ABSORPTION OF OXYGEN BY THE BLOOD. 383 



much water: about 0.56 volumes of the gas for every 100 of 

 the liquid, at a temperature of 20 0. At the temperature 

 of the body the volume absorbed would be still less. This 

 quantity obeys Dalton's law. 



If fresh whipped blood be employed, the quantity of oxy- 

 gen taken up is much greater; this extra quantity 'nust be 

 taken up by the red corpuscles (in possessing whic 7 . whipped 

 blood alone differs from blood serum) and it does not obey 

 Dalton's law. If the partial pressure of oxygen on the sur- 

 face of the whipped blood be doubled, only as much more 

 oxygen will be taken up as corresponds to that dissolved in 

 the serum; and if the partial pressure of oxygen on its sur- 

 face be reduced to one half, only a very small amount of 

 oxygen (J 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 com- 

 bined 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 haemoglobin in the circulating blood 

 which carries around most of its oxygen. The red corpuscles 

 are so many little packages in which oxygen is stowed away. 



The compound formed between oxygen and haemoglobin 

 is, however, a very feeble one; the two easily separate, and 

 always do so when the oxygen pressure in the liquid or gas 

 to which the oxyhaemoglobin is exposed falls below 26 mil- 

 limeters of mercury. Hence, in an air-pump, the blood only 

 gives off some of its small portion of merely dissolved oxy- 

 gen, until the pressure falls to about % of an atmosphere, 

 that is to -^ = 125 mm. (5 inches) of mercury, of which 

 total pressure one fifth (25 millimeters or 1 inch) is due to 

 the oxygen present. As soon as this limit is reached the 

 haemoglobin gives up its oxygen. 



Consequences of the Peculiar Way in which the 

 Oxygen of the Blood is Held. The first, and most im- 

 portant, is that the blood can take up far more oxygen in 

 the lungs than would otherwise be possible. Since blood 

 serum exposed to pure oxygen takes up only 3 volumes for 

 100, blood exposed to the air would take up $ only of that 



