Editing Digestion (section)

==Human digestion process==
{{Main|Human digestive system}}
{{Digestive system diagram}}

The [[human gastrointestinal tract]] is around {{Convert|9|m|abbr=off}} long. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal, and the process of digestion normally takes between 24 and 72 hours.<ref name=KongSingh2008>{{cite journal |vauthors=Kong F, Singh RP |title=Disintegration of solid foods in human stomach |journal=J. Food Sci. |volume=73 |issue=5 |pages=R67–80 |date=June 2008 |pmid=18577009 |doi=10.1111/j.1750-3841.2008.00766.x |doi-access=free }}</ref>

Digestion begins in the [[human mouth|mouth]] with the secretion of saliva and its digestive enzymes. Food is formed into a [[bolus (digestion)|bolus]] by the mechanical [[mastication]] and swallowed into the [[esophagus]] from where it enters the stomach through the action of [[peristalsis]]. [[Gastric juice]] contains [[hydrochloric acid]] and [[pepsin]] which could damage the stomach lining, but [[mucus]] and bicarbonates are secreted for protection. In the stomach further release of enzymes break down the food further and this is combined with the churning action of the stomach. Mainly proteins are digested in stomach. The partially digested food enters the [[duodenum]] as a thick semi-liquid [[chyme]]. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of [[bile]], [[pancreatic juice]] and [[intestinal juice]].  The intestinal walls are lined with [[intestinal villus|villi]], and their [[epithelial cells]] are covered with numerous [[Microvillus|microvilli]] to improve the absorption of nutrients by increasing the [[surface area]] of the intestine. Bile helps in emulsification of fats and also activates lipases.

In the large intestine, the passage of food is slower to enable fermentation by the [[gut flora]] to take place. Here, water is absorbed and waste material stored as [[Human feces|feces]] to be removed by defecation via the [[anal canal]] and [[Human anus|anus]].

=== Neural and biochemical control mechanisms ===
Different [[phases of digestion]] take place including: the [[cephalic phase]], [[gastric phase]], and [[intestinal phase]].

The cephalic phase occurs at the sight, thought and smell of food, which stimulate the [[cerebral cortex]]. Taste and smell stimuli are sent to the [[hypothalamus]] and [[medulla oblongata]]. After this it is routed through the [[vagus nerve]] and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and [[G cell]] (secretes gastrin) activity via [[D cell (biology)|D cell]] secretion of [[somatostatin]].

The gastric phase takes 3 to 4 hours. It is stimulated by [[Gastric distension|distension]] of the stomach, presence of food in stomach and decrease in [[pH]]. Distention activates long and myenteric reflexes. This activates the release of [[acetylcholine]], which stimulates the release of more gastric juices. As protein enters the stomach, it binds to [[hydrogen]] ions, which raises the pH of the stomach. Inhibition of gastrin and [[gastric acid]] secretion is lifted. This triggers G cells to release [[gastrin]], which in turn stimulates [[parietal cell]]s to secrete gastric acid.  Gastric acid is about 0.5% [[hydrochloric acid]], which lowers the pH to the desired pH of 1–3.  Acid release is also triggered by [[acetylcholine]] and [[histamine]].

The intestinal phase has two parts, the excitatory and the inhibitory. Partially digested food fills the [[duodenum]]. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating [[Sympathetic nervous system|sympathetic fibers]] causing the [[Pylorus|pyloric sphincter]] to tighten to prevent more food from entering, and inhibits local reflexes.