VETERINARY PHYSIOLOGY 



In spite of these wide variations in the oxygen pressure in 

 alveolar air the tension of oxygen in the blood remains about 

 the same. 



The difference in the pressure of these gases in the alveolar 

 air, and in the blood going to and coming from the lungs, 

 may be represented as follows in mm. Hg : 



Blood 



This shows that when the blood reaches the lungs the 

 distribution of pressure of the gases is such that, by the laws 

 of diffusion, oxygen will pass from the alveolar air into the 

 blood and carbon dioxide from blood to air ; but that, before the 

 blood has left the lungs, the distribution is such that oxygen 

 should, by the laws of diffusion, pass from blood to lungs and 

 carbon dioxide from lungs to blood, which is exactly the reverse 

 of what occurs. The passage of oxygen to the blood and the 

 passage of carbon dioxide from the blood is, in fact, much 

 greater than could be accounted for by diffusion. 



We musl therefore conclude that the exchange of gases 

 between the alveolar air and the blood is not due entirely to 

 diffusion, but is in part, at least, brought about by the activity of 

 the cells lining the vessels and the alveoli. 



It should, however, be stated that, in spite of these figures, 

 some physiologists maintain that simple diffusion will explain 

 these interchanges \st, because, when the amount of oxygen 

 in the atmosphere i.e. when the partial pressure of falls 

 below a certain point, the gas is no longer taken up by the 

 blood ; and 2nd, because, when the amount and pressure of 

 C0 2 rises, the C0 2 is not given off from the blood. 



The partial pressure of oxygen may be reduced to a half 

 without interfering with the vital processes of the body, and 

 for this reason it is possible for men and animals to live at 

 high altitudes. When men are suddenly subjected to a very 

 marked decrease of pressure, especially if they have to do 

 muscular work, as in climbing, the decreased supply of oxygen 



