THE CIRCULATION OF THE BLOOD. 



321 



of blood, and if sufficiently great may give rise to anemia and pallor. The 

 contractile elements at the periphery of the arterial system, in the so-called 

 arteriole region, therefore regulate the supply of blood to the tissues in 

 accordance with their functional needs. 



Moreover, as will be stated in subsequent paragraphs the degree of 

 contraction of the arteriole muscle influences very markedly the degree of 

 friction which the blood has to overcome in passing from the arteries into 

 the capillaries. If the muscle contracts vigorously the caliber of the arteriole 

 is diminished and the friction increases; if the muscle relaxes, the caliber 

 of the arteriole is augmented and the friction decreases. By virtue of its 

 tonic activity, the arteriole muscle at the periphery of the arterial system 

 offers considerable resistance to the outflow of the blood and this is there- 

 fore spoken of generally as the peripheral resistance, though there is included 

 under this term the resistance 

 offered by the small caliber of 

 the capillary blood-vessel as well. 

 This latter factor is constant, 

 the former variable. 



The Structure, Properties, 

 and Functions of the Capilla- 

 ries. The capillaries are small 

 vessels that connect the arteries 

 with the veins . Though different 

 in structure from a small artery 

 or vein, there is no sharp bound- 

 ary between them, as their struc- 

 tures pass imperceptibly one into 

 the other. A true capillary, 

 however, is of uniform size in 

 any given tissue and does not 

 undergo any noticeable decrease 

 in size from repeated branchings. 

 The diameter varies in different 

 tissues from 0.0045 mm - to -75 mm., just sufficiently large to permit the 

 easy passage of a single red corpuscle. The length varies from 0.5 mm. to 

 i mm. The wall of the capillary (Fig. 149) is composed of a single layer of 

 nucleated endothelial cells with serrated edges united by a cement material. 

 Though extremely short, the capillaries divide and subdivide a number of 

 times, forming meshes or networks, the closeness and general arrangement 

 of which vary in different localities. 



As the endothelial cells are living structures and characterized by irrita- 

 bility, contractility and tonicity, it may be assumed that the capillary wall as 

 a whole is characterized by the same properties. Upon the possession of 

 these properties the functions of the capillary depend. 



The function of the capillary wall is to permit of a passage of the nutritive 

 materials of the blood into the surrounding tissue spaces and of waste 

 products from the tissue spaces into the blood. The structure of the capil- 

 lary wall is well adapted for this purpose. Composed as it is of but a single 

 layer of endothelial cells, the thickness of which defies accurate measurement, 



FIG. 149. CAPILLARIES. THE OUTLINES OP 

 THE NUCLEATED ENDOTHELIAL CELLS WITH THE 

 CEMENT BLACKENED BY THE ACTION OF SILVER 

 NITRATE. (Landois and Stirling.) 



