RESPIRATION 



191 



present, is due to blood protein. In the coelomic fluids of Aplysia and 

 Echinus, for example, there is little protein and buffering is slight (cf. 

 Table 2.12, p. 64). Protein content is low in Urechis plasma, but haemo- 

 globiniferous corpuscles are present and buffering is due largely to the 

 latter. The blood of Urechis absorbs C0 2 by chemical combination up to 

 about 20 mm Hg, above which the dissociation curve becomes parallel to 

 that for H 2 and transport capacity is exhausted. In bloods containing 

 haemocyanin most of the C0 2 is combined with this pigment, and such 

 bloods show high buffering capacity. Haemolymphs of Palinurus, Limulus 

 and Sepia, for example, contain from 10-20 vols of C0 2 . Usually marine 



20 



30 40 50 60 10 SO 90 



Carbon dioxide tension (mm Hg) 



700 



Fig. 4.24. Carbon dioxide Dissociation Curves for Sea Water 

 and Different Bloods 

 Curves: 1 : sea water; 2: Aplysia fasciata; 3: Octopus macropus; 4: Palinurus vulgaris; 

 5: Scomber scombrus; 6: Phoca vitulina. (From various sources.) 



gill-bearing animals are exposed to very low C0 2 tensions (C0 2 0-23 mm 

 Hg) and utilize the steep portion of their dissociation curves (49, 130, 

 134). 



The reaction C0 2 + H 2 ^ H 2 C0 3 is relatively slow and is often 

 catalysed by an enzyme, carbonic anhydrase, found in tissues and cells. 

 Carbonic anhydrase occurs in vertebrate erythrocytes and in many 

 invertebrate tissues. Variable, often high, concentrations are found in gills 

 of fishes, especially in the pseudobranch; in the gills of Loligo, Limulus, 

 Homarus and Libinia (absent, however, in Palinurus); in gills of some 

 polychaetes; and in mantle tissue of lamellibranchs and gastropods (50, 

 63, 107, 146). 



In animals with calcareous shells and skeletons, e.g. bivalves, Crustacea 

 and starfish, the skeleton is an important source of buffer substance. 



