CH. XXIV.] 



COMPOSITION OF THE AIR 



375 



oxygen which goes into the blood is held there in loose combination 

 as oxyhaemoglobin. In the tissues this substance parts with its 

 respiratory oxygen. The oxygen does not necessarily undergo 

 immediate union with 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. Owing to this reserve oxygen, a muscle 

 will contract in an atmosphere of pure nitrogen and yet give off 

 carbonic acid; and a frog will live under the same conditions and 

 give off carbonic acid for several hours. Besides carbonic acid and 

 water, certain other products of combustion are generated ; those like 

 urea and uric acid, which are the result of nitrogenous metabolism, 

 ultimately leave the body in the urine. The carbonic acid and a 

 portion of the water find an outlet by the lungs. 



Inspired and Expired Air. The composition of the inspired or 

 atmospheric air and the expired air may be compared in the following 

 table : 



The nitrogen remains unchanged. The recently discovered gases 

 argon, crypton, etc., 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 about 4 - 5. If the 

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

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

 to be 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 2 ) would give rise to a molecule 

 of carbonic acid (C0 2 ) which would occupy the same volume (Avo- 

 gadro's law). It must, however, be remembered that carbon is not 

 the only element which is oxidised. Fat and proteid contain a 

 number of atoms of hydrogen, which, during metabolism, are oxidised 

 to form water ; a small amount of oxygen is also used in the formation 

 of urea. Carbohydrates contain sufficient oxygen in their own mole- 

 cules to oxidise their hydrogen ; hence the apparent loss of oxygen is 

 least when a vegetable diet (that is, one consisting largely of starch 



* This diminution of volume will cause a slight rise in the proportionate volume 

 of nitrogen per cent. 



