244 Comparative Animal Physiology 



oxygen consumption, and there is no clear-cut distinction between two great 

 groups of animals-poikilothermic and homoiothermic. The initial lack of a 

 temperature-regulating mechanism in xery young animals, followed by an 

 adjustment period during which the temperature control becomes firmly 

 established, is not uncommon in higher vertebrates. The labile respiratory 

 and cardiovascular conditions so pronounced in hibernation among mammals 

 are discussed as a function of temperature in Chapter 10. 



Temperature changes thus prove to be intimately interconnected with the 

 respiratorv pattern of animals, in some instances resulting in adjustment to 

 environmental variation and in others, through metabolic regulation, counter- 

 acting the external change over a significant temperature range. 



The consideration of animal metabolism leads to the conclusion that the 

 rate of oxygen consumption as usually determined under conditions approxi- 

 mating rest may serve as a reference point in defining the respiratory state of 

 an individual, but there are many modifying factors, both within and without 

 the organism, which affect the character of the gas exchange. It is of signifi- 

 cance to understand the part each factor plays in the response of the organism 

 to its oxygen environment, for here lies the basis of important physiological 

 adaptation. 



INTERRELATIONS BETWEEN OXYGEN TENSION AND 

 OXYGEN CONSUMPTION 



General Types of Reactions. The rates of oxygen consumption and of 

 breathing movements are variably dependent on the available oxygen, a func- 

 tion of the gas tension. Many organisms are able to maintain a steady respira- 

 tory state and are independent of oxygen tension over a considerable range, as 

 is man down to an oxygen concentration of about 13 per cent (100 mm. fig) 

 or the earthworm to 3 per cent of atmospheric pressure. These animals are 

 capable of respiratory regulation. Many other animals (Nereis, Homarus, 

 etc.) show a direct dependence of oxygen consumption on oxygen tension, 

 and each variation in gas pressure is reflected in a metabolic change in 

 the organism. The dift'erent responses to decreased oxygen tensions are in 

 some cases attributed to different activity adjustments as, for example, in the 

 mackerel and the toadfish-the sluggishness of the latter corresponding with 

 its ability to live in oxygen-deficient water, compared with the more active 

 mackerel which can sur\'i\e only in an oxygen-rich medium. In other metab- 

 olic comparisons the lack of dependence of oxygen consumption on tension 

 is due to sheer efficiency of the respiratory mechanism at low tensions. This 

 is seen in the chironomid insects, for instance, whose plasma hemoglobin 

 permits them to extract oxygen at \'ery low tensions and to survi\e o.xygen 

 depletion much better than their hemoglobinless relatives, a condition which 

 must afford considerable survival value in oxygen-deficient water. The follow- 

 ing paragraphs will consider significant physiological adajitations to oxygen 

 tension and the \arious ecological consequences involved. 



Dependence of Oxygen Consumption on Ox) gen Tension. 1 he rate of 

 oxygen consumption varies linearly with tension over a wide pressure range 

 in those animals which show little capacity to regulate, e.g.. Nereis, Homarus, 

 Limitlits, Callinectes.^- "' The oxygen consumption is thus dependent on 

 tension, indicating little respiratory control and the probable importance of 



