Vasoconstriction occurs not only in arteries, but also in the venous vasculature, more in big venules than in tiny venules, and especially in the splanchnic region, and is mostly mediated by increased sympathetic nervous system activity and vasopressin production. Vasoconstriction in both arteries and veins can have negative effects on blood flow and therefore tissue oxygenation. Vasoconstriction can occur anywhere within the vascular system, but it most commonly affects small arteries near the surface of the skin (cutaneous arterioles) and large veins back into which they drain (venular valves). Arteries constrict to limit blood flow to tissues that don't need it and allow more rapid movement of blood through areas with higher metabolic demands. Veins constrict to reduce the volume of blood flowing into them, which would otherwise fill up with blood and cause them to swell beyond their normal size. The splanchnic region is a term used to describe the organs inside the abdominal cavity: liver, pancreas, stomach, intestines, and spleen. It should be noted that the splanchnic region includes both arteries and veins; however, for simplicity's sake we will refer only to arteries in this context. The splanchnic region is made up of large and small vessels that supply the muscles, glands, and other tissues in this area.