RESPIRATION 



103 



partial pressure, 80% saturated at approximately 8 mm Hg of 

 oxygen and 50% saturated at 3 •5-4-8 mm Hg of oxygen. 



Temperature and pH both affect the position and shape of the 

 dissociation curves. At low temperatures the haemoglobin has a 

 more pronounced affinity for oxygen the curve becoming steeper 

 (Fig. 32a) and the unloading tension lower (Table 15). This is true 

 for two species, L. terrestris and A. longa (Haughton, Kerkut and 

 Munday, 1958). The dissociation curves of these two species are 

 not identical at the same temperature and it is thought that this 

 may be a reflection of the somewhat different ecology of the two 

 species, one remaining active throughout the year (Liimbricus) and 

 the other aestivating in the summer (Allolobophora). 



Table 15 



Values for Partial Pressures of Oxygen (mm Hg) Required to 



Saturate the Blood 



(From Haughton, Kerkut and Munday, 1958) 



A change in pH from 7-72-7-21 displaces the curve to the right 

 (Fig. 32b), that is to say there is a ''Bohr" effect, an increasing 

 acidity leading to the raising of the unloading tension (Manwell, 

 1959). Thus if carbon dioxide accumulated in the blood and 

 lowered the pH the unloading tension would rise and the respiratory 

 rate would also rise. As mentioned previously, however, carbon 

 dioxide in the atmosphere has no effect on respiration rates, 

 suggesting that the mechanism of acid— base balance effected most 

 likely by the calciferous glands, is efficient in binding excess CO2 

 rapidly. 



Manwell (1959) suggests that these curves indicate that the 

 haemoglobin of Lumbricus is suitable for unloading oxygen at sites 



