PERIPHERAL VENOUS SYSTEM 



1087 



in the wall remains the same. Much as in the classical 

 soap bubble or balloon experiments, the smaller ele- 

 ment will force its contents into the larger element. 

 Although more information is needed to clarify this 

 phenomenon, it appears worthy of mention because 

 of its possible relationship to certain aspects of venous 

 varicosities. 



The phenomenon of more generalized venospasm 

 still remains largely unsolved. An important charac- 

 teristic is that this type of venospasm involves a sig- 

 nificant neurogenic spread which influences both 

 arterial and venous elements so as to throttle blood 

 flow through the entire bed. In experimental venous 

 thrombosis, for example, the reduction in blood flow 

 to an extremity far exceeds anything attributable to 

 the local mechanical block, apparently due to reflex 

 constriction involving all vascular elements of the 

 limb (66). A suggestion as to the factors responsible 

 for the maintenance of venospasm under these con- 

 ditions is found in the observations of Donegan (19). 

 In the presence of good blood flow and well-filled 

 veins, nerve stimulation evoked a brief phasic con- 

 striction of veins which resembled the typical response 

 of smooth muscle to nerve stimulation. When a vein 

 was poorly filled, on the other hand, little detectable 

 response was observed unless intense stimulation was 

 given, whereupon the vein constricted strongly and 

 remained constricted for a period of 6 to 12 min. It 

 is therefore evident that venous distension has an in- 

 fluence on the ability of the venous musculature to 

 contract maximally, which is quite compatible with 

 the influence of venoconstriction on venous distensi- 

 bility, as previously discussed. It leads us to the con- 

 clusion that venospasm is a phenomenon produced 

 when dynamic conditions are such as to displace the 

 vein over to the bottom of the sigmoid distensibility 

 curve (fig. 5). It is still far from clear, nevertheless, 

 as to just what types of conditions of stimulation can 

 evoke such a venospasm. 



ASSESSMENT OF VENOMOTOR ACTIVITY 



In ]'itro Studies 



The most direct method for studying venous smooth 

 musculature is to measure the reactivity of rings of 

 venous tissue in vitro. Rings, circumferential strips, 

 or spirally cut strips (59) are most suitable for such 

 studies; longitudinal preparations exhibit minimal 

 change. It was with such venous rings that Gunn 

 & Chavasse (40) first demonstrated clearly that veins 

 contract strongly in the presence of adrenergic drugs. 

 This adrenergic response has remained somewhat 



of a reference standard for comparing other agents, 

 and for evaluating the effectiveness of other tech- 

 niques for measuring venous tone. The action of many- 

 other agents has been examined in isolated prepara- 

 tions, so that there has developed an appreciable 

 literature on the pharmacology of veins (28, 30, 61). 

 Although the basic importance of these in vitro studies 

 must be recognized, such techniques give us limited 

 insight into the physiological responses of veins in 

 vivo. 



Direct Observation 



The simplest method for detecting activity of the 

 muscular elements of a vein in vivo is to observe 

 changes in the caliber of the vein. Many such obser- 

 vations have been reported and much valid infor- 

 mation has been deduced from such observations. 

 As a general approach to the problem, however, this 

 technique cannot be accepted as reliable. The ob- 

 server can only detect changes in the volume of blood 

 within the vessel. This change in blood volume may 

 be caused by a) changes in flow through the periph- 

 eral vascular bed, b) alterations in the pressure and/or 

 resistance to flow in more central portions of the 

 venous channels, c) changes in the extravascular tissue 

 pressure, or finally d) changes in the venomotor tone 

 in the vessel under observation. Unless the first three 

 factors can be confidently excluded, it is hazardous 

 to infer that the last factor is the crucial variable. 



It is rare that simple observation of veins will jus- 

 tify sound conclusions as to the cause of changes in 

 venous caliber. An exception to this exists in some 

 of the better controlled microscopic studies in which 

 attention is directed toward unique vessels in the 

 microscopic field. In this case the observer has the 

 advantage of being able to assess directly the rate of 

 capillary flow as well as the caliber and flow of vessels 

 in parallel and series with the vessel under study. The 

 contributions of such studies of the "microcirculation" 

 are reviewed in Chapter 27. At this point we would 

 only stress the principle that any observer of blood 

 vessels must be thinking in rigorous hemodynamic 

 terms if he is to make valid inferences from his ob- 

 servations, and refrain from any conclusions in regard 

 to venous tone without reasonably secure evidence 

 that other hemodynamic factors are not the signifi- 

 cant ones. 



Inferences from Venous Pressure 



It has already been emphasized that inferences 

 about venous tone based solely on venous pressure 



