io 9 4 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



One is therefore justified to interpret distensibility 

 data in whatever fashion appears compatible with the 

 responses observed with his particular method. Since 

 there is overwhelming evidence that adrenergic 

 drugs are potent venoconstrictors, a reproducible 

 response to such drugs may be used as a reference 

 standard to which responses evoked by other stimuli 

 may be related. It would nonetheless be desirable 

 to adopt accurately standardized methods for dis- 

 tending veins so that data will have greater validity. 

 In view of the significant stress relaxation and creep 

 phenomena demonstrated by veins, one can scarcely 

 hope to obtain any precise information by making 

 measurements "as soon as the volume seems rea- 

 sonably stable." 



SUMMARY' OF VENOMOTOR RESPONSES 



From the great variety of techniques that have been 

 employed for the assessment of venomotor activity, a 

 wealth of information has been obtained which 

 demonstrates at least qualitativelv the types of 

 stimuli which evoke venomotor responses. In table i 

 are listed responses which stand without controversy 

 as representative of the active responses of the 

 venous system. It should be understood that this 

 listing does not pretend to be comprehensive or 

 cover any significant fraction of the full literature on 

 the subject; in general it has proven convenient to 

 confine the citations in this table to reports which 

 have been referred to for other purposes in our pre- 

 vious discussion. This evidence of a broad spectrum 

 of reactivity suggests that the venous system must 

 play an important function in active regulation of 

 the circulation as a whole. For a discussion of this 

 important aspect of venomotor action, the reader 

 is referred to Chapter 32. The remainder of our 

 comments will be confined to response characteristics 

 which appear to be of some unique importance to 

 the venous system. 



Most investigators of the venous system have been 

 impressed with the fact, first emphasized by Goll- 

 witzer-Meier (36), that the venous system appears 

 to act synergistically with the arterial system. This 

 is emphasized in the evidence presented in table 1 ; 

 with the exception of histamine, all responses listed 

 are direct counterparts of similar reactions known to 

 occur on the arterial side of the circulation. Beyond 

 this qualitative similarity, the interesting question 

 arises as to the relative sensitivity of arteriomotor 



and venomotor systems. The literature contains 

 definite suggestions that the venous system may have 

 a greater sensitivity (55, 92) and also make a greater 

 contribution to the total circulatory response (53), 

 although more information is needed to permit 

 sound generalizations. 



There are a few instances, however, in which the 

 responses of the venous system appear to be unique. 

 One is the influence of 5-hydroxytryptamine pre- 

 viously discussed in reference to figure g. Haddy's 

 data would indicate that this compound is more 

 effective in producing venoconstriction than it is in 

 blocking pre-existing venous constriction resulting 

 from high sympathetic tone, while for the small 

 vessels on the arterial side of the circulation, the 

 svmpathetic blockade can dominate the direct con- 

 strictor action of the compound. Therefore, 5- 

 hydroxytryptamine shares with histamine the capacity 

 to produce both venoconstriction and arteriolar 

 dilation; as a consequence both of these compounds 

 have the capacity to induce edema formation. There 

 is an indication that local tissue acidity may also 

 have such an action (27, 32). An inverse type of 

 dissociation between arterial and venous effects has 

 been reported to occur in circulatory shock, where 

 venoconstrictor mechanisms fail at a point at which 

 arterial constrictor tone is still well maintained (5). 



A particularly interesting dissociation also appears 

 to exist in reference to temperature effects. While cold 

 produces significant constriction of cutaneous 

 arterioles, it is even more effective as a venoconstric- 



