n8 A MANUAL OF VETERINARY PHYSIOLOGY 



inspired. But since expired air is usually warmer than inspired 



(not always in the Tropics) and is saturated with aqueous vapour, 



the volume expired is actually greater. 



Respiratory Quotient. — The proportion which the volume of 



oxygen absorbed bears to the volume of oxygen returned as 



carbon dioxide is termed the Respiratory Quotient. It shows 



the proportion of oxygen required to oxidise carbon, and is 



CO 

 expressed as ~~ . The quotient on a carbohydrate diet, such 



2 



as starch, is high, though rarely greater than i, and generally 



about o*9 ; for in starch the oxygen and hydrogen of the molecule 

 exist in the proportion to form water, so that only the carbon 

 remains to be oxidised. The quotient may, as has been said, 

 be unity, or nearly so, depending upon whether the whole, or 

 nearly the whole, of the 2 is returned as C0 2 . If a purely 

 carbohydrate diet could support life, the R.Q. would be I exactly, 

 as there would be as much of the 2 returned as C0 2 as was taken 

 in ; for example — 



C 6 H 12 6 +60 2 =6C0 2 +6H 2 S-x. 



(Dextrose) ° 



On a fat diet the opposite condition exists : the fat molecule 

 is poor in oxygen, and cannot supply from itself more than 

 one-sixth of that required to oxidise the hydrogen to water ; 

 the remaining five-sixths have to be found from the incoming 

 air, and in consequence less of the oxygen inspired is returned 

 as C0 2 . Thus : 



C 3 H 5 (C 18 H 3 30 2 ) 3 + 8o0 2 = 57 C0 2 + 5 2H 2 ^=0-71. 



(Olein) 8 ° 



Consequently the R.Q. on a fat diet may be as low as 071. 

 On a protein diet, which, in the matter of R.Q., comes midway 

 between carbohydrate and fat, the oxygen in the molecule is 

 still in insufficient quantities to oxidise its own hydrogen to water. 

 It can contribute nearly half the amount required, but the re- 

 mainder must be abstracted from the oxygen inspired. The 

 R.Q. for protein varies, from circumstances which need not detain 

 us here, but is generally taken at 0*8. 



The R.Q. in animals depends, therefore, upon the nature of 

 the diet : on one largely consisting of carbodydrates it approaches 

 unity; on a diet of flesh it approaches that given for protein. 



In herbivora the R.Q. is 09 to ro. 

 In carnivora the R.Q. is 075 to o*8. 

 In omnivora the R.Q. is 0*87. 



The value of the R.Q. lies in its being a measure of the oxi- 

 dation occurring in the body as a whole. As a rule the amount 



