SEC. 4. THE RESPIRATORY CHANGES IN THE LUNGS. 



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 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 hsemoglobin is dependent on the partial pressure of 

 oxygen in the atmosphere to which the haemoglobin is exposed. A 

 solution of hsemoglobin or a quantity of blood will either absorb 

 oxygen and thus undergo association or will 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, and the 

 amount taken up or given off will depend on the degree of the 

 partial pressure. But the law 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 dis- 



