156 ELEMENTARY PHYSIOLOGY less. 



(lower) partial pressure of carbonic acid in the alveolar 

 air and the (higher) partial pressure it has in the venous 

 blood ; but the case is not quite so clear as it is in respect 

 of the entry of oxygen. For the partial pressure of car- 

 bonic acid in alveolar air is not inconsiderable, and its 

 exit from the blood is opposed by the fact that it is in 

 loose combination with some constituent of the plasma. 



The blood thus fully arterialised is whirled away to the 

 tissues, where it becomes once more venous. In the 

 tissues the causes of the change are much more easily 

 understood, for the living tissues are greedy of oxygen, 

 which they stow away in compounds so stable that they 

 give up no oxygen to the vacuum of even the most power- 

 ful pum[) ; the partial pressure of oxygen in tlie tissues 

 may even l)e zero. Hence oxygen readily passes over from 

 the arterial blood. Again, the living ti.ssues are always 

 producing carbonic acid in greater or less amount accord- 

 ing as they are more or less active ; the partial pressure 

 of cj^rbonic acid is therefore high in the tissues and quite 

 sufficient to account for the passage of this gas from them 

 into the neighbouring arterial blood. 



The amount of oxygen left in venous blood is dependent 

 on the varying activity of the tissues and of the quantity 

 of blood which is flowing through them, and this is the 

 reason why the volume of this gas was given (p. 125) as 

 varying from eight to twelve volumes in each hundred 

 volumes of venous blood and indeed it may even vary 

 within wider limits. 



10. The Nervous Mechanism of Respiration. — Of 

 the various mechanical aids to the respiratory process, 

 the nature and workings of which have now been de- 

 scribed, one, the elasticity of the lungs, is of the nature 

 of a dead, constant force. The action of the rest of the 

 apparatus is under the control of the nervoas system, and 

 varies fronx time to time. 



