FUNCTIONS OF BLOOD 93 



function of carbonic anhydrase in this and in many other 

 cases is obscure and worth studying. 



One would expect it to be significant that carbonic anhy- 

 drase is never found outside cells (Robertson and Ferguson, 

 1936; van Goor). In vertebrate blood, where the bicarbonate 

 is mainly present in the plasma, the reacting substance 

 H2CO3 (or C0 2 ) must diffuse into the red cells, and the reac- 

 tion product (CO2 or H 2 C0 3 ) must diffuse out again. Thanks 

 to the small distances and enormous surfaces there is ample 

 time for this diffusion to take place during the passage through 

 capillaries, but it seems an unnecessary complication. 



The transport capacity of blood for oxygen. Water is a very 

 poor transport substance for oxygen, because the solubility is 

 quite low (3.4% by volume from an oxygen atmosphere at 

 15°C) and is even reduced by the presence of dissolved sub- 

 stances. A large number of animals, including all fairly large 

 and highly organized forms so far dealt with, possess special 

 transport substances, combining reversibly with oxygen ac- 

 cording to the O2 tension to which they are exposed. Four 

 groups of such substances are known, all of which are pig- 

 ments containing a heavy metal. The haemoglobins, chloro- 

 cruorins, and haemerythrins contain iron, and the haemo- 

 cyanins copper. They will be considered here from a 

 biological and only incidentally from a chemical or physico- 

 chemical point of view. These have been exhaustively dealt 

 with by Barcroft (1928), L. J. Henderson (1928), Redfield 

 (1933), and Florkin (1934). It will be convenient to deal 

 first with the haemoglobins which are both the best known and 

 the most interesting respiratory pigments. 



Haemoglobin is a proteid with an iron-containing prosthetic 

 group, haematin, in combination with a globulin which defi- 

 nitely differs from one species to another. The unit molecular 

 weight corresponding to 1 atom of iron is, according to 

 Svedberg (1933), 17,250, but haemoglobins differ greatly in 

 the number of units which the actual molecules contain. In 

 the vertebrates the blood-haemoglobins contain 4 such units, 



