Respiratory Functions of Body Fluids 



331 



is high (40 mm. Hg in many mammals), whereas in the water to which the 

 gills of aquatic animals are exposed it is low (0.23 mm. Mg in equilibrium with 

 air). Actually, the corresponding blood CO.. tensions vary nearly in proportion, 

 so that in man there is a gradient of about 6 mm. between venous blood and 

 alveolar air, whereas in fish (fresh-water,^" marine'*-'') the CO2 gradient be- 

 tween venous blood and water is 4-9 mm. In mammals the difference between 

 arterial and venous CO^ indicates an unloading of about 10 per cent of the 

 total in the lungs and loading of a similar amount in body tissues. The total 

 COo in the blood is less and the percentage that is gained and lost in tissues 

 and gills is greater in aquatic vertebrates (Table 58) than in terrestrial verte- 

 brates. 



Reactions to CO^ in Mammalian Blood. The reactions in CO2 transport 

 in mammalian blood can be enumerated as follows: 



1. CO2 goes into solution in the blood, much of it diffusing into red cells. 

 The COo reacts with water in plasma and in red cells 



CO2+H2O ^=i H2CO3 



2. This reaction occurs too slowly to provide for the known rate of loading 

 in the tissues and unloading in the lungs; it is catalyzed by an enzyme, car- 

 bonic anhydrase, which is present in red blood cells 



CO2+H.O ^fbonic anhydrase ^^^^^ 



3. Carbonic acid dissociates according to mass action; it is a weak acid with 

 a pK of 6.1 



HoCO,:^=±rH + -fHC03- 



The dissociation of HCOa" is negligible at blood pH. 



4. Some anions are provided by plasma buffers which take up excess H + , 

 forming weak acids and leaving cations (B + ) to form salts with HCO.i" 



BHC03+H + =;=^H,C03+B ^ 



B proteinate-f-H ~ ^ ^ H proteinate-|-B + 



5. Some HCO3"" leaves the red cells in exchange for CI " from NaCl of 

 plasma. This provides Na+ to form NaHC03 and results in a shift of Cl~ 

 from plasma to cells in the tissues and back out of the red cells in the lungs. 



Tissues 



Lungs 



Plasma 

 CO, 



H2O 



HC O3 



ci- 



Red Cell 



Red Cell 



CO.. 



H.CO, 



H2CO3 



