246 ComparatJx^e Aniinal Physiology 



pressures. That such is sometimes the case is known in those hemoglobin- 

 containing invertebrates whose blood pigments are saturated at very low 

 pressures and which show low oxygen unloading tensions, e.g. Chironornvs 

 (t,/2 sMt.=0.6 mm. llg at CO:-) and the snail, Planorhis (ti/o sat. = l-9 mm. 

 Hgat0CO2). The hemocyanin of Bnsycow blood (Gastropoda) has a tj/:.. ,,,t 

 of 13 mm. llg at CO:-, reducible to less than 3 mm. Hg at 19.6 mm. llg 

 CO..-'-'^ This permits survival in oxygen-deficient water in equilibrium with 

 air at less than 5 per cent of an atmosphere. 



The relationship of oxygen consumption to oxygen tension for most organ- 

 isms is summarized in Figure 55, represented by a hyperbolic curve with no 

 clear break to indicate the critical tension— a plot midway between the extreme 

 non-regulatory Baetis (t..=240 mm. Hg) and the highly regulatory Cloeon 

 (t,.=;32 mm. llg). In his significant review of many such intermediates, 

 Tang-'-^^ relates cc. per gram hour of O^, A, to O2 tension, P, in the equation: 



A= 



K, + K,P 



where K, and Kj are constants. Different values of K indicate differences in 

 the degree of regulation. In the event Ki is very small, A becomes approxi- 

 mately constant; i.e., the oxygen consumption is independent of tension. 



The lack of dependence of oxygen consumption on tension may be attributed 

 in some instances to the respiratory stimulating capacity of oxygen want, acting 

 to increase the breathing rate and respiratory activity. Such a control implies 

 the presence of respiratory centers and, ergo, a higher state of respirator}' 

 development than for non-regulatory animals. A correlation with phylogenetic 

 position, however, cannot be demonstrated."- '"'• ^■''' 



Critical Tensions. The actual presentation of comparative values of critical 

 tensions is no mean task, owing to the vicissitudes with which these x'alues 

 respond to external conditions, and to the variations in the experimental pro 

 cedures under which the determinations are carried out. It has been shown 

 very clearly in the flatworm, Planaria agilis, for example, that the oxygen 

 consumption generally is constant down to a concentration of 3 cc.Oo/1., but 

 if the ()j deprivation is produced very slowly consumption may continue on 

 down to a concentration of 0.5 cc./l. The t^. may be approximately 60 mm. Hg, 

 or 10, depending on the rate of oxygen decrease. A good deal of caution there- 

 fore is urged against too enthusiastic interpretation based on such data as those 

 indicated in Table 43, which includes critical tensions culled from the litera- 

 ture and some recalculated into partial pressures from solubility and saturation 

 data. Absent from this list, of course, are those organisms which show complete 

 dependence on tension and therefore lack t,. values entirely. 



Although a low t,. value undoubtedly connotes higher metabolic efficiency, 

 everything else being equal, other factors are of considerable importance in the 

 matter of low oxygen tolerance— namely, oxygen utilization, metabolic level, 

 and the like. The toadfish is able to extract practically all available oxygen out 

 of his environment, but the mackerel, trout, and similarly active fish require 

 a moderately high oxygen level at all times. The tench can survive indefinitely 

 at ordinary temperatures in water with no more than 0.3 cc. 02/1-, but the 



