THE RESPIRATORY SYSTEM. 28 1 



brought about by the action of the metabolic products set free in the 

 tissue upon the vessels of the organ ; this increases the supply of blood 

 and, therefore, of oxygen to the active tissues. 



The oxidative changes in the body normally take place in the tissue 

 cells and not in the blood itself. When methylene blue is injected 

 into an animal and the animal is killed a few minutes later, the blood 

 is coloured blue, whereas the tissues show no change of colour. 

 Although methylene blue is a comparatively stable substance, the 

 avidity of the tissues for oxygen is so great that they are able to reduce 

 it with the formation of a colourless reduction product. On exposing 

 the tissues to the air, methylene blue is re-formed, and the tissues 

 become deeply stained. 



Another method of demonstrating the fact that oxidation does not 

 take place in the blood is to allow it to stand for a short time, and to 

 observe whether any of its oxyheemoglobin is reduced. In normal 

 animals the reduction is almost negligible, and evidently no oxygen is 

 used up in carrying out metabolic changes in the blood itself. 



SECTION V. 

 THE EFFECT OF CHANGES IN BAROMETRIC PRESSURE. 



(1) The Effect of Lowered Pressure. When a person ascends from 

 near sea level to a height of 5,000 to 10,000 feet or more, he is apt 

 to suffer from symptoms which are generally described as mountain 

 sickness : these symptoms are headache, mental confusion, blueness 

 of the lips, and nausea or vomiting. They may occur when the 

 individual ascends to this height in a train, and are therefore not 

 due to muscular exercise, although they may become more severe when 

 exercise is taken. They are caused entirely by lack of oxygen. With 

 increasing altitude the barometric pressure, and therefore the partial 

 pressure of oxygen in the atmosphere and in the alveolar air, gradually 

 fall ; and when the alveolar pressure of oxygen falls to about 60 mm. 

 Hg, the symptoms just described make their appearance. Similar 

 symptoms are produced in animals and in man, when they are placed in 

 closed chambers and the barometric pressure is gradually reduced. If 

 the individual remains at a high altitude, the symptoms pass off in 

 the course of a day or two, and after a time exercise may be taken 

 without their recurrence. 



The adaptation of the body to the altered conditions is brought 

 about by changes in both the circulatory and respiratory systems. In 

 the first place, the lack of oxygen leads to the passage into the blood of 



