Neural modulation of transcapillary exchange of fluid and solutes in whole-organ preparations.

Activation of sympathetic adrenergic postganglionic fibers that release norepinephrine at the neuromuscular junction can influence resistance, capacitance, and exchange functions in the microcirculation. Although the predominant response is an increase in precapillary resistance that reduces blood flow through the tissue (at constant pressure perfusion), there are two distinct components that can be separated into an initial (1-2-minute) and late (2-15-minute) response. The initial (1-2-minute) response is a decrease in venous and capillary pressures in passive response to decreased flow promoted by an increase in precapillary resistance. Venous pressures are reduced even though postcapillary resistance is increased. The decrease in capillary pressure promotes absorption of fluid from tissue to plasma. Venous constriction results in a translocation of blood from the terminal venous system into the larger venous conduits toward the heart. Total blood volume in an isolated organ system decreases and is manifested as a decrease in volume (plethysmographic) or weight (gravimetric). The decrease in volume is result of three interrelated factors: 1) decrease in flow through the organ, 2) active venoconstriction and a translocation of blood out of the organ, and 3) absorption of fluid from tissue to plasma, which flows out of the organ with venous drainage. With continued sympathetic nervous system stimulation, the late response (3-15 minutes) is a gradual decrease in precapillary resistance that increases blood flow through the organ. Venous and capillary pressures passively increase in association with increases in flow.(ABSTRACT TRUNCATED AT 250 WORDS)