RESPIRATION: THE GASEOUS INTERCHANGES 379 



solved, will vary with the pressure of the gas in question; but 

 another portion, that chemically combined, will remain the same 

 under different pressures. The amount of this second portion de- 

 pends only on the amount of the substance in the liquid which can 

 chemically combine with it, and is totally independent of the 

 partial pressure of the gas. 



5. Bodies are known which chemically combine with certain 

 gases when the partial pressure of these is considerable, forming 

 compounds which break up, or dissociate, liberating the gas, when 

 its partial pressure falls below a certain limit. Oxygen forms such 

 a compound with hemoglobin. 



6. A membrane, moistened by a liquid in which a gas is soluble, 

 does not essentially alter the laws of absorption, by a liquid on one 

 side of it of a gas present on its other side, whether the absorption 

 be due to mere solution or to chemical combinations or to both. 



The Absorption of Oxygen by the Blood. Applying the physi- 

 cal and chemical facts stated in the preceding paragraph to the 

 blood, we find that the blood contains (1) plasma, which simply 

 dissolves oxygen, and (2) hemoglobin, which combines with it un- 

 der some partial pressures of that gas, but gives it up under lower. 



Blood-plasma or, what comes to the same thing, fresh serum, 

 exposed to the air, takes up no more oxygen than so much water: 

 about 0.56 volumes of the gas for every 100 of the liquid, at a 

 temperature of 20 C. At the temperature of the Body the volume 

 absorbed would be still less. This quantity obeys Henry's law. 



If fresh defibrinated blood be employed, the quantity of oxygen 

 taken up is much greater; this extra quantity must be taken up 

 by the red corpuscles and it does not obey Henry's law. If the 

 partial pressure of oxygen on the surface of the defibrinated blood 

 be doubled, only as much more oxygen will be taken up as corre- 

 sponds to that dissolved in the serum; and if the partial pressure 

 of oxygen on its surface be reduced to one-half, only a very small 

 amount of oxygen (one-half of that dissolved by the serum) will 

 be given off. All the much larger quantity taken up by the red 

 corpuscles will be unaffected and must therefore be chemically 

 combined with something in them. Since 90 per cent of their 

 dry weight is hemoglobin, and this body when prepared pure is 

 found capable of combining with oxygen, there is no doubt that it 

 is the hemoglobin in the circulating blood which carries around 



