154 THE BLOOD 



3. THE PLASMA 



As already mentioned on page 147, the blood coagulates a few minutes 

 after it has left the body,, and before a separation of the plasma from the 

 blood corpuscles can take place. Coagulation may be postponed by chilling 

 the blood to C. Then on account of their greater weight the corpuscles 

 sink to the bottom and (especially with horse's blood) a plasma entirely free 

 of corpuscles may be obtained for study. Most investigations on the fluids 

 of the blood relate however to the serum, which is distinguished from the 

 plasma chiefly by the fact that it contains no fibrinogen and less ash, because 

 the fibrin when it separates out carries down with it either mechanically or 

 in chemical combination, some of the ash constituents. 



A. CHEMICAL CONSTITUTION OF THE PLASMA 



Both plasma and serum are clear, faintly yellow fluids with a specific 

 gravity of about 1.028. The specific heat of serum is 0.9401, greater there- 

 fore than that of the whole blood. 



Besides water, plasma contains chiefly proteid substances of different kinds, 

 and mineral constituents. The osmotic tension of plasma is about equal to 

 that of a 0.9 per cent NaCl solution and is dependent mainly on its mineral 

 constituents. The proteid bodies also appear to influence this property, though 

 only to a relatively slight extent. 



The mineral constituents in the serum are dissociated into their ions to 

 the extent of about seventy-five per cent (Bugarsky and Tangl) ; while in the 

 whole blood, electrolytic dissociation amounts to only about forty per cent 

 (Oker-Bloom). 



The electrical conductivity of the whole blood is less than that of the 

 serum, because this property is diminished by the presence of the corpuscles, 

 as in general the conductivity of a solution is diminished by nonconducting 

 particles in suspension (Bugarsky and Tangl, Oker-Blom, et al.). 



The mineral substances in the serum differ essentially from those in the 

 corpuscles, in that sodium salts predominate in the former, potassium salts 

 in the latter. Among the sodium compounds, common salt occurs in greatest 

 quantity (about 0.6 per cent). Besides this, various other inorganic substances 

 have been found in the serum. On the whole the mineral substances in the 

 serum of human blood amount to- about 0.85 per cent. 



The organic substances in the serum amount to about ten times as much, 

 namely, 7.7-9.0 per cent. Among these the proteids are the most important 

 and make up by far the greatest part of the organic matter (about nine- 

 tenths). The chief proteids of the blood plasma are fibrinogen, serum globu- 

 lin and serum albumin. The latter two however are not to be regarded as 

 indivisible substances ; for numerous investigations in recent years have shown, 

 if one may judge by their behavior in salting out, that at least two, probably 

 several, globulins (euglobulin, pseudoglobulin) are present in the blood (cf. 

 page 74) ; and, according to results of fractional heat coagulation, that serum 

 albumin also is a mixture of different proteid substances. 



