THE CHEMISTRY OF RESPIRATION 1115 



reducing oxy haemoglobin, which, is so unstable that it is decomposed by 

 simple physical means such as exposure to a vacuum. 



It was long debated whether the chief processes of oxidation take 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 its blood, if it 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 

 produced, and the tension of this gas in the tissues therefore rises. As 

 Barcroft has pointed out, in cold-blooded animals the dissociation of oxy- 

 haemoglobin with the setting free of oxygen must be largely conditioned 

 by the rise of carbon dioxide tension in the tissues, since at the normal 

 temperature of these animals the evolution of oxygen from haemoglobin 

 is extremely slow. The alteration in reaction of the blood, caused by a rise 

 in C0 2 tension or by the presence of small amounts of lactic acid, markedly 

 quickens the rate at which oxyhsemoglobin gives up its oxygen, as is shown 

 in Fig. 511. The carbon dioxide tension in the tissues may be approximately 

 measured by taking the tension of this gas in fluids such as the bile or urine. 

 Here it may amount to 8 or 10 per cent, of an atmosphere, and since the 

 carbon dioxide in venous blood is rarely above 6 per cent, of an atmosphere, 

 there is a descending scale of tensions from tissue to blood, just as there is 

 an ascending scale in the case of oxygen. This gas therefore passes from 

 the tissues through the lymph into the blood by a simple process of diffusion. 



The carbon dioxide carried by the blood is, like the oxygen, chiefly in a 

 state of chemical combination. From dogs' venous blood we may obtain 

 by means of the pump about 50 c.c. of carbon dioxide per 100 c.c. blood. 

 Water at the temperature of the body, if shaken up with an atmosphere of 

 carbon dioxide at a pressure of 760 mm. Hg., would take up about 50. per 

 cent, of the gas, and the plasma as a mere solvent would take up slightly 

 less. The tension of carbon dioxide in the blood is however much less than 

 1 atmosphere. Shaken up with pure carbon dioxide at a pressure of 1 

 atmosphere, the blood would take up as much as 150 per cent. If we 

 determine the tension of the carbon dioxide in the blood by one of the 

 methods to be described later, we find that in venous blood this gas is at a 

 pressure of only about 5 to 6 per cent, of an atmosphere (about 40 mm. 

 Hg.). Taking the pressure of the carbon dioxide as nV of an atmosphere, 

 and knowing that at a pressure of 1 atmosphere the blood might dissolve 

 50 volumes per cent., it is evident that at gV of an atmosphere the blood 

 would dissolve only 4 volumes per cent., i.e. about 2J volumes. All the 

 rest of the carbon dioxide in the blood must therefore be in combination 

 (cp. Fig. 512). 



