RESPIRATION 8i 



is in equilibrium, lOO c.c. of blood contain over 50 c.c. of carbon 

 dioxide. This combined carbon dioxide exists in the blood 

 in the form of sodium bicarbonate. It has been shown by 

 Parsons that the alkali comes from the dissociation of haemo- 

 globin, which exists in the blood on the alkaline side of its 

 isoelectric point as a sodium salt. Haemoglobin is a weaker 

 acid than its oxygen derivative, and oxygenation therefore 

 favours the displacement of COg in the competition between 

 the weak carbonic acid and the protein anion for a fixed amount' 

 of base (Fig. 21). 



The conditions of carbon dioxide in the blood of marine 

 invertebrates have been recently studied by Collip (1920) and 

 by Parsons and Parsons (1923). The state of affairs existing 

 in these animals is different in some respects from that which 

 is found in the mammal. Collip 's investigations, on repre- 

 sentatives of molluscs, arthropods, annelids, and coelenterates, 

 indicate that in general the amount of carbon dioxide taken up 

 at pressures greater than the low tension of CO2 in ordinary 

 atmospheric air are only such as would be accounted for by 

 physical solution. Practically all the available alkali is com- 

 bined to form bicarbonate at a carbon dioxide tension far 

 below that found even in the arterial blood of the mammal, 

 where the alkali reserve is not used up until comparatively high 

 tensions are attained. This peculiarity is of interest, firstly, 

 in relation to the part played by haemoglobin as an alkaline 

 salt in the blood of the mammal ; secondly, in relation to the 

 rate of metabolism which such an arrangement permits ; and 

 thirdly, as affecting the reaction of the blood which in the 

 normal life of the mammal is kept constant within fairly narrow 

 limits by the buffer action of the dissociated protein. 



These points have been investigated in several genera 

 by Parsons and Parsons (1923), from whose observations 

 emerges a very significant difference between the conditions 

 of carbon dioxide transport in the blood of comparatively 

 active free-living forms such as the crustacean genera Maia 

 and Palinurus or the cephalopod Octopus, and sluggish or 

 sedentary forms such as the mollusc Aplysia and the tunicate 

 Phallusia. In Aplysia and Phallusia the uptake of CO2 is 



G 



