Rat venule mechanical characteristics during venous pressure elevation

The purpose of this study was to compare the in vivo mechanical characteristics during venous pressure elevation of two sizes of venules in the intestinal muscularis of anesthetized Wistar rats. The two venules studied were the first-order venule (1V), ID = 71.2 +/- 3.3 microns [12] (means +/- SE [n]) and the small collecting vein (SCV), ID = 134.4 +/- 9.1 microns [12]. Vessel diameters and servonull pressures were measured as venous pressure was elevated with a snare about the vein of the isolated segment. The distension characteristics of the two vessels were significantly different, with the 1V evidencing greater strains (percent change from control diameter) and in vivo distensibilities [D = (delta lumen area/lumen area)/delta pressure] than the SCV at all pressure elevations above control. The 1V and SCV mean wall elastic moduli (calculated using Rhodin's measurements of venule control wall thickness) reflected these differences: the initial 1V wall Young's modulus was low and changed little during the vessel distension, whereas the SCV modulus was greater at control and increased sharply with strain in a nonlinear fashion. Although a portion of these differences can be attributed to different initial hoop wall stress levels, the SCV elastic modulus was significantly greater than the 1V modulus at comparable levels of wall stress. Both active and passive factors may contribute to these in vivo differences in elastic modulus, but this experiment does not distinguish between them.