THE COMPOSITION OF THE BLOOD 239 



The Formed Elements. By weight, the red corpuscles constitute 

 from 40 to 50 per cent, of the blood, the percentage varying under dif- 

 ferent conditions, while the total weight of the leucocytes and platelets 

 is insignificant. The hemoglobin constitutes from 86 to 94 per cent, by 

 weight of the solids of the red corpuscles, but the physical and chemical 

 relations that it bears to the stroma of the corpuscles are as yet undeter- 

 mined (see ' ' Hemolysis "). Of the remaining constituents of the corpus- 

 cles, from 5 to 12 per cent, consist of proteids, probably chiefly globulins and 

 nucleoproteids ; 0.3 to 0.7 per cent, of lecithin; and about 0.2 to 0.3 per 

 cent, of cholesterin (Hoppe-Seyler). The outer coat of the red corpuscles 

 does not seem to be equally permeable for all substances, and therefore 

 we find the composition of the fluid portion of the cell quite different 

 from that of the plasma about it. The salts of the corpuscles consist 

 largely of potassium phosphate, a little sodium chloride, some magnesium, 

 but no calcium, which is quite different from their proportion in the 

 plasma. Probably many of the other constituents of the plasma, 

 especially urea, penetrate the red corpuscles to a greater or less degree, 

 but most of them, particularly the sugar, remain chiefly in the plasma. 



Hemoglobin, the most characteristic constituent of all the heteroge- 

 neous components of the blood, is a compound proteid, and probably 

 exists combined with some other constituent of the corpuscle> most 

 probably the lecithin. It splits up readily into a proteid, globin, and an 

 iron-containing substance, hemochromogen, which readily takes up oxygen 

 to form hematin. Only about 4 to 5 per cent, of the hemoglobin is 

 hemochromogen, and iron constitutes but about 0. 4 per cent. Hematin 

 may be further split up into other substances, which will be considered 

 in the discussion of ' ' Hemorrhage. ' ' 



The leucocytes consist chiefly of nucleoproteids, with probably some 

 globulin, and they also contain glycogen, lecithin, and cholesterin. The 

 blood-platelets are believed to be largely nucleoproteid, but little is known 

 of their actual composition. 



Blood plasma differs from blood-serum in that the latter is formed 

 from the former through the conversion of the fibrinogen into fibrin. 

 Serum, therefore, contains no fibrinogen, but more fibrin ferment ; other- 

 wise it is practically the same as the plasma. 



Proteids. Fibrinogen has the general properties of a globulin, with 

 also a peculiar tendency to go into the insoluble form, fibrin. (This 

 process will be discussed under " Thrombosis."} In the plasma are also 

 other globulins, one soluble in water (pseudo- globulin), the other insoluble 

 in water (euglobulin}. /Serum-albumin, another proteid of the plasma, 

 probably consists of two or more varieties of albumin. There are also 

 nucleoproteids (prothrombin) and non-coagulable proteids, which being 

 poorly understood have been variously considered as glycoproteids, or 

 mucoids, or albumoses. 



Other Constituents. The fat of the plasma varies much according to 

 the time which has elapsed after the taking of food ; in fasting animals 

 it amounts to from 0. 1 to 0. 7 per cent. The sugar fluctuates less, being 

 normally about 0. 1 per cent. , while the urea has been estimated at 0. 05 

 per cent. Most of the sugar is dextrose ; but probably there is some 

 levulose, possibly some pentose and other forms, and possibly also sugar 

 combined with lecithin (jecorin] or other substances. Soaps, cholesterin, 

 and lecithin also exist free in the plasma. 



Plasma differs strikingly from the corpuscles in that its inorganic 

 substances are chiefly sodium and chlorine, while potassium and phos- 



