BESPIBATION 127 



The nitrogen remains unchanged. The recently discovered gases, 

 argon, crypton, &c., are in the above table reckoned in with the 

 nitrogen. They are, however, only present in minute quantities. 

 The chief change is in the proportion of oxygen and carbonic acid. 

 The loss of oxygen is about 5, the gain in carbonic acid 4^. If the 

 inspired and expired airs are carefully measured at the same tempera- 

 ture and barometric pressure, the volume of expired air is thus rather 

 less than that of the inspired. The conversion of oxygen into carbonic 

 acid would not cause any change in the volume of the gas, for a 

 molecule of oxygen (0^) would give rise to a molecule of carbonic acid 

 (C0 2 ) which would occupy the same volume (Avogadro's law). It 

 must, however, be remembered that carbon is not the only element 

 which is oxidised. Fats contain a number of atoms of hydrogen 

 which during metabolism are oxidised to form water ; a certain small 

 amount of oxygen is also used in the formation of urea. Carbo- 

 hydrates contain sufficient oxygen in their own molecules to oxidise 

 their hydrogen ; hence the apparent loss of oxygen is least when a 

 vegetable diet (that is, one consisting largely of starch and other 

 carbohydrates) is taken, and greatest when much fat and protein are 



eaten. The quotient 2 ,<? lv r is called the respiratory quotient. 

 O 2 absorbed 



Normally it is -=0*9, but this varies considerably with diet, as 

 5 



just stated. 



Gases of the Blood. From 100 volumes of blood about 60 volumes 

 of gas can be removed by the mercurial air-pump (see Appendix). 

 The average composition of this gas from dog's blood is 



The nitrogen in the blood is simply dissolved from the air just 

 as water would dissolve it : it has no physiological importance. The 

 other two gases are present in much greater amount than can be 

 explained by simple solution ; they are, in fact, chiefly present in 

 loose chemical combinations. Less than one volume of the oxygen 

 and about two of carbonic acid are present in simple solution in the 

 plasma. 



Oxygen in the Blood. The amount of gas dissolved in a liquid 

 varies with the pressure of the gas ; double the pressure and the 



