SECTION V 



THE MECHANISMS OF OXIDATION IN THE 



TISSUES 



THE blood in its passage through the capillaries takes up carbon 

 dioxide from the tissues, giving oxygen to the latter in exchange. 

 This interchange is determined by the differences in tension of the 

 gases on the two sides of the capillary wall. Whereas the tension of 

 oxygen in the plasma varies from 100 mm. Hg in arterial to 25 mm. 

 Hg in venous blood, the tension of oxygen in the tissues outside the 

 vessels in most cases approaches 0, as is shown by Ehrlich's methylene- 

 blue experiment described on p. 1186. On the other hand, the ten- 

 sion of carbon dioxide in the tissues, as judged from the examination 

 of fluids such as bile and urine, varies from 6 to 10 per cent, of an 

 atmosphere. The continuous flow of oxygen into, and of carbon 

 dioxide away from, the tissues points to the constant occurrence of 

 oxidative changes in the tissue-cells. By the blood the tissues receive 

 not only oxygen but also food-stuffs, namely, proteins or amino-acids, 

 fats, and sugars, derived from the alimentary canal, or, in starvation, 

 from other parts of the body. The activity of the tissues, whether 

 motor, as in the case of muscle, or secretory, as in the case of glands, is 

 derived from the energy set free in the partial or complete oxidation 

 of these food-stuffs, which occurs within the active cells themselves. 

 A study of the mechanism of oxidation in the body involves there- 

 fore a consideration of the processes which take place within the con- 

 fines of each cell. The question is by no means an easy one. Although 

 we speak of the ' burning ' of food-stuffs, and compare the processes 

 in the body to those which take place in combustion, e.g. in a candle- 

 flame, the analogy is after all a very rough one. In the first place, 

 the food-stuffs, even after absorption, belong to a class of substances 

 which have been designated as dysoxidisable, since they present no 

 tendency to combine with ordinary atmospheric oxygen. Thus 

 sugars, proteins, or fats, if kept free from microbial infection, may be 

 kept for years exposed to the air without undergoing any change. It is 

 true that in certain cases, e.g. in alkaline solutions of sugar, we may 

 obtain slow absorption of oxygen and oxidation of the sugar. The 

 changes are, however, slight and limited in extent. All these food- 

 stuffs are susceptible of combustion if raised to a sufficiently high 



1233 78 



