Influence of extravascular pressure on changes induced in vascular resistance in the small intestine by elevated venous pressure.

The influence of the extravascular pressure on the size of the increase in vascular resistance after elevation of venous outflow pressure (venous-vasomotor response) was studied in an intestinal segment, perfused at a constant rate, in anaesthetized dogs. If pressure in the lumen of the intestine was elevated (spontaneously, pharmacologically, mechanically) or pressure in the plethysmograph was raised, venous-vasomotor responses were either smaller or absent. When pressure in the intestinal lumen was raised, blood volume increments produced in the segment by elevated venous pressure were significantly smaller than those observed in the presence of resting pressure. The presence of a venous-vasomotor response was correlated to the quantitative relationship between the extravascular and the venous pressure. Its induction was dependent on whether the outflow venous pressure was higher than the pressure values in the intestinal lumen or the plethysmograph; in that case it developed to an extent corresponding to the increment in transmural vascular pressure.