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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



dimensions which include many components in ad- 

 dition to those controlling the elasticity of the struc- 

 ture. As a consequence, most of the data in the liter- 

 ature are presented as pressure-volume increments 

 of a system, the total volume of which remains un- 

 defined. 



A significant point in this connection relates to the 

 interpretation of the sigmoid distensibility pattern 

 of the constricted vein (fig. 5). If we accept the in- 

 terpretation given that stretch acts to "pull out" the 

 muscle so that distensibility is eventually restricted 

 by the elastic and fibrous tissue, it follows that at 

 extreme degrees of stretch both constricted and dilated 

 tissues should possess the same length at the same 

 tension. The record in figure 4 accords with this as a 

 first approximation, although actual measurements 

 reveal a minor discrepancy in the recorded lengths. 

 In vivo measurements, by direct injection (2) and 

 by indirect plethysmography studies (39), also sug- 

 gest that with maximum distension the volumes of 

 constricted and dilated veins converge. Burton (15), 

 however, has challenged this interpretation, and 

 offered evidence that constricted vessels retain a 

 smaller caliber than dilated vessels regardless of how 

 much they are stretched. Recent studies on isolated 

 tissues, the dimensions of which can be accurately 

 recorded, have thrown additional light on this prob- 

 lem (77). There appears to be no fixed correlation 

 between the unstretched length of a tissue and the 

 pattern of its distensibility curve. Although repeated 

 stretch of a constricted tissue can eliminate the sig- 

 moid distensibility pattern and yield a convex curve 

 indistinguishable from that of a dilated tissue, this 

 stretch does not erase the shorter unstretched length 

 of the constricted tissue. In other words, the "pulling 

 out" of the muscle by stretching a constricted tissue 

 does not transform it into a dilated tissue. An ulti- 

 mate resolution of the nature of these changes must 

 await a fundamental understanding of tissue elasticity, 

 very probably extending to the molecular level (78). 

 Until that understanding is at hand, we must be 

 cautious not to oversimplify the distensibility charac- 

 teristics of veins. 



Nature of Venous Constriction 



Most references to venous constriction visualize a 

 uniform increase in tonic activity of the muscle so as 

 to reduce the caliber of the vessel, without further 

 definition of the process or processes involved. There 

 is evidence, however, that in addition to increases in 

 venous tone there are more extreme and more sus- 



tained forms of venous constriction which justify the 

 designation of "venospasms." The latter may be 

 localized or may involve more extensive segments of 

 the venous bed. 



The local spasm which frequently is induced by 

 the trauma of venipuncture is a commonplace ob- 

 servation to both the experimentalist and the cli- 

 nician. Since this response cannot be abolished by 

 denervation, it must include a significant local mecha- 

 nism in its genesis. Inversely related to this traumatic 

 venous spasm may be the localized dilation which 

 can be evoked by tapping a vein (33). There are also 

 occasional reports of localized rings of constriction 

 in a venous bed under conditions where there appears 

 to have been uniform stimulation of the bed. Such 

 irregular segments of constriction are shown in figure 

 8, which illustrates intestinal veins following the 

 intra-arterial administration of levarterenol. In ex- 

 treme cases, this pattern of reactivitv may distort the 

 vein so that it resembles a string of sausages, with 

 rings of constriction separating distended segments. 



There are two plausible lines of explanation for 

 this type of segmental venospasm. One could relate 

 to anatomical or functional differentiation of different 

 regions of the vein wall in which certain segments 

 are more sensitive to stimulation or more powerful 

 in the magnitude of response that they can develop. 

 Another possible explanation relates to the Laplacian 

 phenonemon discussed earlier (17). The moment one 

 area of vein reduces its caliber slightly more than an 

 adjacent area, its mechanical ability to compress the 

 lumen increases, even though the tangential tension 



fig. 8. Segment of dog mesentery following the intra-arterial 

 injection of levarterenol, demonstrating the irregular distri- 

 bution of constriction sometimes exhibited by veins. 



