174 ANATOMY AND PHYSIOLOGY 



The serum-albumin is the great tissue builder. The fibrinogen 

 is a fibrin maker (paraglobulin may assist, its use is not fully 

 known). 



Sugars and fats are tissue foods and sources of heat. 



Mineral salts preserve the necessary alkalinity of the blood and 

 assist in the formation of certain tissues (as bone). Sodium 

 chloride (common salt) is the most abundant and to this is due the 

 salinity of the blood. 1 Salinity is an exceedingly important 

 quality of plasma. It is essential to the interchange of fluids 

 between the vessels and the tissues and to the maintenance of 

 the rhythmic action of cardiac muscle. It is also necessary for 

 the preservation of blood corpuscles. Pure water invades them 

 (by osmosis) and so dilutes them that they swell and are de- 

 stroyed. The salinity of plasma is the same as that of the cells, 

 therefore no "diffusion streams" (or osmosis) can occur and the 

 cells are safe. For Coagulation of Blood see p. 217. 



THE CIRCULATORY ORGANS OF THE BLOOD 



This system includes the heart and blood-vessels (arteries, 

 capillaries and veins). They are the organs which contain the 

 blood. 



The heart is a pump. The arteries are elastic tubes which re- 

 ceive the blood directly from the heart. The capillaries are small 

 vessels into which the arteries lead, and the veins carry the blood 

 from the capillaries back to the heart. 



Arteries. Vessels which convey the blood away from the heart. 

 They are flexible tubes whose walls consist of three layers or 

 coats external, middle, and internal (or tunica adventitia, tunica 

 media, and tunica intima). The external coat is composed of 

 fibrous tissue to which the strength and toughness of the vessel is 

 due; the middle is composed of elastic tissue and unstriped muscle 

 fibers, giving to arteries their yielding and contractile chracter; 

 the internal is thin and smooth and is a continuation of the lining 

 of the heart. Arteries of medium size have most muscle tissue, 

 while the larger ones have most elastic tissue. It is owing to 

 their elasticity that arteries remain open when they are empty 

 or cut across. 



1 A "normal saline solution" contains salt in the proportion found in blood. 



