Anatomy and Physiology of the Gastrointestinal Microcirculation

Abstract

The gastrointestinal microcirculation subserves the functional activities of the gastrointestinal tract (transport and motor) that allow for the efficient assimilation of ingested nutrients. Intrinsic regulatory mechanisms ensure that local perfusion and oxygenation is adequate to support gastrointestinal function. The myogenic mechanism contributes to basal vascular tone, while the metabolic mechanism matches blood flow and O2 delivery to the metabolic demands of the postprandial state. Exposure of the gastrointestinal mucosa to noxious material (acid or lipids) elicits a neurogenic hyperemia to wash out and/or neutralize the threat. The mucosal capillaries are of the fenestrated type, allowing for the efficient transendothelial movement of small solutes (e.g., hydrolytic products of food digestion) while restricting the transendothelial movement of plasma proteins. In the preprandial state (nontransporting), the balance of hydrostatic and oncotic pressures across the capillaries ensures the maintenance of a normal interstitial hydration, i.e., the small net capillary filtration is balanced by an equal volume of lymphatic effluent. Solute-coupled fluid transport (absorption/secretion) is associated with appropriate adjustments in transcapillary forces and flows to minimize drastic changes in interstitial volume.