The gastrointestinal system, or GI tract, digests the food and other substances humans consume. In this process, it absorbs nutrients and water the body needs. The GI tract runs in one direction throughout the body, from the mouth to the anus. Under circumstances of infection or toxicity, the upper GI tract might force matter out in the opposing direction to protect the system.
The organs involved in the digestion process include the stomach, intestines, and accessory organs that secrete digestive juices - the pancreas, gall bladder, and liver.
Ingested materials enter the GI tract via the mouth. Food is broken up by the teeth and saliva into pieces that the system can handle. Behind the mouth is the oropharynx, a space continuous with the nasal cavity (nasopharynx) above and the larynx (laryngeal pharynx) below. The oropharynx and mouth are the only anatomical structures in the body to handle both food and air under normal conditions. Chewed food passes from the mouth to the oropharynx for swallowing.
Swallowing is a reflex triggered by the presence of material in the throat. Involuntarily, the palate closes the nasal cavity and the epiglottis, a flap of tissue in the throat, covers the windpipe and vocal cords. These actions are to prevent food from going into the nose or lungs. At the same time, the throat muscles contract to force the material in the throat downward and the esophageal sphincter relaxes to allow the food to continue through the gastrointestinal system. Sphincters are muscles that control organ openings by contracting or relaxing.
A collapsible, muscular tube located behind the windpipe, the esophagus, carries ingested materials from the pharynx by peristalsis, a rhythmic and involuntary contraction and relaxation found in many of the GI organs. The esophagus is lined with a thin tissue called the esophageal epithelium. This tissue is vulnerable where it connects to the stomach due to stomach acid, and it is where the pain of acid reflux, often called heartburn, originates.
The ingested material passed from the esophagus into the stomach, also called the gut, which is made of smooth muscle and lymphatic tissue. Lymphatic tissue is part of the immune system. Immune tissue in the GI tract creates a fast response to any ingested infectious agent, limiting its possibility of entering the blood or causing extensive damage to the internal organs. The digestive function of the gut is carried out by gastric acid and enzymes that chemically digest food, breaking down large matter into smaller pieces. Also in the stomach are parietal cells, which produce intrinsic factor, a molecule necessary for the absorption of vitamin B12. Absorption also occurs in a limited capacity in the stomach, which includes ions, lipid soluble compounds, and some irritants such as aspirin.
Peristalsis moves the partially digested food matter from the stomach into the small intestine for final digestion. The small intestine is named for its diameter, not its length. The three parts of the small intestine are termed the duodenum, jejunum, and ileum. Bile from the gall bladder and digestive enzymes from the pancreas, including trypsin, are secreted into the small intestines. The resulting material of digestion is the components of the ingested material or food: lipids, carbohydrates, and amino acids. Nutrients are absorbed across the mucosa of the intestines into the blood. Folds in the intestinal wall create a larger surface area for this process. The remaining food material is cellulose.
The appendix is attached to the end of the small intestine and the beginning of the large intestine, called the cecum. The appendix is a vestigial organ thought to play a role in the immune system, though it can be removed and should be if it becomes inflamed. The dead end pouch of the appendix can become infected or inflamed if material from the intestines inadvertently gets caught in the tissue.
The large intestines absorb water from the remaining undigested food material. The colon is the larger and last portion of the large intestine, and is also called the bowel. The colon is divided into four parts: ascending, transverse, descending, and sigmoid. The sigmoid colon is followed by the rectum, where food waste, called fecal matter, is stored prior to defecation. When the anal sphincter relaxes, the waste is expelled from the body through the anus.
Defecation is an important process to eliminate intestinal bacteria from the body as well as food waste. Intestinal blockage can result in disease and pain, and hindering the digestive process in a way that reduces absorption can result in malnourishment. All of the body's tissues rely on the water and nutrients absorbed by the GI tract, and the human body's gastrointestinal health relies on appropriate ingestion and waste passage.