Respiratory Functions of Body Fluids 325 



It is probable that this reserve is important at extreme low tide; this does not 

 preclude the possibility that some of the hemoglobin molecules are alternately 

 loaded and reduced as they move from the respiratory gut to body tissues. 



The intracellular hemoglobins of invertebrates may serve as oxygen reserves 

 for times of stress, as in parasitic animals living in regions of low oxygen, for 

 example the larva of G astro philus'''^ and Ascaris.-'^ The dissociation curves of 

 both of these are well to the left, with tensions of half saturation about 0. 1 mm. 

 Hg partial pressure of oxygen. In Ascaris the hemoglobin holds oxygen avidly, 

 that is, unloads very slowly (Table 51), and when the worms are in anaerobic 

 conditions the body wall pigment, but not that in the perienteric fluid, can be 

 seen to be deoxygenated. Similarly in the nematode Nippostrongylus, with a 

 ti/2 sat of less than 0.1 mm. Hg, deoxygenation is seen in low oxygen, but the 

 closely related StrongyJiis dies in low oxygen before the pigment is deoxygen- 

 ated.-^ 



It is apparent from the preceding examples that the hemoglobin-containing 

 invertebrates have part of their oxygen needs supplied from the gas in solution 

 in their blood. In Urechis, for example, the worm uses oxygen only to the 

 equivalent of 1/60 of that held by its hemoglobin per hour. In monoxide- 

 treated animals, usually less than half of the respiration is affected. If the 

 oxygen tension to which hemoglobin is exposed is sufficiently low the pigment 

 will unload oxygen. The reduced molecules may immediately thereafter 

 become oxygenated, or they may wait for hours until oxygen is restored. Thus 

 the question of store versus transport is resolved. In animals like Chironomus 

 and Tiihifex, tissue oxygen is very low; in Planorhis and Daphnia it is higher.''*^ 

 In an animal like Urechis, with no circulatory system, some molecules will be 

 reduced while others are oxygenated, and they will be continuously mixed by 

 the churning of body movements. The added survival time due to hemoglobin 

 at oxygen tensions where saturation is no longer possible may be of marginal 

 significance. A number of marine worms carry on an active life with no 

 blood pigment whatever (Table 49). Some of them, like Chaetopterus, are 

 mud dwellers; Daphnia continues active when its hemoglobin is poisoned. 

 Some invertebrate hemoglobins appear to provide a safety factor of oxygen, 

 and they function at low oxygen tensions; they may act across steep oxygen 

 gradients. The observations on Chironomus and Tuhifex show the futility of 

 inferring function from saturation values of the pigment. Some parasitic 

 species may unload their hemoglobin only under anoxic stress. 



Function of H emery thr'in 

 The oxygen dissociation curves of hemerythrins, although in the low 

 oxygen range, show ti/2 sat values which might be reached by the body 

 tissues of sluggish worms (Table 55). A rise in temperature shifts the O2 

 dissociation curve of Phascolosoma to the right, but changes in pH do 

 not have much effect on it.'"" In Sipunculus the U/2 sat was 8 mm. Hg 

 whether the CO2 was 0.07 or 80 mm. Hg.^" The O^ tension found in the 

 coelomic fluid of Sipunculus in sea water was 32 mm. Hg, at which tension 

 its hemerythrin would remain saturated. The tissue tensions are not known. 

 However, in the mud when the tide is out, Sipunculus is exposed to very low 

 oxygen tensions. At such times, like hemoglobin in Urechis and Arenicola, 

 the hemerythrin may give up its oxygen to the tissues.-*'*' -'^ 



