. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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 ( mm. Mg in equilibrium with air). Actually, the corresponding blood 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 a
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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 ( mm. Mg in equilibrium with air). Actually, the corresponding blood 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+ ^fbonic anhydrase ^^^^^ 3. Carbonic acid dissociates according to mass action; it is a weak acid with a pK of 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 ; 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 b
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