The Circulatory System 179 



remains remarkably constant in its composition and is able effectively 

 to maintain the essential homeostasis of the organism. 



Clotting of Blood. — Blood as normally present in the blood vessels 

 is a liquid tissue, but upon exposure to air through injury to the skin, 

 it is able to form clots which prevent further blood loss. This ability 

 to form clots to stop the flow of blood has an obvious survival value. 

 It means that the animal is not in danger of death by hemorrhaging 

 from every slight injury. Equally important is the prevention of clot- 

 ting within the blood vessels. To accomplish these two completely 

 antagonistic processes, an elaborate series of reactions is necessarily 

 involved. 



Within the blood plasma are substances important in the clotting 

 mechanism ; these are fibrinogen, prothrombin, and calcium salts. The 

 formed elements, the platelets, are also involved in this process. 



When tissue is injured, the platelets disintegrate and both they 

 and the injured tissues produce a complex material, thrombokinase. 

 Thrombokinase, which literally means "clot hastener" actually contains 

 many different materials, one of which is known as cephalin. Throm- 

 bokinase plus the calcium ions of the blood plus prothrombin produces 

 thrombin. The thrombin next reacts with the fibrinogen normally 

 present in the blood to produce fibrin. Fibrin forms a meshwork which 

 entangles the blood cells and thus forms the clot. Briefly this can be 

 summarized as follows : 



Prothrombin -|- thrombokinase + calcium ions > thrombin 



Thrombin + fibrinogen > fibrin 



Fibrin + blood cells > clot. 



One Other essential for the proper clotting of blood is vitamin K, 

 which is necessary for the formation of prothrombin by the liver. This 

 vitamin need not necessarily be in food, for it can be manufactured 

 from foods by bacterial action in the intestine. 



Although this account of the blood-clotting mechanism may seem 

 very complex, it is actually a simplification of the many reactions in- 

 volved. Of equal interest is the fact that the blood does not clot within 

 the blood vessels. So long as all the materials for clotting are present, 

 it does seem remarkable that the formation of clots is actually such a 

 rare phenomenon. Clots do at times form on the walls of blood vessels 

 when an injury occurs. If the clot extends across the blood vessel, 

 it may interfere with circulation ; it is then known as a thrombus. If 

 this thrombus should break loose to float in the circulatory system, 



