70 PHYSIOLOGY OF FARM ANIMALS [CH. 



W'c may also compare the relative aniouuls of the various 

 gases in arterial and venous Ijlood. For this ])urpose a mercurial 

 gas pump is used ; the blood is exi^osed to a vacuum and gas is 

 thereby evolved and may be measured, jootash being employed 

 to absorb the carbon dioxide, and pyrogallic acid to absorb the 

 oxj-gen. It is foimd in the case of l)oth venous and arterial blood 

 that 100 vols, of blood yield about 60 vols, of gas. This 60 vols, 

 of gas is composed approximately as follows : 



From tlio y)ulmonaiy artery From the pulmonary vein 



() ] 2 vols. O 20 vols. 



CO, 46 „ 00, 38 „ 



N 2 „ N, 2 „ 



The purpose of pulmonary respiration is to convert venous 

 blood into arterial blood. The essential difference between these 

 two kinds of blood depends, as has just been shown, on the 

 relative quantities of oxygen and carbon dioxide which they each 

 contain. The passage of these gases from or to the blood in the 

 capillaries of the lungs is regulated normally by the pressure under 

 Avhich the respective gases are present in the air cells of the lungs. 

 Thus oxj'gen is generally present in the air cells under a pressure 

 of about 100 mm. of mercury, and this is quite sufficient to cause 

 the venous blood to become arterialised. In the case of water 

 the amount of ox^^gen (or anj' other gas) dissolved is projDortional 

 to the j)ressure to which the gas is subjected. If the jDressure is 

 doubled the amount of gas dissolved is also doubled. In the 

 case of blood however this relation does not hold, since the 

 quantity of oxygen in blood is not proportional to the pressure of 

 the oxygen in the air to which the air is exposed through the thin 

 lining of the air cells. This is because the oxygen is not merely 

 in solution but is in chemical combination with the haemoglobin 

 of the red corpuscles, forming oxyhaemoglobin. Consequently a 

 given quantity of blood can absorb a much greater amount of 

 oxygen than can the same quantity of water. The effect of a 

 varying ox3^gen pressure on oxyhaemoglobin can be expressed in 

 the form of a curve, called the dissociation curve of oxyhaemo- 

 globin, and this curve varies under different conditions. Thus in 

 an aqueous solution of haemoglobin the quantity of oxj^gen 

 combining with it to form oxyhaemoglobin is very much greater 

 owing to the absence of blood salts which make the compound 

 less stable. Moreover, a rise in the temperature of the blood, or 



