250 Comparative Animal Physiology 



lation set up by the acidosis, on the one hand, and the Bohr effect, on the 

 other, has a good deal to do with the phenomenon. In a series of fresh-water 

 fish investigated for their least tolerable oxygen tension as a function of pH, 

 the majority reached the minimum oxygen level at or near the neutrality 

 point. '"'^ iMost of those studied (largemouthed and smallmouthed black bass, 

 white crappie, yellow perch, rainbow trout, sunfish, and goldfish) were able 

 to extract oxygen at low tensions over a wide pH range-pH 5 to 9 in several 

 cases— whereas the bluegill minnow was limited to pH 7-9, and the steel- 

 colored and blunt-nosed minnows to pH 7-8 (Fig. 57). 



Relation of Least Tolerable Tension to Size and Age. Size and age may 

 alter the capacity of organisms to utilize oxygen at low tensions and thereby 

 affect the critical tension values. Some discrepancies may appear in Table 45 

 as a result of such factors. The oxygen consumption rate of large Planaria is 

 limited below 14-15 volumes per cent (ca. 110 mm. fig), well above the t,. 

 values presented in the table.^-^ In the crayfish, Camharns, higher t,. values 

 were found for larger individuals. ^•'•■"* Animals weighing 4.3, 9.0, and 17.1 gm. 

 showed critical tensions at 32, 48, and 65 mm. Hg, respectively. The larger 

 (older) the animal, the less the range of respiratory regulation. 



Critical Tension and Tissue Oxygen Saturation. The correlation between 

 respiratory regulation and the presence of an oxygen-carrying mechanism, such 

 as the readily saturated hemoglobin transport system in Chironomus, is a rela- 

 tionship which applies directly to interpretations of oxygen consumption- 

 oxygen tension independence. Cloeon (Fig. 55) might be regarded as having 

 a fully saturated oxygen system above 32 mm. Hg tension. This concept has 

 been considered extensively by Maloeuf,-^- who expresses critical tensions as 

 a function of the atmospheric pressure necessary for half saturation of the 

 tissues. Although some skepticism may be in order concerning the advantages 

 gained by Maloeuf's unorthodox and cumbersome method of handling data, 

 largely recalculated from the work of other investigators, something is to be 

 said for his emphasis on the startling dependence of oxygen consumption on 

 tension— Mytihis, for instance, shows no critical tension at 640 mm. Hg— four 

 times atmospheric pressure of oxygen. The possibility that oxygen saturation 

 of the tissues is of great importance in the problem of respiratory regulation 

 is worthy of consideration. Considerable evidence supports the view that 

 single-ceiled organisms as well as tracheates without gas transporting systems 

 may be relatively independent of oxygen tension over a wide oxygen pressure 

 range. Further, animals with transporting systems are on the average about 

 ten times more independent of tension than are those without oxygen trans- 

 porting mechanisms. Although tissue saturation even at low tensions may be 

 important in the responses of one-celled and tracheate organisms, the absolute 

 rate of metabolism and the adequacy of diffusion at low tension are also im- 

 portant factors. The relative independence of those animals with and without 

 circulating systems probably depends primarily on the presence or absence of 

 blood pigments, rather than of circulation itself. 



Oxygen Withdrawal. The ability of animals to withdraw oxygen from 

 their aerial or aquatic media at low pressures governs in part their critical 

 tensions, minimum tolerable oxygen, and indeed their distribution through 

 the ecological adjustment to available oxygen. It is not without some interest 

 to survey the problem of efficiency of oxygen uptake ("utilization" in the sense 



