 

 ing 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 disso- 

 ciation 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 either partially or wholly reduced. The law how- 

 ever 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 fur- 

 ther especially dependent on temperature, a high temperature 

 favouring dissociation, so that at a high temperature less oxy- 

 gen 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 tempera- 

 ture, 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 uni- 

 formly through the solvent, but with the haemoglobin segre- 

 gated into minute isolated masses, bottled up as it were in the 

 individual corpuscles. The haemoglobin of each corpuscle is 



462 



