FUNCTIONS OF BLOOD 97 



respiratory organs, and the t u as high as possible to facilitate 

 the passage of oxygen from the blood to the tissues. The 

 following examples will indicate the compromise struck be- 

 tween the conflicting interests and illustrate the wonderful 

 adaptability of haemoglobin to meet the different and variable 

 cecological conditions. 



When dissociation curves are compared at a standard 

 temperature the differences in t u are very large, but they 

 become much reduced if each curve is made out for the 

 "normal" temperature of the organism in question, which 

 means in other words that haemoglobins are adapted to the 

 temperature at which they have normally to function. This 

 is brought out by comparisons between cold-blooded animals 

 from tropical and temperate climates and between the latter 

 and warm-blooded ones, but it would be interesting and 

 instructive to obtain curves for arctic forms which are quite 

 active and have a fairly high metabolism at temperatures 

 about the freezing point. 



Respiratory environment and blood characteristics. 1. Fairly 

 constant environment. Fishes living in the sea or in rapidly 

 flowing fresh waters which are nearly or completely in diffu- 

 sion equilibrium with the atmosphere are exposed to a very 

 constant environment with a high oxygen tension of 100-160 

 mm and a C0 2 tension usually below 1 mm with which the 

 blood is brought into practical equilibrium during its passage 

 through the gills. Such fishes have a high t x and generally 

 also a fairly high t u at the summer temperature to which the 

 fish is normally exposed. Cases in point are the cod, plaice, 

 and trout at 14-17°C (studied by Krogh and Leitch, 1919), 

 mackerel, sea-robin, and toadfish at 20° (Root, 1931), skate 

 at 10° (Dill, Edwards, and Florkin, 1932) and certain fresh- 

 water fishes, paku, bassara, and haimara, living in rapid 

 streams in the tropics and studied by Willmer at 28-30°C 

 (1934). It is a very interesting fact that in all these fishes 

 C0 2 has a profound influence on the affinity of the blood for 

 oxygen which is manifest and even pronounced at high 2 



