170 BIOLOGICAL CHEMISTRY 



is united with alkali. Finally, carbon dioxide may unite with 

 protein and some of it may be carried in this form. The 

 blood coming away from the tissues contains more bicarbonate 

 than the blood going to the tissues. In the lungs carbon 

 dioxide is removed and some of the bicarbonate is turned into 

 carbonate. The influence of oxyhaemoglobin on the removal 

 of carbon dioxide from carbonates has been mentioned 

 previously (p. 165). It is also possible that carbon dioxide can 

 unite directly with alkaline haemoglobin.* 



Owing to the increase in carbon dioxide the venous blood is 

 slightly more acid than the arterial blood. 



That serum proteins can act in the same way as haemoglobin 

 in competing with carbonic acid for the alkali of the blood is 

 suggested by Moore. As carbon dioxide is removed from the 

 bicarbonate the solution does not become markedly alkaline 

 because the base set free unites with the protein. Thus the 

 presence of protein keeps down the alkalinity and allows the 

 carbon dioxide to come off, but in the absence of protein the 

 rise of alkalinity would prevent the removal of more than a 

 minimal quantity of carbon dioxide, f 



Recent experiments show that the red blood corpuscles 

 carry most of the extra carbon dioxide from the tissues to 

 the lungs. This is probably dependent on the haemoglobin 

 in the corpuscles. { 



INTERNAL RESPIRATION 



Oxygen is carried to the tissues by the blood and carbon 

 dioxide is removed on the return journey of the blood from 

 the tissues. 



During rest there is a continuous oxidation occurring in 

 the tissues, and whenever a tissue becomes active it absorbs 

 more oxygen and gives up more carbon dioxide. The energy 

 changes during oxidation and the possible course of the 

 oxidation have already been indicated, here we are mainly 

 concerned with the exchange of gases between the blood 

 and the tissues. 



Between the blood and tissue cells there is usually a layer 

 of lymph, but that can be regarded merely as an indifferent 

 medium through which diffusion can occur. 



Increased activity causes an increased absorption of oxygen 

 and there are various ways in which that can be brought about. 



* G. A. Buckmaster, Journ. Physiol., 1917, vol. 51, p. 164. 



f B. Moore, J. M. M'Queen and T. A. Webster, Journ. Physiol., 

 1919, vol. 53, proc., p. xxvii. 



J L. W. Smith, J. H. Means and M. N. Woodwell. Journ. Biol. 

 Chem. y 1921, vol. 45, p. 245. 



