THE CIRCULATION OF THE BLOOD 



crease in the amount of blood beyond that supplied during functional inac- 

 tivity or rest. This is accomplished by a relaxation of the muscle-fibers. 

 With the cessation of activity the muscle-fibers again contract and reduce the 

 amount of blood to that required for nutritive purposes only. An increased 

 contraction of the muscle-fibers beyond the average, diminishes the outflow 

 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 enlarged 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 therefore 

 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 functions of the arteries 

 therefore are as follows: (i) to 

 distribute the blood to all regions 

 of the body; (2) to accommodate 

 varying volumes of blood; (3) to 

 convert the intermittent flow of 

 blood as it leaves the heart into 

 a remittent and finally into a con- 

 tinuous and equable flow as it 

 passes into the capillaries; (4) to 

 regulate the volume of blood de- 

 livered to an organ in accordance 

 with its functional activities, and 

 (5) to present that degree of re- 

 sistance necessary to the main- 

 tenance of the normal blood pressure, through the activities of the muscle 

 fibers in the walls of the arterioles. 



The Structure, Properties, and Functions of the Capillaries. The 

 capillaries are small vessels continuous with the arteries on the one hand 

 and with the veins on the other hand. Though different in structure from 

 a small artery or vein, there is no sharp boundary between them, as their 

 structures 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. 147) is composed of a single layer of 



FIG. 147. CAPILLARIES. THE OUTLINES OF 

 THE NUCLEATED ENDOTHELIAL CELLS WITH THE 

 CEMENT BLACKENED BY THE ACTION OF SILVER 

 NITRATE. (Landois and Stirling.} 



