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. 1114. On 

 the other hand, the tension 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 foodstuffs, 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 foodstuffs, which occurs within 

 the active cells themselves. A study of the mechanism of oxidation in the 

 body involves therefore a consideration of the processes which take place 

 within the confines of each cell. The question is by no means an easy one. 

 Although we speak of the ' burning ' of foodstuffs, and compare the pro- 

 cesses 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 guarded 

 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 foodstuffs are susceptible of combustion if raised to a sufficiently high 

 temperature, but in the animal body the processes of oxidation have to go 

 on at a temperature varying between 5 and 40 C., and in a solution which 

 is almost neutral in reaction. It might be said that at the temperature of 

 an ordinary flame the combustion of the foodstuffs is immediate and 

 complete, whereas in the body the oxidation takes place by stages. Recent 



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