510 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



THE BLOOD AS CARRIER OF CARBON DIOXIDE. 



The solubility of carbon dioxide in water at N.T.P. is about 100 

 c.c. in 100 c.c. The solubility in blood is about the same figure, but 

 an additional amount dissolves in the alkali of the medium as carbon- 

 ate and bicarbonate and the proteins of the blood also form an unstable 

 combination with carbon dioxide. Buckmaster l found that the amount 

 of carbon dioxide absorbed by blood rises and falls with the amount 

 of haemoglobin, and concluded that haemoglobin was capable of ab- 

 sorbing considerable quantities of carbon dioxide at pressures between 

 70 and 760 mm. Consequently, haemoglobin has also the function of 

 carrier of carbon dioxide. 



The amount of carbon dioxide in solution in the blood does 

 not approach saturation of the liquid with the gas. 100 volumes of 

 venous blood contain about 46 volumes of carbon dioxide. Most of 

 the carbon dioxide is in combination as carbonates, only about 5 per 

 cent, being actually in solution. This has been determined by a 

 similar method to that used in ascertaining the saturation of haemo- 

 globin by exposing blood to different amounts of carbon dioxide and 

 analysing the gas before and after the experiment. Blood absorbs 

 carbon dioxide at a partial pressure of carbon dioxide higher than 

 5 mm. and gives it up at a partial pressure lower than 5 mm. 



The carbon dioxide exchange in the animal body is in the converse 

 order to that of the oxygen. 



The Blood as a Solvent for Nitrogen. 



Water dissolves 2 c.c. of nitrogen at N.T.P. per 100 c.c. A little 

 more than this amount is dissolved by blood, but the amount of 

 nitrogen in water or in blood exposed to the atmosphere which contains 

 4/5 of its volume as nitrogen is x 2 or i'6 c.c. This amount of 

 nitrogen is always present in blood, whether it be arterial or venous. 



Estimation of Carbon Dioxide in Blood. 



A rough estimation of the carbon dioxide in blood can be made 

 in a similar apparatus to that used in estimating oxygen by Haldane's 

 method. The oxygen is first liberated or estimated. The carbon 

 dioxide is liberated by disconnecting the apparatus, putting tartaric 

 acid in the small tube, again connecting the various parts and 

 upsetting the tartaric acid into the liquid ; the gas must be collected 

 at a constant pressure by attaching the collecting burette at its lower 

 end to a collapsible reservoir to which is connected another burette 

 for reading the pressure. Corrections must be made for the solubility 

 of the carbon dioxide in water. The estimation may also be carried 

 out in the differential apparatus of Barcroft (p. 504). 



The data for carbon dioxide are not so accurate as those for 

 oxygen. 



1 J. Physiol., 1917, 51, 164. 



