BLOOD 477 



plasma contains a small quantity — about 0"4 per cent. — of 

 another globulin (fibrinogen) which coagulates at a low 

 temperature, and which is absent from serum. It is this 

 which undergoes the change from the soluble form to the 

 insoluble form in coagulation. If, by taking advantage of 

 the fact that it is more easily precipitated by sodium chloride 

 than the other proteins, it is separated from them, it may 

 still be made to clot. The source of this substance seems to 

 be the intestine and liver, and when these are removed it is 

 not formed. 



The essential points in coagulation were discovered by 

 Andrew Buchanan in 1845. He showed that something 

 which he called "soluble fibrin" exists in the plasma and that 

 this chancfes to insoluble fibrin. He further showed that 

 the addition of the white cells of the blood brings about the 

 change. 



The process of clotting is due to the action of a 

 substance, thrombin, which does not exist as such in the 

 blood, but which is formed by the union of a precursor with 

 calcium ions. This is proved by the fact that if blood is 

 directly collected in alcohol, it is found to yield no thrombin, 

 although when treated with alcohol after clotting it is rich in 

 this substance. Its precursor may be called prothrombin. 



If calcium salts are precipitated by the addition of 

 oxalates to the blood, clotting does not take place. The 

 mere conversion of the calcium from an ionised state to a 

 non-ionised state, such as that in which it exists in the 

 citrate, prevents clotting. Hence, when unclotted blood is 

 wanted, it may be collected in a vessel containing some 

 potassium oxalate or a solution of sodium citrate (Chemnical 

 Physiology). 



Although prothrombin and calcium ions exist together 

 in the blood, they do not form the thrombin necessary to 

 produce clotting, apparently because an anti-prothrombin is also 

 present. This may be separated from fibrinogen and from 

 prothrombin by heating the plasma to 60° C, which pre- 

 cipitates the fibrinogen and destroys the prothrombin, but 

 leaves the antithrombin unaltered. 



Before clotting can occur, antithrombin must be thrown 



