438 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



THE COAGULABLE PROTEINS OF BLOOD. 



Blood is a fluid which contains proteins, salts, glucose, amino acids 

 and other simple compounds in solution, and red and white blood cor- 

 puscles in suspension. The presence of blood platelets in living blood, 

 i.e. in the blood vessels, is denied by some observers, but they are un- 

 doubtedly present when the blood- is shed, their formation being in- 

 stantaneous. 



I. Clotting of Blood. 



Blood drawn from a blood vessel clots spontaneously into a 

 solid mass or clot. On standing the clot slowly contracts, express- 

 ing the almost colourless blood-serum. The clot consists of the insol- 

 uble protein, fibrin, which has entangled the blood corpuscles. The 

 fibrin, almost free from corpuscles, can be obtained by whipping the 

 blood whilst it clots and washing the fibrin threads with water to 

 remove the entangled corpuscles ; defibrinated blood remains, which 

 contains the corpuscles. Thus 



, Fibrinogen + Corpuscles Whipped f Fibrin 



ni A\ Blood J 



| Defibrinated Blood 



^ Serum I (Serum + Corpuscles) 



This may be readily observed as follows : 



A little freshly drawn blood is collected in two watch glasses. The 

 one is allowed to clot ; after a time the clot contracts, expressing the 

 serum. The other is defibrinated by stirring it with a pin or needle. 

 The fibrin adheres to the needle and defibrinated blood remains. The 

 fibrin can be washed free from corpuscles by water and an almost 

 colourless mass remains. 



II. Factors concerned in Clotting of Blood. 



In the process of clotting the insoluble protein fibrin is formed. 



It arises from fibrinogen, a soluble protein contained in the blood 

 plasma, by the action of fibrin ferment or thrombin. 



Thrombin again doss not exist normally in the blood, but is 

 formed from precursars after the blood is shed in the presence of 

 calcium salts. According to Morawitz the precursors are thrombogen, 

 which exists as such in the plasma, and thrombokinase, which is pro- 

 duced by the corpuscles or platelets, or may come from other tissues. 

 The evidence for the presence of thrombokinase was obtained from 

 experiments with birds' blood ; if it be carefully collected and the cor- 

 puscles separated without damage, it does not clot, but it clots on 

 adding damaged corpuscles or a scraping of muscular tissue. These 

 substances were previously described by Wooldridge under the names 

 B fibrinogen j( = thrombogen), A fibrinogen (= thrombokinase), and 

 C fibrinogen (= fibrinogen). The older observers, Alex. Schmidt, 

 Hammarsten, Arthus, only recognised the stage prothrombin which 

 was converted into thrombin by calcium salts. 



