374 RESPIRATION. - [CH. xxiv. 



respiration. The intermediate steps take place all over the body 

 and constitute what is known as tissue-respiration. The oxygen 

 which goes into the blood is held there in loose combination 

 as oxy haemoglobin. 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 in- 

 spired or atmospheric air 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 

 about 4-5. If the inspired and expired airs are carefully measured 

 at the same temperature 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 ) would give rise to a molecule of carbonic acid (C0 ) 

 which would occupy the same volume (Avogadro'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 



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

 volume of nitrogen per cent. 



