CHAPTER XII 

 BLOOD: BLOOD CLOTTING 



On leaving the blood vessels, the blood clots so as to form a plug, 

 which assists in preventing further hemorrhage. The clotting must 

 therefore be considered as a protective mechanism against excessive 

 draining of blood out of the organism. When the wounded vessels 

 are small, the clotting, along with constriction of the damaged vessels 

 and the formation in them of thrombi containing large numbers of 

 platelets, serves to effect complete stoppage of the hemorrhage even 

 though the blood pressure may not have become materially reduced. 

 The greater loss of blood from larger vessels causes the arterial pressure 

 to fall, and this enables the clot to stiffen and seal the wound before 

 the pressure again rises. When the clotting power of the blood is 

 subnormal, life is endangered by even trivial wounds; under these 

 conditions the smallest surface scratch may continue to bleed exces- 

 sively in spite of whatever local treatment is applied. The most ex- 

 treme degree of this condition occurs in hemophilia, a disease which 

 is characterized by a most interesting family history namely, that 

 although it affects as a rule only certain of the male members of a family, 

 yet it is transmitted from generation to generation by the female side 

 alone. The disease has existed in certain of the royal families of 

 Europe for many generations, which has made it possible by con- 

 sulting the genealogical trees to demonstrate the infallibility of this 

 law of inheritance. 



The clotting of the blood is also either depressed or increased in a 

 variety of physiological and pathological conditions. We shall, however, 

 defer further consideration of these until we have learned something 

 of the nature of the factors which are responsible for the process itself. 



The Visible Changes in the Blood During Clotting 



In a few minutes after it leaves the blood vessels, the blood forms a 

 jelly-like clot, which adheres to the walls of the container in which the 

 blood is collected and soon becomes so solid that the vessel may be 

 inverted without spilling any of the blood. Clotting is now said to be 

 complete. The clot soon begins to contract, and as it does so, drops of 

 clear fluid or serum become expressed and float on the surface of the 



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