224 ORGANIC AGRICULTURAL CHEMISTRY 



Serum Albumin. — The most important protein constit- 

 uent of serum is the albumin, which is like egg albumin in being 

 soluble in water. In considering the absorption and metab- 

 olism of protein food we stated that the absorbed products of 

 protein digestion enter the blood and are metabolized into 

 blood protein, which is then carried to the tissue cells, where 

 oxidation takes place. Although it is not fully estabHshed, it 

 is probable that the serum albumin is the form in which food 

 protein is carried to the cells. If this is true, the relation be- 

 tween serum albumin and food protein is of direct importance. 

 The amount of serum albumin varies in different animals, and 

 in man amounts to about 4.5 per cent. 



Fibrinogen and Fibrin. — The second protein constituent of 

 blood plasma is the fibrinogen, which is a globulin soluble in 

 dilute salt solutions such as the blood plasma. It is present in 

 small amount, probably not over 0.4 per cent, in man. It is 

 also a normal constituent of lymph. 



Clotting. — The special interest in connection with fibrino- 

 gen is its relation to the characteristic clotting property of blood. 

 This action takes place whenever fresh blood comes in contact 

 with the air, and is the natural process by which the flow of 

 blood from a wound is stopped, thus preventing too great loss 

 of this important body fluid. The process of clotting is a 

 complicated one, involving the action of several enzymes. It 

 is not fully established, but the most generally accepted view 

 in regard to the various steps may be given as follows. 



The leucocytes, or white blood corpuscles, together with the 

 blood plates produce, probably with the aid of a kinase, or 

 activator, a ferment known as prothrombin. This prothrombin 

 is the zymogen of another enzyme, and when activated by cal- 

 cium salts, present in the plasma, produces the enzyme called 

 thrombin, or fibrin enzyme. Thrombin then acts directly upon 

 the soluble protein fibrinogen, and the product is an insoluble 

 protein called fibrin. The separation or coagulation of this 

 insoluble protein forms the network or foundation of the blood 

 clot, which contains also the solid suspended constituents of 



