8o 



COMPARATIVE PHYSIOLOGY 



nitrogenous compound) takes place in acid medium. At 

 present the physiological significance of helicorubin is obscure. 

 The Transport of Carbon Dioxide.— We have seen that in 

 the recovery phase of muscle the intake of oxygen is accom- 

 panied by an evolution of carbon dioxide. The removal of 

 carbon dioxide is generally considered under the heading of 

 respiration, though it cannot be assumed without direct evi- 

 dence that the intake of oxygen and the excretion of COo 

 always takes place in the same organs. 



T70 



30 40 



50 60 70 60 

 oj C Oj t-vi *«/m. 36a 



Fig. 21. — Curves relating the amount of CO2 taken up by a given 

 volume of blood to CO2 pressure, showing that oxygenated blood takes up 

 less CO.>, and that therefore oxygenation has made the blood more acid 

 (after J. S. Haldane). 



If the carriage of carbon dioxide in the blood of the mammal 

 is studied by analogous methods to those employed for plotting 

 the dissociation curve for haemoglobin — i.e. exposure of the 

 blood to different tensions of carbon dioxide and estimation 

 of the carbon dioxide absorbed, — it is found that the amount 

 of carbon dioxide which can be taken up by the blood is con- 

 siderably greater than that which would be dissolved by 

 physical solution even at comparatively high tensions. At 

 a partial pressure of 40 mm., which corresponds to the lowest 

 tension of CO2 with which the arterial blood of the mammal 



