RESPIRATION 101 



isolated tissue respiration so any effect of CO is due to haemoglobin 

 blockage in the intact animal. It is clear, therefore, that haemo- 

 globin in the earthworm functions as a carrier of oxygen at all 

 times in fulfilment of function 1. The actual proportion of the 

 body's needs that are carried in this way, however, differ some- 

 what with changing oxygen tensions. At 152 mm Hg (normal 

 atmospheric air) haemoglobin carries approximately 20% of the 

 total requirements, at 76 mm Hg the proportion is 35%, at 

 38 mm Hg 40% and at 19 mm Hg 22%. From these figures 

 Johnson (1942) deduced that the loading pressure of earthworm 

 haemoglobin is higher than 19 mm Hg at 10 °C. The remainder of 

 the earthworm oxygen requirement is carried in physical solution. 

 The respiration of Tiibifex is also decreased by carbon monoxide 

 treatment, about a third of the normal oxygen consumption being 

 depressed when the oxygen tension is high, proportionately less at 

 lower tensions (Dausend, 1931). 



Oxygen Dissociation Curves 



Oxygen carried in combination with haemoglobin to the tissues 

 is there released in response to a relatively low oxygen tension. It is 

 possible to obtain curves that indicate the way in which haemo- 

 globin combines and releases oxygen. Only within the last few 

 years have techniques been worked out for preparing such curves 

 using the small quantities of blood available from earthworms. By 

 the use of spectrophotometric methods, however, Haughton, 

 Kerkut and Munday (1958) and Manwell (1959) have been able to 

 elucidate the proportion of reduced and oxygenated haemoglobin 

 in a mixture of the two forms. 



The curve obtained plotting percentage saturation of haemo- 

 globin w4th oxygen, against partial pressure of oxygen available, 

 is sigmoid in shape. The haemoglobin remains 95% saturated until 

 a very low partial pressure is reached (Fig. 32a, Table 15). There- 

 fore at all normal atmospheric levels of oxygen the haemoglobin 

 of earthworm blood is saturated with oxygen. Only when the 

 external tension drops greatly does the haemoglobin unload, this 

 occurring normally at the tissues. The unloading tension, deduced 

 by Johnson (1942) as rather more than 19 mm Hg oxygen at 10 °C 

 is incorrect, as Manwell (1959) working at the same temperature 

 has shown that Lnmhriciis haemoglobin is 85% saturated at this 



