DETERMINATION OF THE RESPIRATORY EXCHANGE 



the increase of weight in C and D gives the weight of C0 2 . The loss 

 of weight in the animal is obtained by weighing the animal in the 

 beaker before and after experiment. Then since the molecular weight 

 of a gas in grammes measures 22-4 litres under normal condition 



44 



of temperature and pressure, the C0 2 in grammes x _= volume of 



Z2S*4 



CO, given out ; similarly, the loss of weight of C0 2 and water vapour 

 deducted from the loss of weight of the animal gives the weight of 



32 



oxygen taken in; this in grammes x ~o-r= volume of oxygen taken in. 



ZiZi'^t 



Such a method is not convenient for larger animals and for man. 

 The respiratory exchange in such has been investigated by placing 

 the animal in a closed chamber which contains a known quantity of 

 air. This air is circulated, the C0 2 given off being absorbed by caustic 

 alkali, and oxygen gradually added to replace that used up and keep 

 the pressure constant. The oxygen used is known from the amount 

 which has entered the chamber; the amount of C0 2 absorbed is esti- 

 mated by titrating the alkali. In some laboratories, large rooms have 

 been fitted up for the special study of the respiratory exchange in 

 man under varying conditions. 



The respiratory exchange is greatly increased by muscular work, 

 as the following table shows : 



Exposure to a cold wind maydouble the respiraloi}' exchange of 

 the resting man; by stimulating him to muscular acmity it may do 

 much more than this. 



The Respiratory Quotient. The volume of C<^ given out divided 

 by the volume of O 2 taken in gives the respiratory quotient : 



CO 2 by volume 



R.Q.= 



2 by volume 



Thus, in the experiment on man, 280 c.c. of C0 2 was given out, 



350 c.c. of O 2 taken in. 



TheR.Q. ...= --=0-8. 

 350 



The respiratory quotient varies according to the nature of the food being 

 oxidized in the body. On a mixed diet it is found to oe about 0-85. 



