RESPIRATION 129 



the lungs ; that remaining in arterial blood on leaving the pul- 

 monary capillaries is equal to 5 per cent, of an atmosphere or 

 35 mm. of mercury. On the other hand, the pressure of carbon 

 dioxide in the tissues is high — 50 mm. to 70 mm. In conse- 

 quence, there is no difficulty in explaining on physical and 

 chemical grounds the transfer of carbon dioxide from tissues to 

 blood. 



The medium in which this transfer occurs is the lymph which 

 bathes the tissues. This is the carrier between the blood and 

 the cell, for it is in the cell that tissue respiration occurs, and 

 not in the blood. 



As evidence of the important fact that the oxidations take 

 place in the tissues, and not in the blood, two interesting experi- 

 ments may be quoted. If the blood in a frog's body be replaced 

 by saline solution, and the animal kept in an atmosphere of pure 

 oxygen, it will continue to produce carbon dioxide. It is obvious 

 in this case that the C0 2 has been produced from the tissue, as 

 the animal is without blood. A more telling experiment is the 

 following : Methylene blue is a comparatively stable oxygen- 

 holding substance — more so than oxyhemoglobin — and is like- 

 wise an extremely powerful dye. If a solution be injected into 

 the circulation and the animal destroyed, the blood is found dark 

 blue in colour, but the tissues, especially the muscles, are normal 

 in appearance. When, however, they are exposed to the air, 

 they turn a vivid blue. The explanation is that the tissues 

 robbed the methylene blue of oxygen, and formed a colourless 

 reduction product, which on exposure to the air took up oxygen 

 and again formed methylene blue. No more remarkable example 

 of the instantaneous absorption of oxygen by the tissues could 

 be furnished. 



In spite of the imperative necessity which exists for oxygen, 

 it is remarkable how little can be obtained from the body. From 

 muscle none can be obtained; from the various secretions — 

 lymph, bile, urine, milk, saliva — only a very little can be obtained, 

 though in all these cases there is an abundance of carbon dioxide. 

 In the respiration of muscle, which can be readily studied, the 

 muscle preparation of a frog will go on contracting until ex- 

 hausted in an atmosphere of hydrogen, producing carbon dioxide, 

 thereby showing that it is using oxygen, yet neither in the muscle 

 nor in the atmosphere surrounding it does any oxygen exist. 

 The question, therefore, is, How does the oxygen-free muscle 

 obtain oxygen for the production of carbon dioxide ? Few 

 things in the whole range of physiology are more difficult to 

 understand, but it is supposed that when the oxygen reaches 

 the muscle it is at once stored up in some compound or com- 

 pounds among the muscle molecules ; hence this substance has 



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