312 



Comparative Animal Physiology 



at a given Oo tension addition of CO:; facilitates unloading or reduction, while 

 in the lungs as CO^ is given off the uptake of O2 (oxygenation) is facilitated. 

 The effect of CO^ can be duplicated by lowering the pH of the blood. In 

 Table 56 and in Figures 69, 70, and 71, the effects of variations in CO^ tension 

 or in pH on the t]/2 sat of the bloods of several animals are compared. In 

 aquatic animals (e.g., fish), the CO2 effect on the dissociation curve is greater 

 than it is in terrestrial animals (particularly mammals). This is related to the 



LOGp, 



SOX SAT. 



Fig. 69. Effect of pH on log ti/2 s^t of the blood of several animals. 

 Data assembled by Redfield.^"^ 



fact that the normal CO2 tension in blood of aquatic animals is much lower. 

 Also aquatic animals which live in stagnant water usually have hemoglobin 

 which is less sensitive to COo than is that of animals inhabiting fast moving 

 waters. 



Effect of Temperature Changes. A rise in temperature also shifts the oxygen 

 dissociation curve to the right. If the blood of a frog is warmed to 35° C. its 

 O2 dissociation curve is far to the right of man's, whereas at 15° human blood 

 remains saturated at low tensions, hence would be of little use as a carrier. 

 The temperature effect on ti/2 sat in the frog is about twice as great as it is in 



man.^*^" 1 he effects of variations in temperature on tj^ 



are shown in 



Figures 72, 73, and 74. Both the CO2 effect and the temperature effect are of 

 considerable ecological importance. 



Effect of Altitude. At high altitudes the hemoglobin content and the red 

 cell count increase in man so that the oxygen capacity of the blood becomes 

 elevated (Table 57). The tensions for half-saturation of the hemoglobin do 

 not change significantly when measured at a constant pH in man acclimatized 

 to high altitudes;"*- i.e., the nature of the hemoglobin is not altered. Physio- 



