312 THE BLOOD 



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. XXXI.). 



II. Formed Elements 



The formed elements borne by the plasma have a volume about 

 equal to that of the plasma and weigh about the same amount. 



Plasma. Corpuscles. 



Volume . . 52-48 per cent. 48-52 per cent. 



Weiglit . . 3 2 



'fci' 



This may be determined by the haematocrite (see Part II.) 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. 



The Corpuscles 



The blood corpuscles are of three kinds : (i.) coloured corpuscles 

 or erythrocytes ; (ii.) colourless corpuscles or leucocytes ; and 

 (iii.) blood platelets. The colourless corpuscles have been men- 

 tioned already in Chaps. XII. and XVI., and two types of them 

 have been portrayed in Figs. 36 and 37. 



The blood-platelets {q.v.) are oval, colourless, refractile discs, 

 varying somewhat in size, but with an average measurement of 3/x 

 in their longest diameter. There are from 200,000 to 300,000 of 

 them in a cubic millimetre of blood. There is some doubt as to the 

 title of the platelet to be considered a cell. It has no nucleus or 

 chromatin material, and seems to consist of a homogeneous matrix 

 in which highly refractile granules are embedded. These granules 

 stain with basic stains {q.v.) like cresyl blue, and so are called 

 azurophil granules. Many haematologists are of the opinion that 

 the platelet is a complete cell budded off from the giant cells or 

 megakaryocyte of the bone marrow. Others consider that they are 

 fragments of cells or are the nuclei from young erythrocytes, or 

 even are some of the protein matter of the plasma gelatinised. 

 Whatever their origin, their function in the initiation of clotting 

 and in the formation of a firm thrombus is unquestioned {q.v.). 



The erythrocytes are the carriers^ submerged barges into which 

 are packed the oxygen for the tissues and some of the carbon 

 dioxide from the tissues. They are born in the red bone marrow. 



