v] 



THE RESPIRATORY ORGANS 



71 



an increase in the amount of C'ar])oM dioxide also favour dis- 

 sociation. The carbon dioxide like the oxygen in the blood is 

 not simply dissolved in the blood but is in this case in combination 

 with the proteins in the blood ]3lasma, with haemoglobin in the 

 corpuscles as well as with sodium bicarbonate. The dissociation 



Percentage saturation 

 ivith onygen 



10 20 Ju 40 bO dO 7 bu 9u 100 



Oxygen pressure 

 mm. 



Fig. 39. Dissociation curve of oxyhaemoglobin (from Barcroft's 

 Ixc^piratonj Finiction of the Blood). 



curve of carbon dioxide in the blood is in general form similar 

 to that of oxygen. About 21 per cent, of carbon dioxide is 

 dissolved in the blood and is not in a state of chemical combination 

 with the salts or proteins. 



Internal Respiration. In the tissues the arterialised blood 

 l)ecomes converted into venous blood, and the process whereby 

 this occurs is termed internal or tissue respiration. Living 

 tissues undergo a perpetual process of oxidation to a greater 

 ox less extent (according to the activities of the cells con- 

 cerned), and consequently oxygen is being continually absorbed 

 while carbon dioxide is produced and given off into the blood. 

 When the tension of carbon dioxide in the blood increases beyond 

 the normal it has a direct stimulating action upon the respiratory 

 centre in the medulla oblongata and the movements involved in 



