1 84 PHYSIOLOGICAL CHEMISTRY. 



When blood coagulates and the usual clot forms, a light yellow 

 fluid exudes. This is blood serum. It differs from blood plasma 

 in containing a large amount of fibrin ferment, a body of great 

 importance in the coagulation of the blood, and also in possessing 

 a lower protein content. The protein material present in plasma 

 and not found in serum is the fibrinogen which is transformed into 

 fibrin in the process of coagulation and removed. The specific 

 gravity of the serum of human blood varies between 1.026 and 

 1.032. If blood be drawn into a vessel and allowed to remain with- 

 out stirring or agitation of any sort the major portion of the red 

 corpuscles will sink away from the upper surface, causing this 

 portion of the. clot to assume a lighter color due to the predomin- 

 ance of leucocytes. This light-colored portion of the clot is called 

 the " buffy coat." 



Beside the protein constituents already mentioned, other bodies 

 which are found in both the plasma and serum are the following: 

 Sugar (dextrose), fat, enzymes, lecithin, cholesterol and its esters, 

 gases, coloring-matter (lutein or lipochrome) and mineral sub- 

 stances. In addition to these bodies the following substances have 

 been detected in normal human blood : Creatine, carbamic acid, hip- 

 puric acid, paralactic acid, urea and uric acid (urates). Some of 

 the pathological constituents of blood are proteases, leucine, tyro sine 

 and other amino acids, biliary constituents and purine bodies. 



There has recently been considerable controversy regarding the 

 form of the erythrocytes or red blood corpuscles of human blood. 

 It is claimed by some investigators that the cells are bell-shaped or 

 cup-shaped. As the erythrocytes occur normally in the circulation, 

 however, they are probably thin, non-nucleated, biconcave discs. 

 When examined singly under the microscope, they possess a pale 

 greenish-yellow color (see Plate IV, opposite), whereas when 

 grouped in large masses a reddish tint is noted. 



The blood of most mammals contains erythrocytes similar in 

 form to those of human blood. In the blood of birds, fishes, am- 

 phibians and reptiles the erythrocytes are ordinarily more or less 

 elliptical, biconvex and possess a nucleus. The erythrocytes vary 

 in size with the different animals. The average diameter of the 

 erythrocytes of blood from various species is given in the follow- 

 ing table i 1 



1 Wormley's Micro-Chemistry of Poisons, second edition, p. 733. 



