176 



THE BIOLOGY OF MARINE ANIMALS 



capacities are shown by some diving mammals, up to 30% (Table 4.13). 

 Most bloods become saturated with oxygen below atmospheric tension. 

 The affinity for 2 is characterized by the dissocation curve, connecting 

 degree of saturation of the blood (percentage) with tension of 2 . Stand- 

 ard values, which indicate the nature of the dissociation curve and its 

 useful range, are given by tensions of loading and unloading. Loading 

 tension (f z ) refers to the tension at which the blood becomes 95 % saturated 

 with 2 ; tension of unloading (t u ) corresponds to half-saturation. Some 

 oxygen dissociation curves for haemoglobins of different animals are 



10 



20 



30 40 50 60 10 



Oxygen tension (mm Hg) 



80 



90 



Fig. 4.16. Oxygen Dissociation Curves for 

 Haemoglobin of Several Marine Animals 



Curves: 1 : Arenicola marina, 20 C C, pH 7-3; 2: Anguilla bostoniensis, 20 C C pH 7-3; 3: 

 Petromyzon marinus, 20 C C, pH 74; 4: Opsanus tau, 20°C, C0 2 1 mm Hg; 5: skate, 

 10-4°C, C0 2 1 - 0-5 mm Hg; 6: Phoca vitulina, 38°C, C0 2 46-47 mm Hg. (From 

 various sources.) 



shown in Fig. 4.16, and values for tensions of saturation (loading) and 

 half-saturation are listed in Table 4.13. 



Function of Haemoglobin. Haemoglobins of different animals often 

 possess quite dissimilar dissociation curves. The tension of half-saturation 

 indicates the facility with which 2 is transferred from blood to the tissues: 

 a high value for t u increases the availability of 2 to the tissues; a low 

 value indicates that oxygen is available only when supplies in the tissues 

 are nearly exhausted, and tensions are very low. Saturation tension is 

 linked with unloading tension, but does not become functionally signific- 

 ant until oxygen becomes depleted in the external medium, and falls below 

 100 mm Hg. 



The dissociation curve is typically sigmoid in shape, the slope differing 

 greatly from species to species (Fig. 4.16). At one extreme are animals with 

 haemoglobins which possess high t u values and have a low affinity for O s , 

 e.g. seal, porpoise, duck; at the other are animals such as eel and lugworm, 

 the haemoglobins of which show high affinity for 2 and possess low t u 

 and t ( values, Most aquatic poikilotherms have dissociation curves lying 



