98 



THE INDIVIDUAL ORGANISM 



precursor, prothrombin. In the presence of thrombokinase and calcium ions pro- 

 thrombin is changed to the active enzyme thrombin. This enzj-me changes the 

 soluble protein fibrinogen to the insoluble protein fibrin, and clotting results. 



Small amounts of prothrombokinase must continually be liberated into the 

 plasma by the normal destruction of cells and platelets, but there is no resultant 

 clotting within the blood vessels. This is prevented by the presence in plasma of 

 minute amounts of at least two protective substances called antithrombins. These, 

 in the presence of neutral salts, inactivate the small amounts of thrombin pro- 

 duced and thus prevent the formation of fibrin. 



When sludging occurs, or when blood is shed or comes into contact with masses 

 of bacteria or with roughened surfaces in the blood vessels, the platelets first 



Constituenfs 



of Normal 



Blood 



Plasma 



itself 



contains 



Formation 



of 



Thrombokinase 



Formation 



of 

 Thrombin 



Formation 



of 



Fibrin 



PAteletA. 



Prothrombokinase 



PMaAwu*' 



Red Corpuscles 



Leucocytes 

 (playing no part) 



Fibrinogen 



Prothrombokinase + Ca 

 Thrombokinase 



Thrombokinase + Ca + Prothrombin 



7 



Thrombin ••».,„„* 



-\- 



in small amounts 



neutralized by 



Antithrombins 



Thrombin + Fibrinog en 

 (which causes clotting of the blood) 



Fig. 6.7. Diagram of the physiological mechanism of blood clotting. 



clump together and then disintegrate in great numbers. This releases large 

 amounts of prothrombokinase, which is activated to thrombokinase by calcium 

 ions. The thrombokinase changes prothrombin to thrombin in amounts exceeding 

 those inactivated by the antithrombins, and the excess changes fibrinogen to 

 fibrin. The entire process is shown in the accompanying diagram (Fig. 6.7). 



Even this is by no means the whole story. Vitamin K in some unknown manner 

 enables the tissues of the liver to produce the prothrombin of the blood. De- 

 ficiency of this vitamin diminishes or destroys the ability of the blood to clot. 

 Fortunately in man and various other animals vitamin K is synthesized by the 

 intestinal bacteria, so that deficiencies are rare. In persons showing the congenital 

 "disease" hemophilia the blood clots with extreme slowness if at all. This is 

 thought to be caused by failure of the platelets to yield prothrombokinase in 

 sufficient quantity, either because of their small numbers or because they do not 

 disintegrate so easily as in normal individuals. Local application of snake venom 



