274 Comparative Animal Physiology 



vided these forms with an energy-yielding mechanism, and it was only 

 necessary to develop an increased tolerance to the acid metabolites produced, 

 and a way of excreting them. Whereas aerobes ordinarily convert the lactic 

 acid back into glycogen or oxidize it to carbon dioxide (and thereby form an 

 oxygen debt when subjected to a nitrogen atmosphere), anaerobes eliminate 

 the lactic acid and its metabolic counterparts (and contract no oxygen debt). 

 The reduction in metabolic activity also operates in behalf of anaerobes, but 

 apparently this presents no hardship to organisms designed to live as endo- 

 parasites. There seems little evidence in the work performed so far to indicate 

 that anaerobic mechanisms phylogenetically preceded aerobic ones. Until 

 more information is available, we may assume they both arose early and per- 

 haps have developed together in a parallel manner. 



Oxygen Debt. The concept and measurement of the oxygen debt has been 

 applied to both vertebrates and invertebrates. The debt arises during periods 

 of activity when the oxygen supply cannot keep pace with the demand, yet 

 the metabolic processes continue, building up lactic acid and other breakdown 

 products which are oxidized when sufficient oxygen becomes available. The 

 amount of accumulated lactic acid is proportional to the oxygen debt. In man 

 severe exercise may result in a debt of 10 liters or more of oxygen, requiring 

 at least an hour for complete removal of excess lactic acid. 



Invertebrates may be grouped according to two main types of reaction to 

 temporary oxygen deficiency: (1) contraction of an oxygen debt— similar to 

 the situation in the vertebrates, or (2) elimination of acid waste products 

 without benefit of complete oxidation— characteristic of anaerobes— thereby 

 avoiding oxygen indebtedness. Among the former group may be noted the 

 earthworm,^-'-* cockroach, -^-^ grasshopper,-^^ Planaria,-'-^^' and Planorhis,'^'^ all of 

 which repay the oxygen debt formed under partially anaerobic conditions, and 

 Mya, which, interestingly, after low tide utilizes increased quantities of oxy- 

 gen.**^ On the other hand, true anaerobes eliminate lactic and fatty acid 

 metabolites without requiring any oxygen from outside sources. Nereis^^^ and 

 Urechis^^'- react this way. Some organisms characteristically develop only a 

 small oxygen debt and repay it within a very short period of time, e.g., 

 Ascaris,^^''' Drosophila,^* and Chironomus.^^° The oxygen debt of Chironomus 

 calculates to about 0.5 per cent of what the value of oxygen consumed would 

 be if aerobic had replaced anaerobic conditions. One assumes the end-products 

 are otherwise eliminated. The small magnitude of the oxygen debt of Droso- 

 phila in flight is of interest— 0.18 cu. mm./individual. It is paid off within two 

 minutes after the cessation of activity. ^^ 



Respiratory Quotient. A quantitative determination of the metabolic 

 activity of organisms may be obtained from a measurement either of the 

 amount of oxygen consumed or of the quantity of carbon dioxide produced. 

 By comparison of the two— the ratio of volume of carbon dioxide to volume of 

 oxygen, the respiratory quotient (R.Q.)— an estimation can be made of the 

 types of food materials oxidized. Definite R.Q. values are recognized for the 

 main types of organic food;-^^- •'°"- ^^- the R.Q. value of pure carbohydrate 

 (e.g., sucrose) is 1.00, of fat, 0.71, and of protein, about 0.79. For the calcula- 

 tion and interpretation of these values the protein fraction may be determined 

 by means of urinary nitrogen. On an average mixed diet, man has an R.Q. of 

 approximately 0.8 to 0.85. Herbivores naturally tend to have a high R.Q., 

 carnivores a low one. 



