408 THE HUMAN BODY. 



which combines with it under 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 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 must be 

 taken up by the red corpuscles (in possessing which 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 (one half of that dissolved by the serum) will be given 

 off. All the much larger quantity taken up by the red corpus- 

 cles will be unaffected and must therefore be chemically com- 

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

 dry weight is haemoglobin, 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 25 mil- 

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

 gives off some of its small portion of merely dissolved oxygen, 

 until the pressure falls to about -J- of an atmosphere, that is 

 to -3$o. z= 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 passed the haemo- 

 globin gives up its oxygen with a rush. 



Consequences of the Peculiar Way in which the Oxy- 

 gen of the Blood is Held. The first, and most important, 

 is that the blood can take up far more oxygen in the lungs 

 than would otherwise be possible. Since blood serum ex- 

 posed to pure oxygen takes up only 3 volumes for 100, blood 



