314 



Comparative Animal Physiology 



more resistant to low oxygen than older birds because of the greater affinity 



for oxygen.'-" , . i i 



The blood of the bullfrog tadpole becomes saturated with oxygen at lower 

 tensions than does the blood of the adult frog; the tadpole dissociation curve 

 also is more hvperbolic. Changes in pH alter the shape as well as the position 

 of the dissociation curves of the blood of the frog and tadpole.^'- In the tad- 

 pole the curve moves slightly to the left when pH is decreased, whereas in the 

 adult the tensions of loading and unloading are raised as in other animals. 

 Hemoglobin of terrapin embryos also has a higher Oo affinity, that is, a lower 

 t,. o „, than the hemoglobin of adults."'^ Hemoglobins of embryos m general 



lO 20 30 40 



PARTIAL PRESSURE CO, IN mm H3 



Fig. 71. Effects of CO^. on the O, tension for half saturation of blood. © alligator, Dill 

 and Edwards"; O t^ana temporaria, Rana esctilenta, Wolvekamp^*''; A toadfish, + 

 mackerel, Root'"; * carp, Black and Irving'"; IX bowfin, © sucker, p^ brook trout, 

 ^ cathsh. Black'"'; {J paku, Willmer.'" 



arc, therefore, functional at lower oxygen tensions than are adult hemoglobins. 

 Relation of Blood Pigments to Distribution of Amphibians and Fishes. 

 C3xygen dissociation curves for a number of amphibia arc shown in Figure 

 76. Those genera which spend much of their time on land show oxygen 

 dissociation curves to the right of the curves for aquatic forms; i.e., the loading 

 tensions are higher with greater availability of oxygen. The curve of Crypto- 

 branchiis may represent a reversion hom land to water. "^ Similarly, in a series 

 of seven species of turtles, the terrestrial box turtle has the lowest oxygen 

 affinity (tj/^ hui=28.5 mm. Hg), whereas aquatic turtles have higher O2 



