334 RESPIRATORY FUNCTION OF THE BLOOD 



determining the volume of COg absorbed, the vohmie increasing as 

 H+ decreases. 



It has been stated that the bound COg of the blood is found as 

 bicarbonates produced by the combination of COg in solution as 

 H2CO3 with bases of the blood. But the blood, either venous or 

 arterial, has a pfL very near the neutral point, and cannot contain 

 any considerable quantity of free base to be neutralised by the 

 carbonic acid. The mechanism by which base is liberated for 

 combination with the respiratory COg without any considerable 

 change in the pH of the blood has been shown to be dependent on 

 the physical and chemical characteristics of haemoglobin and its 

 compounds. The part played by this compound in the transport 

 of oxygen has been indicated. It is equally important in the 

 transport of carbon-dioxide. 



By virtue of its chemical composition, haemoglobin in solution 

 can dissociate, either as an acid or as a base, according to the pH 

 of the solution ; in other words, it is amphoteric {q.v.). Therefore, 

 in solutions at a pH on the alkaline side of its isoelectric point 

 (about pH 6-8), haemoglobin reacts as a weak acid and can combine 

 with bases. In solutions of |?H less than 6-8 {i.e. more acid), 

 haemoglobin reacts as a weak base and can combine directly with 

 carbon-dioxide. Blood under ordinary conditions in vivo does not 

 reach a ^;H less than 6-8, and, therefore, the haemoglobin in it 

 always reacts as a weak acid, and cannot combine directly with 

 carbon-dioxide. Its action in the transport of the latter is, there- 

 fore, an indirect one. 



When carbon-dioxide enters the blood it reacts with the weak 

 salts of haemoglobin to liberate the free acid haemoglobin and form 

 bicarbonates, i.e. B.Hb + H2CO3 = BHCO3 + H.HB. Haemo- 

 globin is a very weak acid, and exerts a very slight influence on 

 the pH of the red cell contents, Oxy-haemoglobin, on the other 

 hand, acts as though it is a much stronger acid. 



It is known that where a base insufficient for the neutralisation 

 of both is added to a mixture of two acids, the proportion of the 

 salts formed from the two acids depends on the relative " strengths " 

 of the acids and also upon their relative concentrations in the mix- 

 ture. In the cells there appears to be a fixed quantity of base 

 which is distributed between the acids, haemoglogin and carbon- 

 dioxide, according to their relative strength and concentrations. 

 The concentration of haemoglobin can only be varied by variations 

 in total cell volume, as the cell is impermeable to haemoglobin, but 

 the concentration of carbon-dioxide in the red cell depends on the 

 carbon-dioxide tension of the plasma, which again is determined 

 by the COg tension in the lungs or tissues. The concentration of 



