126 A MANUAL OF VETERINARY PHYSIOLOGY 



diet causes a larger respiratory exchange than one of carbo- 

 hydrate or fat. Respiratory exchange is greater in the young 

 than in the old, less during sleep than when awake, greater in 

 warm-blooded than in cold-blooded animals, greater in small 

 than in large animals, and greater at work than at rest. Some 

 of these points will be examined at length in the chapter devoted 

 to their consideration. 



Gaseous Exchange in the Lungs. — There are two theories put 

 forward to account for the transfer of oxygen from the external 

 air to the blood, and the passage of carbon dioxide from the 

 blood to the external air. The first and oldest view is a physico- 

 chemical one. If the partial pressure of the oxygen in the air 

 cells is higher than it is in the blood, oxygen will pass into the 

 blood. 



If the pressure of the carbon dioxide in the blood is greater 

 than it is in the alveolar air, carbon dioxide will be given off 

 to the latter. Until recent years the acceptance of this ex- 

 planation had been very general. When, however, the air in 

 the alveolar cavities was examined, facts were discovered which 

 threw grave doubt on the accepted view. It was found by 

 certain observers that the pressure of oxygen in the arterial 

 blood might be above that of the oxygen in the alveoli, and 

 that of the carbon dioxide in the blood might be below the 

 pressure of that in the pulmonary alveoli. Under these cir- 

 cumstances it was obvious that the theory of diffusion was 

 no longer able to explain the respiratory exchange in the 

 lungs, and a fresh explanation had to be found. The view held 

 by those who reject the physical theory is that the respiratory 

 exchange is carried out by a process of secretion, and this in the 

 present state of our knowledge of physics and chemistry is 

 very difficult to explain. We must look at both views more 

 closely. 



The picture of the diffusion theory is outlined above. The 

 blood, robbed of 40 per cent, of its oxygen in the tissues, and 

 carrying over 16 per cent, increase of carbon dioxide, makes its 

 way back to the lungs in a partially reduced condition. Here it 

 circulates through the vast capillary system spread over the 

 alveoli of these organs, and is brought as closely as possible into 

 contact with the alveolar air in the ultimate air-passages. Nothing 

 but the capillary wall and the delicate moist membrane of the 

 alveolus separates the blood from the air, and through this wall 

 the oxygen instantaneously passes, in consequence of its partial 

 pressure in the blood being low and that in the alveoli high. 

 In the blood it is taken up by the plasma, and then absorbed by 

 the haemoglobin of the red cells, with which, as we have already 

 seen, it forms a weak chemical compound. Concurrently with 



