324 - Multicellular Animals, Especially Man 



i 



Fig. 17-6. Delicate network of fibrin fibers deposited after thrombin (thrombose) has been added to a 

 fibrinogen solution. A similar network, formed in normally coagulating whole blood, enmeshes the cor- 

 puscles and fluid components and provides the thrombus, or clot, with a considerable degree of solidity. 

 The magnification in this electronmicrograph is 30,000 diameters. (Courtesy of Clinton Van Zandt Hawn, 

 Keith R. Porter, and the Rockefeller Institute for Medical Research.) 



tact with a damaged tissue, liberate the en- 

 zyme that initiates the whole series of clot- 

 ting reactions. First, thromboplastin is pro- 

 duced and this (together with ionic calcium) 

 leads to the production of thrombase. Then, 

 finally, thrombase catalyzes the production of 

 the thrombin clot. 



Recent studies on hemophilic blood have 

 revealed a very specific defect. Such blood 

 contains little, if any, of the normal plasma 

 globulin component, thromboplastinogen. 

 Now, in fact, the control of hemophilia can 

 be achieved by periodic injections of con- 

 centrates of the missing protein, which also 

 has been called the antihemophilic globulin. 



Although some factors still remain un- 

 known, the foregoing account of coagulation 

 is substantiated by a large accumulation of 



evidence. The synthesis of prothrombase is a 

 well-recognized function of the liver, and 

 both prothrombase and thrombase have been 

 extracted from blood in highly active, dry 

 form. Like other enzymes, thrombase is 

 readily destroyed by heat (50° C), although 

 the precursory protein, prothrombase, is 

 much more heat stable. Thromboplastino- 

 genase, which normally initiates the clotting 

 reactions, appears to come in small part from 

 the cells of the damaged tissues; but mainly 

 it comes from the blood platelets, which dis- 

 integrate whenever blood comes into contact 

 with a foreign surface — except when such a 

 surface is exceedingly smooth. Thus if blood 

 is drawn directly from a vein into a test tube 

 that has been coated internally with a film of 

 oil, coagulation is delayed for an hour or 



