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HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



Rhythmical active vasomotion is the third type of 

 activity seen in vascular beds. It has been observed in 

 arteries, arterioles, precapillary sphincters, and veins. 

 This regular alternation of contraction and relaxation 

 of vascular smooth muscle cells has been shown to 

 continue after denervation (129). It becomes more 

 marked in the most peripheral vessel, the best example 

 of it on the arterial side being at the level of the pre- 

 capillary sphincters. It is the predominant type of 

 activity shown by veins. In general, the more normal 

 conditions are, the more outstanding is the rhythmical 

 active vasomotion. 



Nicoll & Webb (88) offer several reasons to give 

 support to the hypothesis that this rhythmical ac- 

 tivity is the result of an inherent property of smooth 

 muscle cells rather than the response of vascular 

 muscle to a rhythmical discharge from the vasomotor 

 center or from humoral influences or physical condi- 

 tions. The reasons are these: a) Terminal arterioles, 

 precapillary sphincters, and veins exhibit rhythmical 

 active vasomotion after denervation, b) This type of 

 vasomotion is most highly developed in the venous 

 muscular coat and precapillary sphincters, neither of 

 which appears to be under direct control of vasomotor 

 nerves, c) Adjacent vessels vary independently in the 

 rate and magnitude of their rhythmical activitv. 



Rhythmical active vasomotion in veins is fre- 

 quently powerful, reducing the vascular lumen to 

 one-third or one-fourth of its resting diameter at the 

 peak of contraction. The rate at which the contrac- 

 tions and relaxations occur is usually much faster 

 than that observed in arterial vessels. 



In later reports, Chambers & Zweifach (19, 22) dis- 

 cussed vasomotion and its relation to fluid exchange 

 across the capillary wall. The term "vasomotion" was 

 still used only in reference to spontaneous contraction 

 and relaxation of the metarteriole and its branches. 

 Vasomotion in the precapillary offshoots was said to 

 produce alternate periods of varying hydrostatic pres- 

 sure, thus greatly influencing fluid exchange in the 

 capillary bed. When vasomotion was reduced or ab- 

 sent, blood pressure in the arterioles was spread 

 through the numerous capillaries of the bed resulting 

 in a slower flow through the capillaries and a subse- 

 quent accumulation of fluid in the collecting venules. 

 Such a situation would create a sufficient back pres- 

 sure to favor outward filtration. When vasomotion 

 was active, blood flow went primarily through the 

 arteriovenous pathways, bypassing the capillary ves- 

 sels and producing a rapid flow in collecting venules. 

 This bypassing of capillaries would favor drainage 

 from the capillary network, a condition which would 



increase inward filtration. In summarizing the sig- 

 nificance of vasomotion in fluid exchange in the 

 capillary bed, they state that the delicate vasomotor 

 adjustments, which vary the surface area over which 

 hydrostatic pressure may cause outward filtration, 

 play a greater role than the differences between 

 hydrostatic and colloidal pressure. Osmotic uptake, 

 which is responsible for inward filtration, is depend- 

 ent upon and reinforced by adequate venous outflow, 

 a factor influenced by vasomotion. 



Webb & Nicoll (130) refer to rhythmical active 

 vasomotion as being the outstanding activity in the 

 entire minute vascular bed and regard it as the prin- 

 cipal factor of a local nature that regulates blood flow, 

 and probably pressure, in the capillaries. All types of 

 anesthesia reduce or abolish active vasomotion of the 

 smaller vessels, and the authors suggest that this may 

 be the reason why such activity is overlooked in many 

 vascular studies. Active vasomotion is greatly reduced 

 in conditions in which vascular flow is sluggish or 

 irregular, or when the arterial pressure is low. Active 

 vasomotion in arterioles can be augmented by sudden 

 increases in intra-arteriolar pressure. 



Active vasomotion was further discussed by Xicoll 

 & Webb (89) in a paper in which investigations to 

 determine the effect of environmental changes on 

 active vasomotion were described. Arteries and the 

 largest arterioles were said to show two types of active 

 vasomotion, one being a slowly developing diameter 

 change dependent on tonus and the other, a rapid 

 diameter change dependent on the response of vascu- 

 lar muscle to nerve excitation. The smaller arcuate 

 and terminal arterioles showed active vasomotion 

 independent of nerve connection, and no classifica- 

 tion of this activity into tonus changes or contractile 

 responses was possible. However, two different types 

 of muscular activity were seen in these small vessels, 

 one being peristaltic waves sweeping along the ter- 

 minal arterioles and the other being localized contrac- 

 tion of the precapillary sphincters. 



The effect that active vasomotion has on blood flow 

 through capillary beds depends on both its intensity 

 and its duration. When constriction is not great 

 enough to close the lumen completely, plasma and 

 platelets continue to flow through the capillary vessels 

 while the cellular elements are held back. This is re- 

 ferred to as "plasma skimming.'" When contraction of 

 the vascular muscle is great enough to occlude the 

 lumen, blood flow into the capillary nets is necessarily 

 curtailed. The occlusion is normally temporary, result- 

 ing in intermittent flow through the capillary nets. 



Veins and venules in the bat wing with smooth 



