TRANSPORT OF THE RESPIRATORY GASES 95 



8/x diameter in man, whereas the axes of the eUiptical amphibian 

 corpuscle may be as large as 25/x and ISfj, {Rand), 53/Lt and 28/x 

 {Necturus). The corpuscles of fishes range between 20jLt x 14/x 

 (dogfish) and 10/x x 7/x (white perch). In reptiles and birds the 

 size range is approximately the same, those of birds being 

 smaller than those of reptiles. The smallness of the corpuscles 

 determines the minimum diameter that is possible in the capil- 

 laries. The finer the capillaries the closer the oxygen-carrying 

 corpuscles can come to the tissue cells. 



The advantages of a respiratory pigment in the blood are 

 common to all animals. The first of these is to increase the 

 oxygen-carrying capacity of the blood. If blood were water, it 

 could carry only about 0-2 ml. of oxygen and 3-0 ml. CO2 in 

 each hundred ml. Vertebrate bloods can carry between 5 and 

 30 ml. O2/IOO ml. blood (Table 1). The importance of this in- 

 creased capacity is clear and it has been estimated that if all the 

 oxygen were carried in physical solution instead of in com- 

 bination with haemoglobin, a man would need either 30 times 

 the volume of blood or the blood would have to circulate 30 

 times more rapidly. The oxygen-carrying capacity is greater in 

 mammals and birds than among cold-blooded vertebrates, but 

 even within these groups there are marked variations. Diving 

 mammals such as seals have the highest capacity, the lowest 

 being found in those fish in the antarctic which do not possess 

 any haemoglobin. Mountain-living mammals, e.g. the llama, 

 have high oxygen capacities. The oxygen capacity of the blood 

 from active fishes is higher than that of sluggish species and very 

 high oxygen capacities have been recorded in some fish which 

 resort to air breathing. 



The haemoglobin functions by giving up oxygen where the 

 tension is lowest and absorbing it at high tensions. The relation- 

 ship between tension and percentage saturation of the blood is 

 not a linear one, however, and this is important for it increases 

 the usefulness of the pigment, as is apparent from the dis- 

 sociation curves shown in fig. 27. The sigmoid shape of this 

 curve ensures that over the effective working range the amount 

 of oxygen given up for relatively small changes in tension is very 



