When food is ingested, it is masticated in the mouth and transferred to the stomach where further digestion of the food takes place. Once the food settles in the stomach, it triggers a neural response which informs the parasympathetic nervous system to initiate the various processes involved in digestion.
The splanchnic vessels supplying blood to the gastrointestinal tract then open up to allow for an increase in blood flow to the intestines in preparation for the increased level of activity which occurs during the process of digestion. The parasympathetic nervous system also increases the motility of the gastrointestinal tract and stimulates production of digestive secretions from the pancreas and the liver into the tract.
This re-routing of blood supply into the splanchnic circulation during digestion leads to a reduction in the amount of blood available to the brain. This causes a relative cerebral ischemia, giving rise to drowsiness, especially following a heavy meal. This phenomenon is physiological and perfectly normal. The usual pattern of blood distribution returns once the process of digestion is substantially completed.
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