THE RESPIRATION 303 



way the oxygen of the air is passed to the blood more rapidly 

 than the CO. 



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

 some physiologists maintain that simple diffusion will explain 

 these interchanges, 1st, because, when the amount of oxygen 

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



It has been ascertained by experiment that the pressure 

 of oxygen in the air may fall to about half its usual pressure 

 without interfering with the oxygenation of the blood. If it 

 falls below this oxygen is not taken up. Hence it is possible 

 for men to live at high altitudes where the oxygen tension 

 is much reduced. But under these conditions the rate of 

 respirations and the rate of blood flow through the lungs 

 have to be accelerated, and hence hyperpnoea is apt to be 

 induced, especially on exertion. 



II. INTERNAL RESPIRATION. 



Having seen how the blood gets its oxygen and gets rid of 

 its carbon dioxide in the lungs, it is necessary to consider 

 the exchanges of these gases between the blood and the 

 tissues. 



1st. Passage of Oxygen from Blood to Tissues. 



In studying the physiology of muscle, which may be taken 

 as a type of all the active tissues, it was seen that oxygen 

 is constantly being built up into the muscle molecule, and 

 that the living tissues have such an affinity for oxygen that 

 they can split it off from such pigments as methylene blue. 

 The tension of oxygen in muscle is therefore always very 

 low. We have seen that the tension of oxygen in arterial 

 blood is nearly 100 mm. Hg, therefore, when the blood is 

 exposed to a low tension of oxygen in the tissues, the oxygen 

 comes off from the blood and passes into the tissues by the 

 ordinary laws of diffusion. 



But it must be remembered that this takes place in three 

 stages. 



(1) The tissue elements are always taking up oxygen 



