FORMED ELEMENTS 241 



accounted for by the adsorption of salts by the protoplasmic 

 colloids. Of these salts one of the most important is sodium 

 bicarbonate on account of its power of neutralising acid. This 

 has been termed its " buffer " value a term which, although 

 faulty, has crept into the writings of physiologists and clinicians 

 and seems firmly ensconced there because it is handy. As Prof. 

 Bayliss points out, a railway buffer absorbs shock but not the 

 engine, while NaHCO 3 absorbs the acid. 



The amount of NaHCO 3 present in plasma has been called the 

 alkali reserve of the body. How does it act ? The addition of 

 acid to bicarbonate may be represented by the equation 



NaHCO 3 +HA=NaA +H 2 O +CO 2 . 



Until nearly all the bicarbonate has been acted on by the acid, 

 no increase in acidity can be detected. This is a mechanism of 

 great value to the organism. Acids are constantly being produced 

 in the tissues, especially in muscle. Unless the organism had an 

 alkali reserve, the concentration of hydrogen ions would so increase 

 after muscular exercise, for instance, that a serious derangement 

 of metabolism would ensue (see Preservation of Neutrality, 

 Chap. XXX.). 



II. Formed Elements. 



The formed elements borne by the plasma have a volume of 

 half the plasma and weigh about the same amount. 



Plasma. Corpuscles. 



Volume 2 1 



Weight 3 2 



This may be determined by the haematocrite (see p. 422) or by 

 an ingenious method due to Stewart. He made use of the fact 

 that the presence of corpuscles reduces the electrical conductivity 

 of plasma in proportion to their number. 



They are of two kinds, viz. : the leucocytes and erythrocytes. 

 The former have been discussed in Chaps. XII. and XV. They 

 play a further part in stopping bleeding from a wound. As we 

 saw (p. 237) the thigmocytes (altered leucocytes) on undergoing 

 cytolysis set free thrombin. The blood contributes in a second 

 way towards the arrest of haemorrhage, viz. : by the agglutination 

 of certain cells to form a plug closing the opening. The primary 

 event is the local adhesion of the thigmocytes to the edge of the 

 wound. These altered cells acquire stickiness and any other 

 formed element coming into contact with them adheres. In this 

 way a plug is formed. 



B.B. 16 



