446 • BLOOD AND LYMPH. 



pathological conditions much importance is given to the deter- 

 mination of the non-protein nitrogen, that is to say, the nitrogen 

 contained in the urea, uric acid, creatin, creatinin, ammonia 

 salts, and amino-acids. This nitrogen amounts to about 20 to 30 

 milhgrams per 100 grams of blood, of which about one-half is the 

 nitrogen of urea. 



Proteins of the Blood-plasma. — The general properties and 

 reactions of proteins and the related compounds, as well as a classi- 

 fication of those occurring in the animal body, are described briefly 

 in the Appendix. This description should be read before attempt- 

 ing to study the proteins of the plasma and the part they take in 

 coagulation. Three proteins are usually described as existing in 

 the plasma of circulating blood, namely, fibrinogen, paraglobulin, 

 or, as it is sometimes called, "serum-globuhn," and serum-albumin. 

 The first two of these proteins, fibrinogen and paraglobulin, belong 

 to the group of globulins, and hence have many properties in com- 

 mon. Serum-albumin belongs to the group of albumins, of which 

 egg-albumen constitutes another member. The proteins constitute 

 from 6 to 8 per cent, of the plasma, and investigations have shown 

 that this percentage is maintained with great constancy, indicat- 

 ing that the concentration in protein is a matter of essential im- 

 portance in the functions of the blood. Something is known re- 

 garding the origin and special functions of the fibrinogen, as will 

 be described below, but we have httle or no satisfactory informa- 

 tion regarding the origin, fate, and functions of the serum-albumin 

 and paraglobuhn. Formerly it was assumed that they represent 

 nutritive material absorbed from the alimentary canal and sup- 

 plied to the tissue-cells by the blood, but investigation has shown 

 that this view is not tenable. The plasma proteins may be re- 

 duced in quantity by experimental methods, * and in such cases it 

 is found that they are regenerated promptly, at fii-st rapidly and 

 then more slowly, the normal concentration being reached after 

 several days. It is to be presumed that normally these proteins 

 are being used constantly and as constantly being replaced, but we 

 know Httle at present of this part of their history. The view 

 usually advanced in regard to their functional value in the blood 

 is that they confer a certain viscosity which is important to it 

 physically as a circulating liquid, and secondly, they exert a small 

 but constant osmotic pressure which is of importance in preventing 

 the water of the blood from passing out of the vessels into the 

 tissue fluids. 



Serum-albumin. — This substance is a typical protein. It can 

 be obtained readily in crystaUine form from the horse's blood. 



* Whipple, Smith, and Belt, "American Journal of Physiology," 52, 54, and 

 72, 1920. 



