416 BLOOD AND LYMPH. 



Blood-serum and Defibrinated Blood. In connection with the 

 explanation of the term "blood-plasma " just given it will be con- 

 venient to define briefly the terms "blood-serum" and "defibrin- 

 .ated blood." Blood, after it escapes from the vessels, usually clots 

 or coagulates; the nature of this process is discussed in detail on 

 page 452. The clot, as it forms, gradually shrinks and squeezes out 

 a clear liquid to which the name blood-serum is given. Serum re- 

 sembles the plasma of normal blood in general appearance, but dif- 

 fers from it in composition, as will be explained later. At present 

 we may say, by way of a preliminary definition, that blood-serum is 

 the liquid part of blood after coagulation has taken place, as blood- 

 plasma is the liquid part of blood before coagulation has taken place. 

 If shed blood is whipped vigorously with a rod or some similar object 

 while it is clotting, the essential part of the clot namely, the fibrin 

 forms differently from what it does when the blood is allowed to 

 coagulate quietly ; it is deposited in shreds on the whipper. Blood 

 that has been treated in this way is known as defibrinated blood. It 

 consists of blood-serum plus the red and white corpuscles, and as far 

 as appearances go it resembles exactly normal blood; it has lost, 

 however, the power of clotting. A more complete definition of these 

 terms will be given after the subject of coagulation has been treated. 



Reaction of the Blood. When the blood is tested with litmus 

 paper it gives a distinct alkaline reaction, and if titrated with a 

 weak acid a considerable amount of the acid may be added before 

 the reaction, as tested by litmus, becomes neutral. On account 

 of these facts it was formerly believed that the liquid of the blood, 

 the blood-plasma, is markedly alkaline, owing to the presence in 

 it of sodium carbonate. It is now realized that this view was 



founded on a wrong conception of the cause of changes in reac- 



+ 

 tion. A liquid is acid when the hydrogen ions (H) in solution 



are in excess of the hydroxyl ions (OH) and it is alkaline when the 

 hydroxyl ions are in excess. Complete neutrality is obtained when 

 the concentration in hydrogen ions is equal to that of the hy- 

 droxyl ions. Acids are defined as compounds which, in aqueous 

 solution, undergo dissociation more or less completely with the 

 formation of hydrogen ions. HC1 dissociates, for example, into 

 + 



H and Cl, and the strength of the acid is proportional to the 

 number of hydrogen ions present in the solution. Alkalies, on the 

 other hand, are compounds which on dissociation yield hydroxyl 



ions either directly or indirectly. NaOH on dissociation yields 

 + 



Na and OH. Sodium carbonate also gives an alkaline solution, 



+ + 

 owing to a secondary reaction. Na 2 CO3 dissociates into Na, Na, 



and C0 3 , but the acid ion CO 3 then reacts with water to form 



