134 THE BLOOD 



Proteins. Fibrinogen is the substance in plasma which is converted into 

 fibrin on coagulation. It belongs to the class of proteins called globulins. 

 It is precipitated from plasma with serum globulin by saturation with MgSO 4 

 and NaCl. It is soluble in dilute salt solutions, but is not soluble in water. 

 It can be distinguished from serum globulin by a number of special reactions: 

 i. Its coagulation temperature is lower, 55 to 56 C. 2. It is completely 

 precipitated by saturation with NaCl as well as with MgSO 4 . 3. It gives 

 rise to an insoluble protein, fibrin. It may be, however, that fibrinogen is 

 not a simple protein, but a mixture or loose chemical combination of two 

 or more proteins. Fibrinogen is present in plasma to the extent of 0.2 to 

 o . 5 per cent. 



Serum globulin or paraglobulin is completely precipitated by MgSO 4 ; in- 

 completely by NaCl, and coagulates at a temperature of 75 C. It is like- 

 wise soluble in dilute salt solutions, but insoluble in water. It is present in 

 plasma in from 3 . 5 to 4 per cent. 



Serum albumin is the protein which predominates in human plasma. 

 It is readily obtained in crystalline form; is soluble in MgSO 4 and NaCl 

 solutions, but insoluble in saturated ammonium sulphate solutions; and 

 coagulates in neutral or acid solutions at from 73 to 75 C. 



Extractives. The extractives are the nitrogen-containing substances, 

 such as urea, uric acid, creatin, creatinin, etc.; glycogen, dextrose, choles- 

 terin, etc., a total of 0.5 to 0.6 per cent. The dextrose content amounts 

 to from o.i to o . 15 per cent. 



Ferments are also found in blood; first, a diastatic ferment converting 

 amyloids into sugars; second, a glycolytic ferment causing a disappearance 

 of sugar; third, a fat- splitting ferment, lipase; and fourth, fibrin ferment 

 (thrombin), or its precursor, prothrombin. 



Inorganic Substances. The blood plasma contains about 0.85 per cent, 

 of inorganic salts distributed as follows, the sodium chloride predominating: 



Parts in 1000 of Plasma. 



Chlorine 3-536 



Sulphuric acid 0.129 



Phosphoric acid 0.145 



Potassium 0.314 



Sodium 3-410 



Phosphate of lime 0.298 



Phosphate of magnesia 0.218 



Oxygen 0.455 



8-505 



The Serum. The serum is the liquid part of the blood or of the 

 plasma which remains after the fibrin has been formed and removed. It is 

 a transparent, yellowish, faintly alkaline fluid, with a specific gravity of 

 from 1025 to 1032. Serum may be obtained from blood corpuscles by allow- 



