The function of the digestive system is to break down complex carbohydrates, protein and fat into nutrient molecules that can be absorbed by the epithelium of the gastrointestinal tract to nourish the body, and then rid the body of the indigestible remains.
The gastrointestinal tract is divided into four distinct parts: the esophagus, stomach, small intestine, and large intestine (colon). They are separated from each other by special muscles called sphincters which normally stay tightly closed and which regulate the movement of food and food residues from one part to another. Each part of the gastrointestinal tract has a unique function to perform in digestion, and as a result each part has a distinct type of motility and sensation.
The Enteric Nervous System (ENS) located in the sheaths of tissue lining the gastrointestinal tract manages every aspect of digestion, from the esophagus to the stomach, small intestine and colon. The ENS is a network of neurons, neurotransmitters and proteins communicating between neurons and other support cells controlling the gastrointestinal neuromuscular activities. As seen in the figure below, the command neurons and interneurons are spread throughout two layers of gut tissue called the myenteric plexus and the submuscosal plexus.
These command neurons control the pattern of activity in the gut through signals sent to the muscle layer (muscularis) which is composed of two layers of smooth muscle; circular and longitudinal. The ensuing electrical activities of the smooth muscle cells results in muscle contraction/relaxation that narrow/relax the lumen (central passageway) for peristalsis and segmentation and dictate the transit through the gastrointestinal tract.
After a meal, muscle contractions initiated by these command neurons of the distal stomach begin and propagate as a ring like contractions toward the pyloric sphincter. The pyloric region of the stomach holds about 30 ml of chyme (semi fluid mass of partly digested food) and with each peristaltic wave about 3 ml or less of chyme is emptied into the small intestine with the remainder being retropulsed. Titration and reduction in particle size occurs as the food is retropulsed. Liquids tend to empty from the stomach much more rapidly than solids. For a mixed liquid/solid meal, liquids begin to empty within 5 min, while the solid portion has a lag time of 15 to 60 min. The lag time for solid emptying is due to the time necessary for the stomach to triturate the food into small enough particles (1 to 5 mm in diameter) to pass through the pylorus, which acts as a sieve. Additionally, the composition of a meal influences gastric emptying. For example, a carbohydrate rich meal empties quicker from the stomach than a fat rich meal. Gastric emptying time following a 500 Kcal meal (50% of the Kcal from fat) is approximately four hours but as the Kcal or fat content increase this emptying time also increases.
The majority of digestion and absorption of nutrients ends up taking place in the small intestine – a convoluted tube which extends from the pyloric sphincter to the ileocecal valve where it joins the large intestine. In the fed state as the chyme enters the small intestine from the stomach it simply gets moved backward and forward a few centimeters at a time by alternating contraction and relaxation of circular smooth muscle. This segmental contraction gradually moves the intestinal content distally at a rate that is appropriated for efficient digestion and absorption of the nutrients. Once the digestive process is complete, peristaltic activity returns. The segmenting movements wane and the duodenal mucosa releases the hormone motilin. As the motilin blood concentration rises, peristaltic contractions initiated in the duodenum or proximal jejunum sweep slowly along the length of the small intestine in about two hours. This pattern of segmental and peristaltic activity is called the migrating motor complex (MMC).
Through the MMC, each day, about a liter of material is introduced into the colon, largely water, salt and fiber that has not been absorbed. The large intestine frames the small intestine on three sides and extends from the ileocecal valve to the anus. In terms of digestive system functioning, its major function is to absorb most of the remaining water from indigestible food residues, stores the residues temporarily and then eliminate them from the body as semisolid feces. The seemingly rapid transit observed in the small intestine is slowed down to a sluggish crawl as we hit the large intestine with it taking up to 24 hours for the material to move from the cecum to the rectum. When the material gets to 100 grams in the rectum it triggers a bowel movement.
Disruption in these patterns cause symptoms such as bloating, vomiting, constipation, or diarrhea which are associated with subjective sensations such as pain, bloating, fullness, and urgency to have a bowel movement.