BLOOD 313 



arteries. All the tissues of the body are penetrated by a 

 system of very fine blood-vessels called capillaries, and it is 

 through the walls of these capillaries that the nutrient 

 material passes and thus feeds the cells of the tissues. 



Physically, blood is an opaque, red liquid consisting of a 

 colorless solution holding in suspension several kinds of 

 solids, one of them red in color. These red colored particles 

 are so numerous that they give the blood a red appearance. 

 The clear solution is called plasma. The suspended solids 

 are red corpuscles, white corpuscles, and some other small 

 bodies which need not be considered. 



(a) Plasma is a clear, transparent, colorless or slightly 

 yellow, partly viscid liquid, consisting largely of water which 

 holds in solution or in suspension proteins, fats, dextrose, 

 lecithin, mineral salts, urea, uric acid, enzymes, and gases. 

 The proteins are principally serum albumin, which is the 

 most important constituent, and is probably the source of the 

 body proteins. Fibrinogen is another protein, and although 

 not present in a large amount, is very imporant. This will 

 be discussed later under coagulation. Fats are present in 

 minute globules. The inorganic salts are mostly sodium and 

 potassium chlorides, carbonates, sulphates and phosphates, 

 together with calcium and magnesium phosphates. The 

 reaction of the blood is slightly alkaline, due to sodium 

 carbonate and phosphate. Urea and uric acid are waste 

 products. 



(6) Red Corpuscles, Erythrocytes, occur in the blood to 

 the extent of about 5,000,000 per cubic millimeter in man. 

 They are disk- or bell-shaped, about 7.5 microns in diameter 

 (0.0075 mm.) and about 2 microns thick (0.002 mm.) (Fig. 78). 

 They consist of a framework of protein material called 

 stroma and coloring matter called haemoglobin. The latter 

 compound can combine with oxygen, carbon monoxide, and 

 some other gases. It will crystallize, acts as a weak acid, 

 and is composed of a protein called globin and an iron com- 

 pound which is the real coloring matter. Haemoglobin when 

 combined with oxygen is called oxyhemoglobin. The union 

 is a weak chemical one in the proportion of one molecule 

 of haemoglobin to one molecule of oxygen. The union is 



