414 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-senun " 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 448. 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 interpretation of the significance of the color 

 changes given by litmus, and on an inadequate conception of the 

 cause of acidity and alkalinity. 



A neutral solution is one in which the hydrogen and hydroxyl ions exist in 

 equal concentrations. If the hydrogen ions are in excess the solution is acid. 

 If the hydroxyl ions are in excess the solution is alkahne. In neutral solu- 

 tions, such as pure water, it is known also that the product of the concentration 

 of hydrogen ions by the concentration of hydroxyl ions is a constant. 



H X OH = K 



Hence it follows that if the value of K is known a determination of the con- 

 centration of either the hydrogen or the hydroxyl ions suffices to give the 

 value of the other factor. The methods of determining hydrogen ion concen- 

 tration are more exact and it is customary in giving the reaction of blood or of 

 any body liquid, whether acid or alkaline, to express it in terms of the hydrogen 

 ion concentration. For neutral solutions, such as pure water, the value of K 

 equals 1 X lO"!^. In such a solution, therefore, the hydrogen ion concentra- 

 tion would be 1 X 10-7 normal, or 0.0000001 gram per liter. The concentra- 

 tion of the hydroxyl ions would be the same. If, in a given liquid, the concen- 

 tration of hydrogen ions was found to be 1 X 10-^ or 0.00000001 gram, the 



