Blood volume changes in microcirculation of rat intestine caused by carotid sinus baroreceptor reflex.

This series of experiments quantified the role of the rat small intestinal arterioles in capacity changes during bilateral carotid occlusion (BCO) and compared them with previous venular studies. We also determined the role of autoregulation in arteriolar constriction during BCO. First-order, second-order, and third-order arteriolar diameter changes were measured during changes in arteriolar pressure and/or sympathetic activity (via BCO). The results indicated that an arteriolar pressure drop caused an immediate significant transient diameter decrease of 14% (P < 0.001), followed by an average steady-state diameter increase of 11% (P < 0.001) over control because of autoregulation. During BCO, the arterioles demonstrated an initial 8-22% decrease in diameter (P < 0.05). The largest first-order vessels were the least responsive to changes in pressure and BCO, while the smallest third-order vessels were the most responsive. Apparently autoregulation, and not sympathetic activity, was responsible for most of the arteriolar constriction during BCO. We also developed an anatomic model of the rat intestinal vasculature which revealed that venules hold 70% of the microcirculatory volume and are responsible for 80% of the total blood shift during BCO. Venular constriction and, to a minor degree, arteriolar constriction result in a 14% decrease in microcirculatory intestinal blood volume during BCO.