BLOOD 



ing the platelet to be a sphere of about 1 micron diameter, the 

 surface area would be of the order of 3 X 10^ A^. This would 

 provide space for approximately 500,000 molecules of a protein 

 the size of prothrombin, if oriented endwise. It is not intended 

 to imply that prothrombin is, in fact, the interacting species; 

 consideration of the clotting system might suggest a more likely 

 component whose frequency of occurrence in plasma might be 

 one or two orders of magnitude less than that of prothrombin. 

 In this connection, the molecular ratio of prothrombin to 

 fibrinogen presents a striking contrast to that normally found for 

 an enzyme and its substrate. With the use of data obtained with 

 relatively impure thrombin, it can be calculated that one mole- 

 cule of prothrombin is capable of catalyzing the conversion of 

 100,000 molecules of fibrinogen to fibrin. In terms of the 

 enzymatic reaction for which it is the substrate, the high molar 

 concentration of prothrombin must favor the formation of the 

 enzyme-substrate complex and consequently a rapid rate of 

 thrombin formation. 



Summary 



The fragmentary glimpse of blood in its cellular and molec- 

 ular dimensions lends perspective to the understanding of the 

 biochemical functions of this disperse tissue. The paramount 

 importance of the oxygen transport mechanism is confirmed by 

 the overwhelming molecular superiority of hemoglobin in com- 

 parison to plasma proteins. The contribution of albumin in the 

 maintenance of the circulating fluid volume of the blood, and the 

 size of the transport facility which derives from the affinity of 

 albumin for a diversity of small molecules and ions (without 

 significantly interfering with colloid osmotic pressure) need 

 hardly be stressed when viewed in these terms. In contrast to 

 the above, as well as to the /3-metal combining globulin and the 

 7-globulins, all of whose physiological interactions can be char- 

 acterized in terms of classical mass law chemistry, are the hor- 

 mones, enzymes, and other species which exert their influence 



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