PERIPHERAL VENOUS SYSTEM 



IO77 



constrictor mechanisms and, on the basis of changes 

 in arterial and venous pressure following spinal 

 transection, inferred that the nervous system must be 

 of importance in maintaining venous tone. Donegan 

 (19) studied this innervation in greater detail and 

 established that it was sympathetic in nature. As with 

 other sympathetic pathways, localization is rather 

 gross, with a given vein segment responding to stim- 

 ulation from several adjacent spinal segments. There 

 is now clear evidence of a tonic constrictor activity 

 of this sympathetic innervation, since venous dilation 

 occurs with sympathectomy (9, 54) or with sym- 

 patholytic drugs. This adrenergic sympathetic influ- 

 ence appears to be purely constrictor without any 

 dilator component (62). Conversely, there appears 

 to be no evidence of parasympathetic innervation of 

 veins. Although pharmacological doses of cholinergic 

 drugs may influence venous musculature, neither 

 parasympathetic stimulation nor atropinization have 

 any effect on venous tone (31). Even in such a highly- 

 specialized vascular function as penile erection, para- 

 sympathetic control appears to be restricted to the 

 arterial side of the circulation, with the veins playing 

 a purely passive role (47). 



Venous Valves 



A unique feature of the venous system is the pres- 

 ence of venous valves. The dramatic simplicity with 

 which the nature of this valve action can be demon- 

 strated in the veins on the dorsum of the hand remains 

 one of the classical observations of physiology. Clini- 

 cally, the role of the venous valves in the lower ex- 

 tremities have received particular attention. The 

 superficial veins of the leg, lacking protection from 

 surrounding muscle, are often subjected to prolonged 

 hydrostatic loads. This excessive distension of the 

 vessels may eventuate in valvular incompetence. The 

 consequences of this valvular incompetence and its 

 relation to venous varicosities and varicose ulcers has 

 been analyzed extensively in the literature on periph- 

 eral vascular surgery. (See Burch, Chapter 36.) 



This focusing of attention on the venous valves of 

 the extremities has distorted an appreciation of the 

 significance of valves in the venous system as a whole. 

 For example, a widely prevalent notion is typified by 

 the following statement from a leading textbook of 

 histology (46): "Valves are especially abundant in 

 the veins ot the extremities and they are generally 

 absent from the veins of the thorax and abdomen." 

 In actual fact, valves or valve-like structures have 

 been reported in most segments of the venous system, 



although generalizations are difficult because of the 

 marked species variation which has been reported 

 (29). The distinguishing feature of the valves in the 

 extremities is not their presence but the degree of 

 competence which thev exhibit; venous valves in 

 areas not confronted with severe hydrostatic strains 

 are usually more rudimentary and therefore less 

 easily demonstrated. 



An example of the latter type of valvular structure 

 in abdominal veins is illustrated in figure 1 from a 

 preparation made by Dr. Darrell Davis. This is a 

 photograph of a plastic cast of venous vessels obtained 

 by retrograde injection of a segment of dog intestine. 

 Numerous valve impressions are clearly indentifiable 

 on this preparation. Just before each point of junction, 

 most of the tributaries contain a valve. In every in- 

 stance the injection mass terminates with a bilobed 

 indentation which clearly represents a valve. Blood 

 traversing this venous bed must pass through a series 

 of valves before gaining access to the portal vein. 



It should be appreciated that figure 1 also demon- 

 strates a relative incompetency of these valves, in that 

 the injection mass has readily passed beyond a num- 

 ber of the valves. Accordingly, in spite of the profusion 

 of valves in this bed, it is reasonably easy to reverse 

 the flow of blood in the intestine. If artery and vein of 

 a loop of dog intestine are sectioned and a circuit 

 re-established whereby the intestinal vein is connected 

 to an arterial supply and the artery led out to a route 

 of venous drainage, a substantial retrograde flow is 

 observed for several minutes, eventually becoming 

 reduced as massive edema develops from the abnormal 

 capillary pressure relationships. Measurement of 

 pressure gradients and flow demonstrates that retro- 

 grade flow encounters a resistance of three to ten 

 times the vascular resistance to forward flow during 



fig. 1 . Plastic cast of mesenteric veins of a dog demonstrating 

 multiple valves. (Preparation made by Dr. Darrell Davis.) 



