View source for Digestion

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==Overview==
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'''Digestion''' is the process of [[metabolism]] where by a biological entity processes a substance in order to [[Chemistry|chemically]] and mechanically convert the substance for the [[body]] to use.  In [[mammal]]s, preparation for digestion begins with the [[cephalic phase]] in which [[saliva]] is produced in the [[mouth]] and [[digestive enzyme]]s are produced in the [[stomach]].  Mechanical and chemical digestion begin in the mouth where food is [[Mastication|chewed]], and mixed with [[saliva]] to break down [[starch]]es.  The stomach continues to break food down mechanically and chemically through the churning of the stomach and mixing with enzymes.  [[Absorption (chemistry)|Absorption]] occurs in the stomach and [[gastrointestinal tract]], and the process finishes with [[Defecation|excretion]].

Digestion occurs at the multicellular, cellular, and sub-cellular levels in animals and some plants. This process takes place in the digestive system, [[gastrointestinal tract]], or alimentary canal.  The digestive system as a whole is a one-way tube with accessory organs like the [[liver]], [[gallbladder]], and [[pancreas]] that assist in the chemical processes involved in digestion.

Digestion is usually divided into mechanical processing to reduce the size of food particles and chemical action to further reduce the size of particles and prepare them for absorption.  In most [[vertebrate]]s, digestion is a multi-stage process in the digestive system, following [[ingestion]] of the raw materials, most often other organisms. The process of ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four separate processes:
# Ingestion: placing food into the mouth
# Mechanical digestion & chemical digestion: [[mastication]] to tear and crush food, and churning of the stomach. Addition of chemicals ([[acid]], [[bile]], [[enzyme]]s, and water) to break down complex molecules into simple structures
# Absorption: movement of nutrients from the digestive system to the circulatory and lymphatic capillaries through [[osmosis]], [[active transport]], and [[diffusion]]
# Egestion: Removal of undigested materials from the digestive tract through [[defecation]]

Underlying the process is muscle movement throughout the system, [[swallowing]] and [[peristalsis]].

==Human digestion process==

===Phases of human digestion===

*[[Cephalic phase]] - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the [[cerebral cortex]]. Taste and smell stimulus is sent to the [[hypothalamus]] and [[medulla oblongata]]. After this it is routed through the [[vagus nerve]].
*Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distention of the stomach and alkaline [[pH]]. Distention activates long and myentric reflexes. This activates the release of [[acetylcholine]] which stimulates the release of more [[gastric juice]]s. As protein enters the stomach, it binds to [[hydrogen]] ions, which raises the [[pH]] of the [[stomach]] to an [[Alkalinity|alkaline]]level. This triggers [[G cell]]s to release [[gastrin]], which in turn stimulates [[parietal cell]]s to secrete [[Hydrochloric acid|HCl]].  HCl release is also triggered by [[acetylcholine]] and [[histamine]]. 
*Intestinal phase - This phase has 2 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.

===Oral cavity===

{{main|Mouth (human)}}
[[Image:Stomach colon rectum diagram.svg|thumb|left|[[Human]] [[Gastrointestinal tract]]]]
In humans, digestion begins in the [[Mouth (human)|oral cavity]] where food is [[Mastication|chewed]]. [[Saliva]] is secreted in large amounts (1-1.5 litre/day) by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue. There are two types of saliva.  One is a thin, watery secretion, and its purpose is to wet the food.  The other is a thick, mucous secretion, and it acts as a lubricant and causes food particles to stick together and form a [[bolus]].  The saliva serves to clean the oral cavity and moisten the food, and contains digestive [[enzyme]]s such as salivary [[amylase]], which aids in the [[Hydrolysis|chemical breakdown]] of [[polysaccharide]]s such as [[starch]] into [[disaccharide]]s such as [[maltose]].  It also contains mucin, a [[glycoprotein]] which helps soften the food into a [[Bolus (digestion)|bolus]].

[[Swallowing]] transports the chewed food into the [[esophagus]], passing through the [[oropharynx]] and [[hypopharynx]]. The mechanism for swallowing is coordinated by the swallowing center in the [[medulla oblongata]] and [[pons]]. The reflex is initiated by touch receptors in the pharynx as the bolus of food is pushed to the back of the mouth.

===Esophagus===<!-- Please do not change the spelling of Esophagus to Oesophagus - see WP:ENGVAR for why; regional spellings should not be switched back and forth -->

{{main|Esophagus}}
The [[esophagus]], a narrow, muscular tube about 20 centimeters (8 inches) long, starts at the pharynx, passes through the [[thorax]] and [[Thoracic diaphragm|diaphragm]], and ends at the [[Cardia|cardiac orifice]] of the [[stomach]].  The wall of the esophagus is made up of two layers of [[smooth muscle]]s, which form a continuous layer from the esophagus to the [[rectum]] and contract slowly, over long periods of time. The inner layer of muscles is arranged circularly in a series of descending rings, while the outer layer is arranged longitudinally.  At the top of the esophagus, is a flap of tissue called the [[epiglottis]] that closes during swallowing to prevent food from entering the [[trachea]] (windpipe).  The chewed food is pushed down the esophagus to the stomach through [[Peristalsis|peristaltic]] contraction of these muscles.  It takes only seconds for food to pass through the esophagus, and little digestion actually takes place.

===Stomach===

{{main|Stomach}}
The food enters the [[stomach]] after passing through the [[Cardia|cardiac orifice]]. In the stomach, food is further broken apart, and thoroughly mixed with a [[gastric acid]] and digestive enzymes that break down proteins.  The acid itself does not break down food molecules, rather, the acid provides an optimum pH for the reaction of the enzyme [[pepsin]].  The [[parietal cell]]s of the stomach also secrete a [[glycoprotein]] called [[intrinsic factor]] which enables the absorption of [[Cyanocobalamin|vitamin B-12]].  Other small molecules such as [[alcohol]] are [[Absorption (digestive)|absorbed]] in the stomach as well by passing through the membrane of the stomach and entering the [[circulatory system]] directly.

{{main|Histology of stomach}}
The transverse section of ailimentary canal reveals four distinct and well developed layers called serosa, muscular coat, submucosa and mucosa.
[[Serosa]]: It is the outermost thin layer of single cells called mesothelial cells.
[[Muscular coat]]: It is very well developed for churning of food. It has outer longitudinal, middle smooth and inner oblique muscles.
[[Submucosa]]: It has connective tissue containing lymph vessels, blood vessels and nerves.
[[Mucosa]]: It contains large folds filled with connective tissue. The gastric glands have a packing of lamina propria. [[Gastric glands]] may be simple or branched tubular secreting mucus, hydrochloric acid, pepsinogen and renin.

===Small intestine===

{{main|Small intestine}}
After being processed in the stomach, food is passed to the [[small intestine]] via the [[Pylorus|pyloric sphincter]]. The majority of digestion and [[absorption (digestive)|absorption]] occur here as [[chyme]] enters the [[duodenum]].  Here it is further mixed with three different liquids: 
# [[bile]], which [[Emulsion|emulsifies]] [[fat]]s to allow absorption, [[Neutralization|neutralizes]] the chyme, and is used to excrete waste products such as [[bilin (biochemistry)|bilin]] and [[bile acid]]s (which has other uses as well).  It is not an enzyme, however.
#[[pancreatic juice]] made by the [[pancreas]]
#intestinal enzymes of the alkaline mucosal membranes. The enzymes include: [[maltase]], [[lactase]] and [[sucrase]], to process [[sugar]]s; [[trypsin]] and [[chymotrypsin]] are also added in the small intestine

Most nutrient absorption takes place in the small intestine.  As the [[pH|acid level]] changes in the small intestines, more enzymes are activated to split apart the molecular structure of the various nutrients so they may be absorbed into the circulatory or lymphatic systems.  Nutrients pass through the small intestine's wall, which contains small, finger-like structures called [[Villius|villi]], each of which is covered with even smaller hair-like structures called [[Microvillus|microvilli]]. The [[blood]], which has absorbed nutrients, is carried away from the small intestine via the [[hepatic portal vein]] and goes to the [[liver]] for filtering, removal of toxins, and nutrient processing.  

The small intestine and remainder of the digestive tract undergoes [[peristalsis]] to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes.  When the circular muscles contract, the [[lumen (anatomy)|lumen]] becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter. In the stomach there is another phase that is called Mucus.

===Large intestine===

{{main|Large intestine}}
After the food has been passed through the small intestine, the food enters the [[large intestine]]. The large intestine is roughly 1.5 [[meter]]s long, with three parts: the [[cecum]] at the junction with the [[small intestine]], the [[Colon (anatomy)|colon]], and the [[rectum]]. The colon itself has four parts:  the [[ascending colon]], the [[transverse colon]], the [[descending colon]], and the [[sigmoid colon]].  The large intestine absorbs water from the [[bolus (digestion)|bolus]] and stores [[feces]] until it can be [[Defecation|egested]].  Food products that cannot go through the [[villi]], such as [[cellulose]] ([[dietary fiber]]), are mixed with other waste products from the body and become [[feces]].

===Carbohydrate digestion===

[[Carbohydrates]] are formed in growing plants and are found in grains, leafy vegetables, and other edible plant foods. The molecular structure of these plants is complex, or a [[polysaccharide]]; poly is a prefix meaning many. Plants form carbohydrate chains during growth by trapping carbon from the atmosphere, initially [[carbon dioxide]] (CO<sub>2</sub>).
Carbon is stored within the plant along with water (H<sub>2</sub>O) to form a complex starch containing a combination of carbon-hydrogen-oxygen in a fixed ratio of 1:2:1 respectively.

Plants with a high sugar content and table sugar represent a less complex structure and are called [[disaccharide]]s, or two sugar molecules bonded. Once digestion of either of these forms of carbohydrates are complete, the result is a single sugar structure,  a [[monosaccharide]]. These monosaccharides can be absorbed into the blood and used by individual cells to produce the energy compound [[adenosine triphosphate]] (ATP).

The digestive system starts the process of breaking down polysaccharides in the mouth through the introduction of [[amylase]], a digestive enzyme in [[saliva]]. The high acid content of the stomach inhibits the enzyme activity, so carbohydrate digestion is suspended in the stomach. Upon emptying into the small intestines, potential hydrogen (pH) changes dramatically from a strong acid to an [[alkaline]] content. The pancreas secretes [[bicarbonate]] to neutralize the acid from the stomach, and the mucus secreted in the tissue lining the intestines is alkaline which promotes digestive enzyme activity. Amylase is present in the small intestines and works with other enzymes to complete the breakdown of carbohydrate into a monosaccharide which is absorbed into the surrounding capillaries of the [[villi]].

Nutrients in the blood are transported to the liver via the [[hepatic portal circuit]], or loop, where final carbohydrate digestion is accomplished in the [[liver]]. The liver accomplishes carbohydrate digestion in response to the hormones [[insulin]] and [[glucagon]]. As blood glucose levels increase following digestion of a meal, the pancreas secretes insulin causing the liver to transform glucose to [[glycogen]], which is stored in the liver, adipose tissue, and in muscle cells, preventing [[hyperglycemia]]. A few hours following a meal, blood glucose will drop due to muscle activity, and the pancreas will now secrete glucagon which causes glycogen to be converted into glucose to prevent [[hypoglycemia]].

Note: In the discussion of digestion of carbohydrates; nouns ending in the suffix -ose usually indicate a sugar, such as [[lactose]]. Nouns ending in the suffix -ase indicates the enzyme that will break down the sugar, such as [[lactase]]. Enzymes usually begin with the [[substrate]] (substance) they are breaking down.  For example: malt'''ose''', a [[disaccharide]], is broken down by the enzyme malt'''ase''' (by the process of [[hydrolysis]]), resulting in a two [[glucose]] molecules, a [[monosaccharide]].

===Fat digestion=== <!--Fat digestion redirects here-->

The presence of fat in the small intestine produces hormones which stimulate the release of [[lipase]] from the pancreas and bile from the [[gallbladder]]. The lipase (activated by acid) breaks down the fat into [[monoglycerides]] and [[fatty acids]]. The bile [[emulsifies]] the fatty acids so they may be easily absorbed.

[[Short chain fatty acids|Short-]] and [[medium chain fatty acids]] are absorbed directly into the blood via intestine capillaries and travel through the [[portal vein]] just as other absorbed nutrients do. However, [[long chain fatty acids]] are too large to be directly released into the tiny intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine villi and reassembled again into [[triglycerides]]. The triglycerides are coated with [[cholesterol]] and protein (protein coat) into a compound called a [[chylomicron]]. 

Within the villi, the chylomicron enters a [[lymphatic]] capillary called a [[lacteal]], which merges into larger lymphatic vessels. It is transported via the lymphatic system and the [[thoracic duct]] up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left [[subclavian vein]]. At this point the chylomicrons can transport the triglycerides to where they are needed.

===Digestive hormones===

There are at least four hormones that aid and regulate the digestive system:  
* [[Gastrin]] - is in the [[stomach]] and stimulates the [[gastric gland]]s to secrete [[pepsinogen]](an inactive form of the enzyme [[pepsin]]) and [[hydrochloric acid]]. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low [[pH]] .
* [[Secretin]] - is in the [[duodenum]] and signals the secretion of sodium bicarbonate in the [[pancreas]] and it stimulates the [[bile]] secretion in the [[liver]].  This hormone responds to the acidity of the chyme. 
* [[Cholecystokinin]] (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the [[gall bladder]]. This hormone is secreted in response to fat in chyme.
* [[Gastric inhibitory peptide]] (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.

===Significance of pH in digestion===

Digestion is a complex process which is controlled by several factors. [[pH]] plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus, pH is typically about 6.8, very weakly acidic. [[Saliva]] controls pH in this region of the digestive tract. [[Salivary amylase]] is contained in saliva and starts the breakdown of carbohydrates into [[monosaccharides]]. Most digestive enzymes are sensitive to pH and will not function in a low-pH environment like the stomach. Low pH (below 5) indicates a strong acid, while a high pH (above 8) indicates a strong [[Base (chemistry)|base]]; the concentration of the acid or base, however, does also play a role.

pH in the stomach is very acidic and inhibits the breakdown of [[carbohydrates]] while there. The strong acid content of the stomach provides two benefits, both serving to [[Denaturation (biochemistry)|denature]] proteins for further digestion in the small intestines, as well as providing [[Innate immune system|non-specific immunity]], retarding or eliminating various [[pathogens]].

In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralise the acidic conditions from the stomach. Also the [[pancreatic duct]] empties into the duodenum, adding [[bicarbonate]] to neutralize the acidic [[chyme]], thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline, creating a pH of about 8.5, thus enabling absorption in a mild alkaline in the environment.

==Specialized organs in non-human animals==

Organisms have [[Evolution|evolved]] specialized organs to aid in the digestion of their food, modifying [[tongue]]s, teeth, and other organs to assist in digestion.  Certain insects may have a [[crop (anatomy)|crop]] or enlarged [[esophagus]], while birds and cockroaches have developed [[gizzard]]s to assist in the digestion of tough materials.  [[Herbivore]]s have evolved [[cecum]]s (or an [[abomasum]] in the case of [[ruminant]]s) to break down [[cellulose]] in plants.

==See also==

*[[Nutrition]]

==References==

*[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GITract.html#pancreas Kimball's Biology Pages, Digestion]
*[http://www.njacs.org/demobcce8.pdf Chemistry lecture]
*[http://ajpgi.physiology.org/cgi/content/full/285/5/G804 American Journal of Physiology, article]

==External links==

*[http://www2.ufp.pt/~pedros/qfisio/digestion.htm  Human Physiology - Digestion]
*[http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/index.htm NIH guide to digestive system]
[[Category:Digestive system]]
[[Category:Metabolism]]


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