Blood flow and intestinal motility.

The effect of motility on blood flow varies with the nature of the motility and the initiating stimulus, and the mechanical compressing effect on contractions can be modified by intricate local regulatory mechanisms. Although rhythmic contractions usually decrease instantaneous arterial inflow and increase instantaneous venous outflow, mean blood flow may increase, decrease, or remain unchanged. A decrease in mean flow may indicate the predominance of the mechanical effects whereas an increase may be due to an active muscular hyperemia. Both tonic contractions and gut distention decrease total wall blood flow if mechanical compression overrides compensatory mechanisms; the effect is more pronounced on mucosal than on muscularis flow. Oxygen consumption and the capillary filtration coefficient may increase, decrease, or remain unchanged, depending on the degree of distention. The effects of chemicals depend on the interplay and predominance of their vascular, metabolic, and motility actions. When drawing conclusions regarding the effect of motility on blood flow, one must consider the role of local neural and chemical factors, alterations in the metabolic activity of different layers of the gut wall, collateral blood flow, and the mechanical effects of contractions and increased lumen pressure. Increased blood flow has little effect on motility, but severe or total ischemia and hypoxia produce a transient rise in motility followed by a prolonged paralysis.