THE CHEMISTRY OF RESPIRATION 1187 



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 p.c. 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 



12-5 25 37-5 SO 



FIG. 497. Curves showing the rate at which oxyhaemoglobin is reduced on 

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

 presence of CO 2 and of lactic acid. Ordinates = percentage saturation of 

 oxyhaemoglobin. Abscissas = time in minutes. (MATHISON.) 



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 ^o f an atmosphere, and 

 knowing that at a pressure of 1 atmosphere the blood might dis- 

 solve 50 volumes per cent., it is evident that at -^ of an atmosphere 

 the blood would only dissolve f volumes per cent., i.e. about 2J 

 volumes. All the rest of the carbon dioxide in the blood must there- 

 fore be in combination (cp. Fig. 498). 



The carbon dioxide is contained chiefly in the plasma, though a 

 certain amount is also in combination in the corpuscles. It is evident 

 that part of the carbon dioxide at any rate must be in combination with 



