72 



COMPARATIVE PHYSIOLOGY 



the initial part of the curv^e ; and differences in salt-content 

 and pH. play a part in determining differences in the form of 

 the dissociation curve from the blood of different species. 

 The effect of acid is of physiological importance, since it implies 

 (see Fig. i6) that the readiness of the blood to give up oxygen 

 is greater under the conditions — presence of CO2 — normally 

 associated with oxygen want. Rise in temperature also 

 increases the dissociation of oxy-haemoglobin at low tensions. 

 Thus the warm blood of the mammal or bird is better suited 

 as a carrier of oxygen to the tissues than a pure solution of 



20»-*OSObOV0^0 90**>0 



Fig. 16. — Dissociation curves of mammalian blood (after Parsons). 



haemoglobin at atmospheric temperature in the absence of 

 salts. Seeing that the haemoglobins of different species are 

 not identical, it is of interest to inquire into the extent to which 

 the properties of different haemoglobins can be correlated 

 with the conditions in which a given species pursues its exist- 

 ence. Investigations with this end in view have been carried 

 out by Krogh and Leitch (1919) on fishes, and by Barcroft 

 and Barcroft (1924) on the polychaete, Arenicola. 



Krogh and Leitch compared the oxygen dissociation curves 

 of the blood of several species of fishes between io°-20° C. in 

 the presence and absence of CO2 to make the data as complete 



