PATTERNS OF THE A-V PATHWAYS 



897 



fig. 3. Arcuate patterns in the terminal vascular bed. [From 

 Nicoll & Webb (89).] 



tween the outside diameter of the branch at its 

 junction and along its length. Terminal arterioles 

 originate mainly from the smallest arcuate vessels, 

 but may also arise from any of the arcuate arterioles 

 or a small artery (see fig. 3). 



The capillaries form extensive nets, and the distri- 

 bution of blood within the nets from any particular 

 terminal arteriole is limited. Local conditions, which 

 must be considered to be a major factor of control, 

 constantly change the paths of blood flow through 

 the capillary bed. 



Venous vessels show an arcuate pattern that 

 roughly follows that of the arteriolar vessels. At the 

 point where a capillary vessel joins a venule, a valve 

 may often be seen, although in many instances no 

 such structure is evident. Nicoll and Webb suggest 

 that since the muscular coat of the venule begins in 

 the immediate vicinity of the valve, this site may be 

 considered as the true junction between capillary 

 and venule. 



The flow of blood through the capillary nets is con- 

 trolled chiefly by activity of the terminal arterioles. 

 When they are dilated, flow is rapid and continuous 

 in the capillary nets. Constriction of a terminal 

 arteriole necessarily stops the flow of blood through 

 the capillary vessels supplied by it. When the numer- 

 ous terminal arterioles which supply an interconnec- 

 tive network of capillary vessels are contracting and 

 relaxing intermittently and aphasically, the flow of 

 blood into collecting venules may be continuous. Ces- 

 sation of flow from venous capillaries into venules is 

 often produced when resistance to inflow is met be- 

 cause of a closed valve at the junction of the two 



converging vessels. Forward flow is seen on opening 

 of the valve. 



Nicoll and Webb offer several features of both the 

 anatomical arrangement and the behavior of vessels 

 in terminal vascular beds as the regulators of blood 

 flow and blood pressure at this level, a) The arcuate 

 pattern of arterioles provides a means for intrinsic 

 regulation of flow and pressure. The roughly concen- 

 tric organization of the arcuate systems, made up of 

 anastomosing vessels of the same size, serve as volume 

 reservoirs for capillaries. Such an arrangement assures 

 an adequate blood supply for capillary nets which 

 does not fluctuate widely with changes in flow and 

 pressure in single arterial vessels. The authors con- 

 sider such an arrangement to be necessary in a system 

 in which the demand for blood varies and in which 

 some of the distributing vessels are distensible, thus 

 allowing increases in pressure to be absorbed in the 

 stretched vessels rather than to contribute to increased 

 flow, b) The angle of origin formed by an arteriole in 

 reference to its parent vessel affords a means by 

 which pressure may be abruptly reduced. Also, this 

 manner of branching off at a 90-degree angle or more 

 assures an adequate pressure head for each outlet from 

 a given vessel. This arrangement, coupled with the 

 fact that a capillary bed receives blood from several 

 terminal arterioles, results in equal pressure in all 

 capillaries regardless of their distance from their 

 arterial supply. Capillary pressure, sufficient for 

 proper function, can be maintained with minimal 

 arterial pressure, c) Nicoll and Webb believe that the 

 Indian Club formation at the arteriolar origins is 

 most important in pressure regulation. The actual 

 size of the orifice of each arteriole aids in reducing 

 pressure from artery to arteriole. The variability in 

 the size of the orifice, which depends on contraction or 

 relaxation of the muscle cells which form it, adds 

 another means of control of pressure in small arteries 

 and arterioles. It is possible that the contraction and 

 relaxation of the muscular elements at arteriolar 

 origins is determined by intra-arterial pressure. This 

 myogenic response would afford another intrinsic 

 mechanism whereby the pressure and flow through 

 capillary nets could be kept at a constant level inde- 

 pendent of wide variations among these values in 

 arterial vessels, d) Neural control of larger arteries 

 does not seem to be important in the regulation of 

 capillary blood flow, e) Active vasomotion in the 

 terminal arterioles causes blood flow through capillary 

 nets to alternate between very vigorous flow and no 

 flow at all. Local conditions determine the degree and 



