74 PRINCIPLES OF ANIMAL NUTRITION. 
the products of previous metabolism may be added to those formed 
during the experiment. This is especially true of the carbon diox- 
ide, particularly in work experiments, where the rate and volume 
of respiration are largely affected. During severe work, there may 
be more or less accumulation of this gas in the blood, while, on the 
other hand, the increased respiration in an immediately following 
period of rest may reduce the proportion in the blood below the 
normal. The oxygen is thought to be far less subject to this error 
than the carbon dioxide, and therefore to be a more accurate indi- 
cator of the total metabolism. 
Tue Respiratory QuotTientT.—This name was given by Pfliger 
to the ratio of the volume of carbon dioxide excreted to the volume 
of oxygen consumed in the same time. It is frequently represented 
by the abbreviation R.Q., or by the symbol a 
It is obvious that this ratio will vary with the nature of the 
material metabolized. Thus the oxidation of a carbohydrate, e.g. 
dextrose, will give rise to a volume of carbon dioxide equal to that of 
the oxygen consumed, since, as the following equation shows, each 
molecule of oxygen gives rise to a molecule of carbon dioxide: 
C,H,,0,+ 60, = 6CO,+6H,0. 
In this case the respiratory quotient is equal to unity. On the 
other hand, when fat is oxidized, a portion of the oxygen combines 
with the hydrogen of the fat to form water, and the volume of ear- 
bon dioxide produced is less than that of the oxygen employed. 
Representing the process by the equation used by Chauveau,* viz., 
2057H490,+ 1630, = 114C0, + 110H,0, 
1 
the respiratory quotient is a =0.6993. Computed from the aver- 
age percentage composition of animal fat as given on p. 61, it 
equals 0.7069. 
The proteids of the food, as we have seen, are not completely 
oxidized in the body, a portion of their carbon, along with all their 
nitrogen, being excreted in ~he form of urea and other organic 
* La Vie et 1’Energie chez |’ Animale. 
