BLOOD AND LYMPH 215 



ferricyanide, and that the carbon dioxide is liberated by 

 adding an acid. The amount of gas is estimated by measuring 



t/ i/ 



the increased pressure in the tube in which the gas has been 

 given off. 



About 60 c.c. of gas measured at C. and 760 mm. pres- 

 sure can be extracted from 100 c.c. of blood. The proportion 

 of the gases varies in arterial and in venous blood. 



AMOUNT OF GASES PER HUNDRED VOLUMES OF BLOOD 



Arterial Blood. Venous Blood. 



Oxygen 20 12 



Carbon dioxide . 40 46 



There are two ways in which gases may be held in such a 

 fluid as the blood 



1st. In simple solution. 



2nd. In chemical combination. 



Oxygen. At the temperature of the body the blood can 

 hold in solution less than 1 per cent, of oxygen. Now the 

 amount of oxygen actually present is about 20 per cent. So 

 that by far the greater quantity of the gas is not in solution. 

 We have already seen that it is in loose chemical union with 

 haemoglobin. 



Carbon Dioxide. In the animal body the blood can dissolve 

 about 2| per cent, of carbon dioxide. But it may contain 

 as much as 46 per cent., and this is uniformly distributed 

 between plasma and corpuscles. Hence the greater part of 

 the gas must be in chemical combination. Analysis of the 

 ash of the plasma shows that the sodium is more than sufficient 

 to combine with the chlorine and phosphoric acid, and is 

 thus available to take up carbon dioxide, as the carbonate 

 Na 2 C0 3 and the bicarbonate NaHC0 3 . Sodium carbonate 

 and basic sodium phosphate are therefore present together 

 in the plasma. 



If carbon dioxide is passed into a solution of sodium 

 phosphate it appropriates a certain amount of the sodium, 

 changing Na 2 P0 4 to NaHP0 4 . This is what happens in the 

 tissues where C0 2 is abundant. In the lungs, where the blood 



