1064 



PHYSIOLOGY 



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

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

 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 oxyhaemoglobin gives up its oxygen, as is shown in Fig. 499. 



The 



FIG. 499. Curves showing the rate at which arterial blood is reduced on bubbling 

 through a gas free from oxygen, and the effect on the rate of the presence of 

 C0 2 and of lactic acid. Ordinates = percentage saturation of oxyhsemoglobin. 

 Abscissae = time in minutes. (MATHISON.) 



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 descend- 

 ing 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. 



