RESPIRATION 



187 



often more easy to demonstrate the augmented oxygen consumption 

 than the augmented output of carbonic acid. This is due to several 

 causes : (1) carbonic acid is soluble in the tissues in which it is pro- 

 duced; (2) any change in the chemical reaction of the tissues alters 

 the amount of carbonic acid which they give out to the blood ; if, for 

 instance, it becomes more alkaline it retains a portion of its carbonic 

 acid. 



The preceding table shows the variations which take place in the 

 oxygen intake of several organs as the result of activity produced by 

 widely different forms of stimulus (Barcroft). 



The relation of the oxygen taken in to the carbonic acid given out 

 is well shown in the following experiment performed by Zuntz on the 

 leg of the dog : 



Two points must be noted in considering the above table : 



(1) That the exchange of gases was decreased on cutting the 

 nerves. The decrease in metabolism was greater even than the 

 figures show, for the blood flow through the leg was decreased. 



(2) That the oxygen exchange and the carbonic acid exchange 

 changed in about the same proportions. The ratio of the carbonic 



14'4 



acid given out to the oxygen taken in was with the nerves intact 



13'2 



10-1 



and 



10-45 



with the nerves cut. 



Effect of reduced Oxygen Tension on Tissue Respiration. When 

 the oxygen tension in the blood is reduced, the tissues still take up the 

 same quantity of oxygen as before and give out as much, or slightly 



