BLOOD 



In contemplating these examples of the transport functions 

 of blood, several striking features deserve emphasis. The 

 hemoglobin-oxygen interaction and its localization in the red 

 cell represents a signally efficient mechanism for providing 

 relatively large amounts of oxygen to the tissues. The lipo- 

 proteins represent the answer to the problem of transporting 

 substances which are physically incompatible with the aqueous 

 environment of the body. The transport of iron typifies the 

 conversion to a state of chemical or physiological inertness of 

 substances which might otherwise be toxic during transport in 

 the blood stream. In this connection, Cartwright and Wintrobe 

 (8) have shown that iron ascorbate could be injected into the 

 blood stream without incident so long as the binding capacity 

 of the circulating metal-combining protein was not exceeded. 

 If more than the stoichiometric amount of iron were introduced, 

 toxic reactions attributable to free iron were immediately 

 observed. Furthermore, the excess iron did not remain in the 

 blood ; the plasma iron concentration could not be raised above 

 the iron-binding capacity. Finally, tight chemical binding to 

 specific carrier substances allows for directed transport to 

 specific sites. Selectivity derives from the chemical characteris- 

 tics of the unloading reaction. Dissociation of the transported 

 substance may result from chemical modification of the bound 

 material (as by reduction in the case of iron), from the action of 

 a specific tissue component with higher affinity for the bound 

 material, from reduction of the concentration of the unbound 

 species as the result of utilization in metabolic reactions, or 

 indeed from any combination of circumstances which displace 

 the mass law equilibrium in the direction of the dissociated 

 components. 



Over-all, however, the specificity of many of these inter- 

 actions remains their most striking and least understood 

 characteristic. Some advances have been made toward eluci- 

 dating the steric nature of the binding site on the protein and its 

 relation to the small ion. Studies with related series of com- 

 pounds have shed light on the structural parts of the small ion 



663 



