i;2 BIOLOGICAL CHEMISTRY 



can be taken as absolute proof that there is no oxygen tension 

 in the tissues. 



On the other hand there is some evidence that an oxygen 

 tension does exist in the tissues. Verzar * carried out experi- 

 ments in which the rate of oxidation in the tissues was followed 

 whilst the animal breathed gas mixtures with less and less 

 oxygen in them. If there is no oxygen tension in the tissues 

 the rate of absorption of oxygen by the tissues should fall as 

 soon as the oxygen tension in the blood falls below its usual 

 value. He found that with some tissues the oxygen con- 

 sumption fell with a small decrease in oxygen tension in the 

 lungs but that with others the oxygen consumption remained 

 at the same level until there is a marked decrease in the 

 alveolar oxygen tension. 



The results indicate that skeletal muscle has a low oxygen 

 tension, not more than 19 mm. mercury at most, whilst the 

 submaxillary gland has a considerable oxygen tension which 

 can be only a little below the oxygen tension in the vein from 

 the gland. 



From these figures it is legitimate to conclude that dunng 

 resting metabolism the tissues have an oxygen tension at 

 least equal to the decrease of tension necessary to cause a 

 decrease in their rate of oxygen absorption. 



If it can be shown that there is an oxygen tension in the 

 tissue it is obvious that increased use of oxygen during activity 

 will cause a fall in oxygen tension and an increased absorption 

 of oxygen from the blood. 



An increased rate of oxidation without an increase in the 

 oxygen tension can be produced in three ways : 



(1) By an increase in oxidisable substance. 



(2) By a quicker removal of the products of oxidation. 



(3) By the liberation of an increased amount of oxidising 

 enzyme. 



In discussing these three we must remember that complete 

 oxidation is a series of changes and that an increase in rate of 

 one reaction may allow the whole series to proceed more quickly. 



(i) We must distinguish between the initial changes and the 

 later changes during activity. Increase in activity is associated 

 with increase in oxygen intake and in carbon dioxide output. 



If we examine a quick reaction such as the contraction of 

 voluntary muscle we are faced by the difficulty that ordinary 

 analytical methods are not quick enough to follow the 

 rapid changes, but by electro-chemical methods it can be 

 demonstrated that chemical changes such as an increase 

 * F. Verzar, Journ. PhysioL, 1912, vol. 45, p. 39. 



