504 ANIMAL BIOCHEMISTRY 



iitili/ed by most other animals. Actually it is surprising that the diges- 

 tion coefficients are as high as those tabulated when the assumptions 

 and inaccuracies are considered. 



When an animal feeder wishes to calculate the total digestible nu- 

 trients of a given feed or mixture of feeds, he merely multiplies the 

 amount of such nutrient in 100 lb. of ration by its digestion coefficient 

 and then combines these values to obtain the total amount of nutri- 

 ents available to the animal organism, as indicated in Table 21-4. 

 The value so obtained is a useful index of the caloric value of a feed 

 when the digestion coefficients are known for the ration and the ani- 

 mal concerned. Differences between species have already been dis- 

 cussed. In addition, the digestibility of one feed component may be 

 affected by the presence and amounts of other components. The level 

 of feed intake also affects the results. Moreover, losses in methane 

 and urine may amount to 15 per cent of the heat content of the ration 

 and are ignored by this method of calculation. Nevertheless, in feed- 

 ing practice wide use has been made of the total digestible nutrients 

 in comparing rations and their components for feeding domestic ani- 

 mals. 



Respiratory Quotients 



Indirect calorimetry with animals is based in part on measurements 

 of oxygen consumed and carbon dioxide evolved. Oxygen uptake is 

 determined by calculation from the change in either the volume or 

 the pressure that occurs when oxygen is utilized. Since carbon dioxide 

 is expelled at the same time, a correction is made after a separate 

 determination of the carbon dioxide. Or this latter gas is continuously 

 absorbed and an effect on pressure thus avoided. The carbon dioxide 

 is determined either by absorption with calcium oxide and weighed 

 or by absorption in potassium hydroxide solution from which it is 

 liberated with an excess of acid and its volume measured. Respiration 

 studies involve chambers for animals and man or face masks for 

 human beings with provision for sampling of the inspired and expired 

 gases. 



The proportions of oxygen and carbon dioxide vary with the nature 

 of the materials metabolized as revealed in the following balanced 

 reactions: 



(C6Hio05)a' + 6A"02 -^ 6XCO2 + 5ZH2O + heat 



starch 



C51H98O6 + 72.5O2 -^ 5ICO2 + 49H2O + heat 



tripalmitin 



