306 Comparative Animal Physiology 



the test. However, Webb pointed out that some tunicates which lack vana- 

 dium still produce tunicin. It appears certain that the vanadium in tunicate 

 blood has no respiratory function. 



OXYGEN TRANSPORT BY BLOOD PIGMENTS 



Function of Hemoglobin 



Oxygen Capacity. The first requisite of an oxygen transporter is ability to 

 combine with enough oxygen to supply the needs of the animal. The oxygen 

 capacity is the measure of the amount of oxygen combined with blood or with 

 blood cells when they are saturated. It is usually expressed as volumes per cent 

 of oxygen in whole blood or cells, and is determined by measuring the amount 

 of oxygen combined after the blood sample is saturated with air. In experi- 

 mental equilibration of some blood cells, particularly of nucleated erythrocytes, 

 the amount of oxygen consumed by the cells is considerable and must be taken 

 into account. The oxygen capacity depends largely on the amount of hemo- 

 globin or other pigment in the blood or blood cells. 



Table 55 shows the oxygen capacity of the blood of a number of animals. 

 The oxygen capacity of the blood of mammals and birds is usually between 15 

 and 20 volumes per cent. In some but not all of the diving mammals (seal, sea 

 lion, porpoise), the cells have unusually high Oo capacity. Also the llama, even 

 at sea level, has a high O2 capacity. Among cold-blooded vertebrates the O2 

 capacities are lower, usually between 5 and 12 volumes per cent. In a few 

 fishes which inhabit sluggish, acid ponds and can resort to air breathing (hassa 

 and electric eel), unusually high O2 capacities have been reported. ^^'^ Among 

 the invertebrates the O2 capacity also corresponds to the amount of respiratory 

 pigment. Individual variation within a species is great. Where there is a 

 significant amount of pigment, as in Arenicola, Urechis, Spirographis, Caii- 

 dina, Octopus, Loligo, and a few others, the oxygen capacity of the blood is 

 roughly ten times as much as it would be without the pigment. In many 

 other animals the blood at equilibrium with air holds only about 0.5 to 2 

 volumes per cent. Sea water dissolves 0.538 volumes per cent of oxygen at 20° 

 when saturated with air.^^ 



Oxygen Dissociation Curve. In no animal is the blood exposed directly to 

 atmospheric tensions of oxygen (150 mm. Hg). Most respiratory pigments 

 become saturated at much lower oxygen tensions. The most important differ- 

 ences among the hemoglobins of different animals are in the tensions at which 

 they load and unload oxygen. These tensions determine the range of useful- 

 ness of a particular pigment. 



In the lungs of man the blood is exposed to oxygen at a partial pressure of 

 approximately 100 mm. Hg. When the blood leaves the lungs it carries 19 

 volumes per cent of oxygen at 80 mm. Hg and 96 per cent of its hemoglobin 

 is saturated. In the capillaries the blood passes through tissues where the 

 oxygen pressure is low (5-30 mm. Hg). Here 25 to 30 per cent of the oxygen 

 is unloaded, and venous blood returning to the heart carries 14 volumes per 

 cent of oxygen at about 40 mm. Hg pressure. 



The relation of the oxygen held by the hemoglobin and the partial pressure 

 of oxygen is best seen by plotting the per cent saturation of the hemoglobin 

 against oxygen tension, the so-called oxygen dissociation curve. This curve is 



