THE BLOOD 99 



tion external respiration. We have already seen that oxyhaemo- 

 globin is only a loose compound, and in the tissues it parts with 

 its oxygen. The oxygen does not necessarily undergo immediate 

 union vdth carbon to form carbonic acid, and with hydrogen to form 

 water, but in most cases, as in muscle, is held in reserve by the tissue 

 itself. Ultimately, however, these two oxides are formed : they are 

 the chief products of combustion. Certain other products which 

 represent the combustion of nitrogenous material (urea, uric acid, &c.) 

 ultimately leave the body by the urine. All these substances pass into 

 the venous blood, and the gaseous products, carbonic acid, and a 

 portion of the water find an outlet by the lungs. 



Inspired and Expired Air. — The composition of the inspired and 

 the expired air may be compared in the following table : — 



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 (O,) would give rise to a molecule of carbonic acid 

 (CO2), 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 proteid are 



eaten. The quotient ^ ' ^ — . .,- is called the respiratory quotient. 



