PERIPHERAL VENOUS SYSTEM 



IO83 



sent elastic distension of the vein segment through 

 increase in its circumference and length. 



The inference sometimes encountered, however, 

 that the vein remains at "zero" pressure until it is 

 "filled" is quite unrealistic. Some finite pressure is 

 required to restore the wall to its cylindrical shape. 

 For veins in situ, filling the vein must also overcome 

 tissue pressure, and local tissue pressure will itself 

 be augmented by the swelling of the vein. Finally, as 

 fluid starts to fill the vein, hydrostatic pressures will 

 be created unless the vein is perfectly horizontal. 

 Consequently, if one records intraluminal pressure 

 accurately while fluid is progressively added to an 

 empty vein in vivo, pressure starts to rise almost 

 immediatelv and does not show any recognizable 

 inflection when the vein is "filled" (fig. 3). 



The pressure-volume curves obtained from venous 

 segments, or the tension-length curves obtained from 

 strips or rings, are most commonly described as 

 curvilinear with considerable convexity toward the 

 length or volume axis. Clark (17) was the first to 

 recognize that these two types of measurement must 

 be related by the Laplacian relationship whereby 

 in a cylindrical tube, wall tension ( T) increases as a 



function of pressure (P) and radius (R ) : 

 T = P X R 



It follows that at small radii, relatively less wall 

 tension is created by a given pressure increment than 

 at large radii. The pressure-volume curve shows 

 correspondingly less curvature than the tension- 

 length curve. 



Figure 3 illustrates the volume change due to radial 

 distension as contrasted with the volume increase due 

 to elongation. It is apparent that most of the volume 

 increment results from radial distension within the 

 physiological range of venous pressures. It is only 

 above pressures of 30 to 40 cm H 2 that radial dis- 

 tension becomes restricted to the relatively low degree 

 of distensibilitv which is characteristic of longitudinal 

 distension. A further important characteristic of the 

 longitudinal distension of veins, that is not shown by 

 such data, is the spiral twist which veins exhibit when 

 they are subject to sufficient pressure to produce 

 significant longitudinal stretch. Presumably because 

 of the spiral structures within the vein wall, there is a 

 definite rotation of one end of the vein in respect to 

 the other end as the vein lengthens. Teleologically, 



70 



60 



so-: 



40 — 



30- 



20- 



10 



fig. 3. Volume distensibility of a segment of a dog's jugular vein in vivo 88.8 mm in initial length, 

 prepared by double ligation, cannulation, and ligation of side branches through small skin incisions 

 with as little disturbance of the surrounding tissue as possible. Length measurements were obtained 

 directly with calipers; radial distension was calculated from the known length and volume. Fluid 

 added at the rate of 0.4 cc/min. 



