The only route by which glucose normally enters the glucose pool is through the hepatic vein, which receives glucose either from hepatocytes during fasting or from the gut via the portal vein in the absorptive phase of a meal. During fasting, when the peripheral plasma insulin concentration falls below the level that is required to stimulate or permit free entry of glucose into insulin-dependent cells (such as muscle, fat, and connective tissues), outflow from the glucose pool is restricted to noninsulin-dependent tissues such as the brain and erythron.
