Respiratory Functions of Body Fluids 323 



therefore, appears to be more dependent on its hemoglobin than is Linnbriciis. 

 Nereis diversicolor consumes approximately ten times as much oxygen as 

 Lumhricus, under comparable conditions. 



Chirononius larvae and tubificid worms can live at the bottom of ponds 

 where the oxygen content approaches zero. Those species of Chironomus 

 larvae which lack hemoglobin are not so resistant to low oxygen as those with 

 hemoglobin. Using a microspectrometric method for oxyhemoglobin deter- 

 mination, Leitch^^'^ found that at 20° in the absence of CO^. the blood of larvae 

 of Chironovius sp. was half saturated at the exceedingly low oxygen tension 

 of 0.17 mm. Hg. In the presence of 1 per cent CO^ it was 38 per cent saturated 

 at this tension. The very low t,/^ snt was confirmed (0.6 mm. Hg) by Fox*'" 

 with Chironomus riparus, although the decreased oxygen affinity with COo 

 was not confirmed. The oxygen capacity of the blood is 6 volumes per cent, 

 and the total combined oxygen could supply the needs of the larvae for only 

 12 minutes; hence function as an oxygen store is unimportant.^"^ From the 

 very low tj/o sat it might be concluded that tissues could scarcely make use of 

 oxygen delivered at such low tensions. However, the oxyhemoglobin bands 

 disappear in vivo when the oxygen in the water corresponds to 13 mm.**" In 

 Chironomus plumosus the O- consumption is normal in the presence of CO 

 down to Oo tensions of approximately 75 mm. (3 cc. O2/I.) but is inhibited 

 below this level (Fig. 78).^^ It was also indicated''''^' ^■'- that in payment of an 

 oxygen debt after a period of anoxia the excess oxygen consumption in Chiron- 

 omus thummi is sensitive to CO in concentrations which poison the hemo- 

 globin. Here, then, is evidence that the Chironomus hemoglobin unloads at 

 oxygen tension of less than 1 mm. Hg (0.079 per cent atm., according to Fox 

 ^"), and, at the other extreme, evidence that the hemoglobin functions in 

 oxygen transport at atmospheric tensions outside the animal. The unloading 

 tension of blood in vitro cannot be used to determine the oxygen tension in 

 the medium at which a pigment is functional. It appears certain that in chiron- 

 omid larvae living in low oxygen the oxygen tension in the tissues is very low 

 and that a steep oxygen gradient exists from water to tissues. The oxidative 

 enzyme pattern in the tissues must differ from that in the vertebrates to make 

 possible functioning at partial oxygen pressures of 0.5 mm. Hg. In a chiron- 

 omid Tantytarsus, which lives in lakes where the water is never more than 

 half saturated with air, the hemoglobin functions in transport only if the 

 dissolved oxygen is less than 25 per cent air saturation. ^^^ 



In Tuhifex also the hemoglobin is half saturated at 0.6 mm. Hg, but in vivo 

 the bands disappear at oxygen concentrations near the limit of the Winkler 

 method of analysis (about 10 mm. Hg**"). The oxygen consumption as a func- 

 tion of O2 concentration declines steeply below 1-1.5 cc. Oo/l. (te=approxi- 

 mately 37 mm.). In the presence of CO the respiration is reduced by about 

 one-third at higher oxygen tensions, proportionately less at lower tensions-^ 

 (Fig. 79). Two thirds of the normal oxygen requirement is supplied by oxygen 

 uncombined in solution in the body fluid. As in Chironomus, the ti/2 sat is 

 far below the tension in water, where oxygen transport by hemoglobin is 

 important and the gradient from water to tissue is very steep. 



Some species of Daphnia contain hemoglobin in varying amounts; the 

 hemoglobin content of the blood increases when the oxygen in the water is 

 low. However, Daphnia can swim actively when the O2 is so low that the 



