THE CHEMISTRY OF RESPIRATION 1063 



with the atmosphere, take up oxygen from it to saturation, or to a point 

 not far removed from it. 



The blood, thus laden with oxygen, travels to the left side of the heart, 

 and from there is sent through the arteries to all parts of the body. It 

 must be remembered that neither in the lungs nor in the tissues does the 

 haemoglobin come in actual contact with the source of the oxygen, nor 

 with the cells which it is to supply. In both cases the interchange is effected 

 through the intermediation of the plasma and, in the tissues, of the lymph 

 as well. Since the tissue- elements are constantly using up oxygen, they 

 absorb any oxygen that is present in the surrounding lymph. There is, in 

 consequence, a descending scale of oxygen tensions from red blood- corpuscle 

 through plasma, vessel wall, lymph, and tissue- element. The cell draws from 

 the lymph, and the lymph from the plasma, so that the oxygen tension in the 

 plasma sinks. This has the same effect as if we put the red corpuscles in a 

 mercurial pump and lowered the pressure of gas. The immediate result is an 

 evolution of oxygen, which is taken up by the plasma, to be in turn passed 

 on to the lymph and the tissue-cell. 



Under normal circumstances a blood- corpuscle never stays long enough 

 in the proximity of the tissues to lose its whole store of oxygen. If, how- 

 ever, the further supply of oxygen to the blood be prevented, as in asphyxia, 

 the last traces of oxygen disappear from the blood. The enormous avidity 

 of the tissues for oxygen is shown by the following experiment (Ehrlich). If 

 a saturated solution of methylene blue be injected into the circulation of a 

 living animal and the animal be killed ten minutes later, it is found on first 

 opening the body that most of the organs present their natural colour, 

 although the blood is a dark blue colour. On exposure to the atmosphere 

 all the organs acquire a vivid blue colour. The avidity of the tissues for 

 oxygen has been so great that they have been able to decompose the methy- 

 lene-blue molecule, with the formation of a colourless reduction-product, 

 which on exposure to the air undergoes oxidation again and re-forms 

 methylene blue. If the tissues are able to effect the reduction of a com- 

 paratively stable body like methylene blue, it is easy to understand their 

 power of reducing oxyhsemoglobin, which is so unstable that it is decom- 

 posed by simple physical means such as exposure to a vacuum. 



It was long debated whether the chief processes of oxidation took place 

 in the blood or in the tissues. Our experiences with muscle would alone 

 serve to convince us that, in some tissues at any rate, processes of oxidation 

 take place, and the methylene-blue experiment shows that these processes of 

 oxidation are intense in all the chief organs of the body. It has been found 

 moreover that it is possible to keep a frog alive after substituting normal 

 saline solution for his blood, if he be placed in absolutely pure oxygen, and 

 that in this case indeed the metabolism of the animal goes on as actively as 

 before. As the frog has no blood, it is evident that its metabolic processes, 

 consisting of the taking up of oxygen and the giving out of carbon dioxide, 

 must have their seat in the tissues. 



As a result of the oxidative changes in the tissues carbon dioxide is 



