{"id":2553,"date":"2026-06-08T12:17:24","date_gmt":"2026-06-08T12:17:24","guid":{"rendered":"https:\/\/edmpackz.com\/?p=2553"},"modified":"2026-06-08T12:17:24","modified_gmt":"2026-06-08T12:17:24","slug":"human-digestive-system","status":"publish","type":"post","link":"https:\/\/edmpackz.com\/?p=2553","title":{"rendered":"Human digestive system"},"content":{"rendered":"
<\/header>\n
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<\/div>\n
“Digestive system” and “alimentary system” redirect here. For digestive systems of non-human animals, see\u00a0Digestion<\/a>.<\/div>\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Human digestive system<\/th>\n<\/tr>\n
<\/a><\/span><\/p>\n
Human digestive system<\/div>\n<\/td>\n<\/tr>\n
Details<\/th>\n<\/tr>\n
Identifiers<\/th>\n<\/tr>\n
Latin<\/a><\/th>\nsystema digestorium<\/i><\/td>\n<\/tr>\n
MeSH<\/a><\/th>\nD004064<\/a><\/td>\n<\/tr>\n
TA98<\/a><\/th>\nA05.0.00.000<\/a><\/td>\n<\/tr>\n
TA2<\/a><\/th>\n2773<\/a><\/td>\n<\/tr>\n
TH<\/a><\/th>\nH3.04<\/a><\/td>\n<\/tr>\n
FMA<\/a><\/th>\n7152<\/a><\/td>\n<\/tr>\n
Anatomical terminology<\/a><\/p>\n
[edit on Wikidata<\/a>]<\/small><\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The\u00a0human digestive system<\/b>\u00a0consists of the\u00a0gastrointestinal tract<\/a>\u00a0plus the accessory organs of digestion (the\u00a0tongue<\/a>,\u00a0salivary glands<\/a>,\u00a0pancreas<\/a>,\u00a0liver<\/a>, and\u00a0gallbladder<\/a>).\u00a0Digestion<\/a>\u00a0involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the\u00a0cephalic phase<\/a>, the\u00a0gastric phase<\/a>, and the\u00a0intestinal phase<\/a>.<\/p>\n

The first stage, the cephalic phase of digestion, begins with secretions from\u00a0gastric glands<\/a>\u00a0in response to the sight and smell of food, and continues in the\u00a0mouth<\/a>\u00a0with the mechanical breakdown of food by\u00a0chewing<\/a>, and the chemical breakdown by\u00a0digestive enzymes<\/a>\u00a0in the\u00a0saliva<\/a>. Saliva contains\u00a0amylase<\/a>, and\u00a0lingual lipase<\/a>, secreted by the\u00a0salivary glands<\/a>, and\u00a0serous glands<\/a>\u00a0on the tongue. Chewing mixes the food with saliva to produce a\u00a0food bolus<\/a>\u00a0to be\u00a0swallowed<\/a>\u00a0down the\u00a0esophagus<\/a>\u00a0to enter the\u00a0stomach<\/a>. The second stage, the gastric phase, takes place in the stomach, where the food is further broken down by mixing with\u00a0gastric juice<\/a>\u00a0until it passes into the\u00a0duodenum<\/a>, the first part of the\u00a0small intestine<\/a>. The intestinal phase where the partially digested food is mixed with\u00a0pancreatic digestive enzymes<\/a>\u00a0completes the process of digestion.<\/p>\n

Digestion is helped by the chewing of food carried out by the\u00a0muscles of mastication<\/a>, the tongue, and the\u00a0teeth<\/a>, and also by the\u00a0contractions<\/a>\u00a0of\u00a0peristalsis<\/a>, and\u00a0segmentation<\/a>. Gastric juice containing\u00a0gastric acid<\/a>, and the production of\u00a0mucus<\/a>\u00a0in the stomach, are essential for the continuation of digestion.<\/p>\n

Peristalsis is the rhythmic contraction of\u00a0muscles<\/a>\u00a0that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract. This initially results in the production of\u00a0chyme<\/a>\u00a0which when fully broken down in the small intestine is absorbed as\u00a0chyle<\/a>\u00a0into the\u00a0lymphatic system<\/a>. Most of the digestion of food takes place in the small intestine. Water and some\u00a0minerals<\/a>\u00a0are reabsorbed back into the blood in the\u00a0large intestine<\/a>. The waste products of digestion (feces<\/a>) are\u00a0excreted<\/a>\u00a0from the\u00a0rectum<\/a>\u00a0via the\u00a0anus<\/a>.<\/p>\n

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Components<\/h2>\n<\/div>\n
<\/a>
Adult digestive system<\/figcaption><\/figure>\n

There are several organs involved in the digestion of food. The organs that are outside of the gastrointestinal tract (GI tract) but associated with digestion, are known as the\u00a0accessory digestive organs<\/b>\u00a0and include the\u00a0mouth<\/a>, and\u00a0tongue<\/a>, and glandular organs \u2013 the\u00a0salivary glands<\/a>, the\u00a0liver<\/a>,\u00a0gall bladder<\/a>\u00a0and\u00a0pancreas<\/a>.[<\/span>1]<\/span><\/a><\/sup>\u00a0Other components considered are the\u00a0teeth<\/a>\u00a0and\u00a0epiglottis<\/a>.[<\/span>2]<\/span><\/a><\/sup>\u00a0A number of\u00a0sphincters<\/a>\u00a0in the GI tract are also involved in digestion, including those of the esophagus (esophageal sphincters<\/a>) and stomach (pyloric sphincter<\/a>).<\/p>\n

The largest structure of the digestive\u00a0system<\/a>\u00a0is the GI tract. This starts at the mouth and ends at the\u00a0anus<\/a>, covering a distance of about nine metres (30\u00a0ft).[<\/span>3]<\/span><\/a><\/sup><\/p>\n

A major digestive organ is the\u00a0stomach<\/a>. Within its\u00a0mucosa<\/a>\u00a0are millions of embedded\u00a0gastric glands<\/a>. Their secretions as\u00a0gastric juice<\/a>\u00a0are vital to the functioning of the organ.<\/p>\n

Most of the digestion of food takes place in the\u00a0small intestine<\/a>\u00a0which is the longest part of the GI tract but has a smaller diameter than the large intestine.[<\/span>2]<\/span><\/a><\/sup><\/p>\n

The largest part of the GI tract is the\u00a0large intestine<\/a>, of which the\u00a0colon<\/a>\u00a0is the main part. Water is absorbed here and the remaining waste matter is stored prior to\u00a0defecation<\/a>.[<\/span>4]<\/span><\/a><\/sup><\/p>\n

There are many specialised\u00a0cells<\/a>\u00a0of the GI tract. These include the\u00a0taste receptors<\/a>, various cells of the gastric glands,\u00a0centroacinar cells<\/a>\u00a0of the\u00a0pancreas<\/a>,\u00a0enterocytes<\/a>\u00a0lining the intestinal epithelium, and\u00a0microfold cells<\/a>\u00a0also known as mucosal cells, mainly found in\u00a0gut-associated lymphoid tissue<\/a>\u00a0of the small intestine.[<\/span>5]<\/span><\/a><\/sup><\/p>\n

Some parts of the digestive system are also part of the\u00a0excretory system<\/a>, including the large intestine.[<\/span>6]<\/span><\/a><\/sup><\/p>\n

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Mouth<\/h3>\n<\/div>\n
\"3D<\/a>
3D medical illustration explaining the oral digestive system<\/figcaption><\/figure>\n

The\u00a0mouth<\/a>\u00a0is the first part of the\u00a0upper gastrointestinal tract<\/a>\u00a0and is equipped with several structures that begin the first processes of digestion.[<\/span>7]<\/span><\/a><\/sup>[<\/span>2]<\/span><\/a><\/sup>\u00a0These include salivary glands, teeth and the tongue. The mouth consists of two regions; the vestibule and the oral cavity proper.[<\/span>8]<\/span><\/a><\/sup>\u00a0The vestibule is the area between the teeth, lips and cheeks, and the rest is the oral cavity proper.[<\/span>9]<\/span><\/a><\/sup>[<\/span>8]<\/span><\/a><\/sup>\u00a0Most of the oral cavity is lined with\u00a0oral mucosa<\/a>, a\u00a0mucous membrane<\/a>\u00a0that produces a lubricating\u00a0mucus<\/a>, of which only a small amount is needed. Mucous membranes vary in structure in the different regions of the body but they all produce a lubricating mucus, which is either secreted by surface cells or more usually by underlying glands. The mucous membrane in the mouth continues as the thin mucosa which lines the bases of the teeth. The main component of mucus is a\u00a0glycoprotein<\/a>\u00a0called\u00a0mucin<\/a>\u00a0and the type secreted varies according to the region involved. Mucin is viscous, clear, and clinging. Underlying the mucous membrane in the mouth is a thin layer of\u00a0smooth muscle tissue<\/a>\u00a0and the loose connection to the membrane gives it its great elasticity.[<\/span>10]<\/span><\/a><\/sup>\u00a0It covers the cheeks, inner surfaces of the\u00a0lips<\/a>, and floor of the mouth, and the mucin produced is highly protective against\u00a0tooth decay<\/a>.[<\/span>11]<\/span><\/a><\/sup><\/p>\n

The roof of the mouth is termed the\u00a0palate<\/a>\u00a0and it separates the oral cavity from the nasal cavity. The palate is hard at the front of the mouth since the overlying mucosa is covering a plate of\u00a0bone<\/a>; it is softer and more pliable at the back being made of muscle and connective tissue, and it can move to swallow food and liquids. The\u00a0soft palate<\/a>\u00a0ends at the\u00a0uvula<\/a>.[<\/span>12]<\/span><\/a><\/sup>\u00a0The surface of the\u00a0hard palate<\/a>\u00a0allows for the pressure needed in eating food, to leave the nasal passage clear.[<\/span>13]<\/span><\/a><\/sup>\u00a0The opening between the lips is termed the oral fissure, and the opening into the throat is called the\u00a0fauces<\/a>.[<\/span>14]<\/span><\/a><\/sup><\/p>\n

At either side of the soft palate are the\u00a0palatoglossus muscles<\/a>\u00a0which also reach into regions of the tongue. These muscles raise the back of the tongue and also close both sides of the fauces to enable food to be swallowed.[<\/span>15]<\/span><\/a><\/sup>:\u200a1208\u200a<\/span><\/sup>\u00a0Mucus helps in the mastication of food in its ability to soften and collect the food in the formation of the bolus.<\/p>\n

\n

Salivary glands<\/h4>\n<\/div>\n
<\/a>
The main salivary glands<\/figcaption><\/figure>\n

There are three pairs of main\u00a0salivary glands<\/a>\u00a0and between 800 and 1,000 minor salivary glands, all of which mainly serve the digestive process, and also play an important role in the maintenance of dental health and general mouth lubrication, without which speech would be impossible.[<\/span>16]<\/span><\/a><\/sup>\u00a0The main glands are all\u00a0exocrine glands<\/a>, secreting via ducts. All of these glands terminate in the mouth. The largest of these are the\u00a0parotid glands<\/a>\u2014their secretion is mainly\u00a0serous<\/a>. The next pair are underneath the jaw, the\u00a0submandibular glands<\/a>, these produce both serous fluid and mucus. The serous fluid is produced by\u00a0serous glands<\/a>\u00a0in these salivary glands which also produce\u00a0lingual lipase<\/a>. They produce about 70% of the oral cavity saliva. The third pair are the\u00a0sublingual glands<\/a>\u00a0located underneath the tongue and their secretion is mainly mucous with a small percentage of saliva.<\/p>\n

Within the oral mucosa, and also on the tongue, palates, and floor of the mouth, are the minor salivary glands; their secretions are mainly mucous and they are innervated by the\u00a0facial nerve<\/a>\u00a0(CN7<\/a>).[<\/span>17]<\/span><\/a><\/sup>\u00a0The glands also secrete\u00a0amylase<\/a>\u00a0a first stage in the breakdown of food acting on the carbohydrate in the food to transform the starch content into maltose. There are other serous glands on the surface of the tongue that encircle\u00a0taste buds<\/a>\u00a0on the back part of the tongue and these also produce lingual\u00a0lipase<\/a>. Lipase is a\u00a0digestive enzyme<\/a>\u00a0that catalyses the\u00a0hydrolysis<\/a>\u00a0of\u00a0lipids<\/a>\u00a0(fats). These glands are termed\u00a0Von Ebner’s glands<\/a>\u00a0which have also been shown to have another function in the secretion of\u00a0histatins<\/a>\u00a0which offer an early defense (outside of the immune system) against microbes in food, when it makes contact with these glands on the tongue tissue.[<\/span>16]<\/span><\/a><\/sup>[<\/span>18]<\/span><\/a><\/sup>\u00a0Sensory information can stimulate the secretion of saliva providing the necessary fluid for the tongue to work with and also to ease swallowing of the food.<\/p>\n

<\/div>\n
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Saliva<\/h5>\n<\/div>\n
Main article:\u00a0Saliva<\/a><\/div>\n

Saliva<\/a>\u00a0moistens and softens food, and along with the chewing action of the teeth, transforms the food into a smooth\u00a0bolus<\/a>. The bolus is further helped by the lubrication provided by the saliva in its passage from the mouth into the esophagus. Also of importance is the presence in saliva of the digestive enzymes amylase and lipase. Amylase starts to work on the\u00a0starch<\/a>\u00a0in\u00a0carbohydrates<\/a>, breaking it down into the simple\u00a0sugars<\/a>\u00a0of\u00a0maltose<\/a>\u00a0and\u00a0dextrose<\/a>\u00a0that can be further broken down in the small intestine. Saliva in the mouth can account for 30% of this initial starch digestion. Lipase starts to work on breaking down\u00a0fats<\/a>. Lipase is further produced in the pancreas where it is released to continue this digestion of fats. The presence of salivary lipase is of prime importance in young babies whose pancreatic lipase has yet to be developed.[<\/span>19]<\/span><\/a><\/sup><\/p>\n

As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth.[<\/span>20]<\/span><\/a><\/sup>\u00a0It also has an\u00a0immunological<\/a>\u00a0role in supplying antibodies to the system, such as\u00a0immunoglobulin A<\/a>.[<\/span>21]<\/span><\/a><\/sup>\u00a0This is seen to be key in preventing\u00a0infections<\/a>\u00a0of the salivary glands, importantly that of\u00a0parotitis<\/a>.<\/p>\n

Saliva also contains a glycoprotein called\u00a0haptocorrin<\/a>\u00a0which is a binding protein to vitamin B12<\/sub>.[<\/span>22]<\/span><\/a><\/sup>\u00a0It binds with the vitamin in order to carry it safely through the acidic content of the stomach. When it reaches the duodenum, pancreatic enzymes break down the glycoprotein and free the vitamin which then binds with\u00a0intrinsic factor<\/a>.<\/p>\n

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Tongue<\/h4>\n<\/div>\n
<\/a>
<\/figcaption><\/figure>\n

Food enters the mouth where the\u00a0first stage<\/a>\u00a0in the digestive process takes place, with the action of the\u00a0tongue<\/a>\u00a0and the secretion of saliva. The tongue is a fleshy and\u00a0muscular<\/a>\u00a0sensory organ<\/a>, and the first sensory information is received via the taste buds in the\u00a0papillae<\/a>\u00a0on its surface. If the taste is agreeable, the tongue will go into action, manipulating the food in the mouth which stimulates the secretion of saliva from the salivary glands. The liquid quality of the saliva will help in the softening of the food and its enzyme content will start to break down the food whilst it is still in the mouth. The first part of the food to be broken down is the starch of carbohydrates (by the enzyme amylase in the saliva).<\/p>\n

The tongue is attached to the floor of the mouth by a ligamentous band called the\u00a0frenum<\/a>[<\/span>10]<\/span><\/a><\/sup>\u00a0and this gives it great mobility for the manipulation of food (and\u00a0speech<\/a>); the range of manipulation is optimally controlled by the action of several muscles and limited in its external range by the stretch of the frenum. The tongue’s two sets of muscles, are four\u00a0intrinsic muscles<\/a>\u00a0that originate in the tongue and are involved with its shaping, and four\u00a0extrinsic muscles<\/a>\u00a0originating in bone that are involved with its movement.<\/p>\n

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Taste<\/h5>\n<\/div>\n
Main article:\u00a0Taste<\/a><\/div>\n
<\/a>
Cross section of\u00a0circumvallate papilla<\/a>\u00a0showing arrangement of nerves and taste buds<\/figcaption><\/figure>\n

Taste<\/a>\u00a0is a form of\u00a0chemoreception<\/a>\u00a0that takes place in the specialised\u00a0taste receptors<\/a>, contained in structures called\u00a0taste buds<\/a>\u00a0in the mouth. Taste buds are mainly on the upper surface (dorsum) of the tongue. The function of taste perception is vital to help prevent harmful or rotten foods from being consumed. There are also taste buds on the\u00a0epiglottis<\/a>\u00a0and upper part of the\u00a0esophagus<\/a>. The taste buds are innervated by a branch of the facial nerve the\u00a0chorda tympani<\/a>, and the\u00a0glossopharyngeal nerve<\/a>. Taste messages are sent via these\u00a0cranial nerves<\/a>\u00a0to the\u00a0brain<\/a>. The brain can distinguish between the chemical qualities of the food. The five\u00a0basic tastes<\/a>\u00a0are referred to as those of\u00a0saltiness<\/a>,\u00a0sourness<\/a>,\u00a0bitterness<\/a>,\u00a0sweetness<\/a>, and\u00a0umami<\/a>. The detection of saltiness and sourness enables the control of salt and acid balance. The detection of bitterness warns of poisons\u2014many of a plant’s defences are of poisonous compounds that are bitter. Sweetness guides to those foods that will supply energy; the initial breakdown of the energy-giving carbohydrates by salivary amylase creates the taste of sweetness since simple sugars are the first result. The taste of umami is thought to signal protein-rich food. Sour tastes are acidic which is often found in bad food. The brain has to decide very quickly whether the food should be eaten or not. It was the findings in 1991, describing the first\u00a0olfactory<\/a>\u00a0receptors that helped to prompt the research into taste. The olfactory receptors are located on cell surfaces in the\u00a0nose<\/a>\u00a0which bind to chemicals enabling the detection of smells. It is assumed that signals from taste receptors work together with those from the nose, to form an idea of complex food flavours.[<\/span>23]<\/span><\/a><\/sup><\/p>\n

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Teeth<\/h4>\n<\/div>\n
Main article:\u00a0Human tooth<\/a><\/div>\n

Teeth<\/a>\u00a0are complex structures made of materials specific to them. They are made of a bone-like material called\u00a0dentin<\/a>, which is covered by the hardest tissue in the body\u2014enamel<\/a>.[<\/span>13]<\/span><\/a><\/sup>\u00a0Teeth have different shapes to deal with different aspects of\u00a0mastication<\/a>\u00a0employed in tearing and chewing pieces of food into smaller and smaller pieces. This results in a much larger surface area for the action of digestive enzymes. The teeth are named after their particular roles in the process of mastication\u2014incisors<\/a>\u00a0are used for cutting or biting off pieces of food;\u00a0canines<\/a>, are used for tearing,\u00a0premolars<\/a>\u00a0and\u00a0molars<\/a>\u00a0are used for chewing and grinding. Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be\u00a0swallowed<\/a>\u00a0to make its way down the\u00a0upper gastrointestinal tract<\/a>\u00a0to the stomach.[<\/span>24]<\/span><\/a><\/sup>\u00a0The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles.[<\/span>25]<\/span><\/a><\/sup>[<\/span>20]<\/span><\/a><\/sup><\/p>\n

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Epiglottis<\/h4>\n<\/div>\n
Main article:\u00a0Epiglottis<\/a><\/div>\n
<\/a>
<\/figcaption><\/figure>\n

The\u00a0epiglottis<\/a>\u00a0is a flap of\u00a0elastic cartilage<\/a>\u00a0attached to the entrance of the\u00a0larynx<\/a>. It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth.[<\/span>26]<\/span><\/a><\/sup>\u00a0Its laryngeal surface faces into the larynx. The epiglottis functions to guard the entrance of the\u00a0glottis<\/a>, the opening between the\u00a0vocal folds<\/a>. It is normally pointed upward during breathing with its underside functioning as part of the pharynx, but during swallowing, the epiglottis folds down to a more horizontal position, with its upper side functioning as part of the pharynx. In this manner it prevents food from going into the trachea and instead directs it to the esophagus, which is behind. During swallowing, the backward motion of the tongue forces the epiglottis over the glottis’ opening to prevent any food that is being swallowed from entering the larynx which leads to the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong\u00a0cough reflex<\/a>\u00a0in order to protect the lungs.<\/p>\n

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Pharynx<\/h3>\n<\/div>\n
Main article:\u00a0Pharynx<\/a><\/div>\n

The\u00a0pharynx<\/a>\u00a0is a part of the\u00a0conducting zone<\/a>\u00a0of the\u00a0respiratory system<\/a>\u00a0and also a part of the digestive system. It is the part of the throat immediately behind the\u00a0nasal cavity<\/a>\u00a0at the back of the mouth and above the esophagus and\u00a0larynx<\/a>. The pharynx is made up of three parts. The lower two parts\u2014the\u00a0oropharynx<\/a>\u00a0and the\u00a0laryngopharynx<\/a>\u00a0are involved in the digestive system. The laryngopharynx connects to the esophagus and it serves as a passageway for both air and food. Air enters the larynx anteriorly but anything swallowed has priority and the passage of air is temporarily blocked. The pharynx is innervated by the\u00a0pharyngeal plexus of the vagus nerve<\/a>.[<\/span>15]<\/span><\/a><\/sup>:\u200a1465\u200a<\/span><\/sup>\u00a0Muscles in the pharynx<\/a>\u00a0push the food into the esophagus. The pharynx joins the esophagus at the oesophageal inlet which is located behind the\u00a0cricoid cartilage<\/a>.<\/p>\n

\n

Esophagus<\/h3>\n<\/div>\n
Main article:\u00a0Esophagus<\/a><\/div>\n
<\/a>
Esophagus shown in yellow passing behind the trachea and the heart<\/figcaption><\/figure>\n

The\u00a0esophagus<\/a>, commonly known as the foodpipe or gullet, consists of a muscular tube through which food passes from the pharynx to the stomach. The esophagus is continuous with the laryngopharynx. It passes through the posterior\u00a0mediastinum<\/a>\u00a0in the\u00a0thorax<\/a>\u00a0and enters the\u00a0stomach<\/a>\u00a0through a hole in the\u00a0thoracic diaphragm<\/a>\u2014the\u00a0esophageal hiatus<\/a>, at the level of the tenth\u00a0thoracic vertebra<\/a>\u00a0(T10). Its length averages 25\u00a0cm, varying with an individual’s height. It is divided into cervical,\u00a0thoracic<\/a>\u00a0and\u00a0abdominal<\/a>\u00a0parts. The pharynx joins the esophagus at the esophageal inlet which is behind the\u00a0cricoid cartilage<\/a>.<\/p>\n

At rest the esophagus is closed at both ends, by the\u00a0upper and lower esophageal sphincters<\/a>. The opening of the upper sphincter is triggered by the\u00a0swallowing reflex<\/a>\u00a0so that food is allowed through. The sphincter also serves to prevent back flow from the esophagus into the pharynx. The esophagus has a mucous membrane and the epithelium which has a protective function is continuously replaced due to the volume of food that passes inside the esophagus. During swallowing, food passes from the mouth through the pharynx into the esophagus. The epiglottis folds down to a more horizontal position to direct the food into the esophagus, and away from the\u00a0trachea<\/a>.<\/p>\n

Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known as\u00a0peristalsis<\/a>.[<\/span>2]<\/span><\/a><\/sup>\u00a0The lower esophageal sphincter is a muscular sphincter surrounding the lower part of the esophagus. The\u00a0gastroesophageal junction<\/a>\u00a0between the esophagus and the stomach is controlled by the lower esophageal sphincter, which remains constricted at all times other than during swallowing and vomiting to prevent the contents of the stomach from entering the esophagus. As the esophagus does not have the same protection from acid as the stomach, any failure of this sphincter can lead to\u00a0heartburn<\/a>.<\/p>\n

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Diaphragm<\/h3>\n<\/div>\n

The\u00a0diaphragm<\/a>\u00a0is an important part of the body’s digestive system. The muscular diaphragm separates the\u00a0thoracic cavity<\/a>\u00a0from the\u00a0abdominal cavity<\/a>\u00a0where most of the digestive organs are located. The\u00a0suspensory muscle<\/a>\u00a0attaches the ascending duodenum to the diaphragm. This muscle is thought to be of help in the digestive system in that its attachment offers a wider angle to the\u00a0duodenojejunal flexure<\/a>\u00a0for the easier passage of digesting material. The diaphragm also attaches to, and anchors the liver at its\u00a0bare area<\/a>. The esophagus enters the abdomen through a\u00a0hole in the diaphragm<\/a>\u00a0at the level of\u00a0T10<\/a>.<\/p>\n

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Stomach<\/h3>\n<\/div>\n
Main article:\u00a0Stomach<\/a><\/div>\n
<\/a>
Areas of the stomach<\/figcaption><\/figure>\n

The\u00a0stomach<\/a>\u00a0is a major organ of the gastrointestinal tract and digestive system. It is a consistently J-shaped organ joined to the esophagus at its upper end and to the duodenum at its lower end.\u00a0Gastric acid<\/a>\u00a0(informally\u00a0gastric juice<\/i>), produced in the stomach plays a vital role in the digestive process, and mainly contains\u00a0hydrochloric acid<\/a>\u00a0and\u00a0sodium chloride<\/a>. A\u00a0peptide hormone<\/a>,\u00a0gastrin<\/a>, produced by\u00a0G cells<\/a>\u00a0in the\u00a0gastric glands<\/a>, stimulates the production of gastric juice which activates the digestive enzymes.\u00a0Pepsinogen<\/a>\u00a0is a precursor enzyme (zymogen<\/a>) produced by the\u00a0gastric chief cells<\/a>, and gastric acid activates this to the enzyme\u00a0pepsin<\/a>\u00a0which begins the digestion of\u00a0proteins<\/a>. As these two chemicals would damage the stomach wall, mucus is secreted by innumerable gastric glands in the stomach, to provide a slimy protective layer against the damaging effects of the chemicals on the inner layers of the stomach.<\/p>\n

At the same time that protein is being digested, mechanical churning occurs through the action of peristalsis, waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes.\u00a0Gastric lipase<\/a>\u00a0secreted by the chief cells in the fundic glands in the gastric mucosa of the stomach, is an acidic lipase, in contrast with the alkaline pancreatic lipase. This breaks down fats to some degree though is not as efficient as the pancreatic lipase.<\/p>\n

The\u00a0pylorus<\/a>, the lowest section of the stomach which attaches to the\u00a0duodenum<\/a>\u00a0via the\u00a0pyloric canal<\/a>, contains countless glands which secrete digestive enzymes including gastrin. After an hour or two, a thick semi-liquid called\u00a0chyme<\/a>\u00a0is produced. When the\u00a0pyloric sphincter<\/a>, or valve opens, chyme enters the duodenum where it mixes further with digestive enzymes from the pancreas, and then passes through the small intestine, where digestion continues.<\/p>\n

The\u00a0parietal cells<\/a>\u00a0in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption of\u00a0vitamin B12<\/a>. Vitamin B12 (cobalamin), is carried to, and through the stomach, bound to a glycoprotein secreted by the salivary glands \u2013\u00a0transcobalamin I<\/a>\u00a0also called haptocorrin, which protects the acid-sensitive vitamin from the acidic stomach contents. Once in the more neutral duodenum, pancreatic enzymes break down the protective glycoprotein. The freed vitamin B12 then binds to intrinsic factor which is then absorbed by the enterocytes in the ileum.<\/p>\n

The stomach is a distensible organ and can normally expand to hold about one litre of food.[<\/span>27]<\/span><\/a><\/sup>\u00a0This expansion is enabled by a series of\u00a0gastric folds<\/a>\u00a0in the inner walls of the stomach. The stomach of a newborn baby will only be able to expand to retain about 30 ml.<\/p>\n

\n

Spleen<\/h3>\n<\/div>\n
Main article:\u00a0Spleen<\/a><\/div>\n

The\u00a0spleen<\/a>\u00a0is the largest lymphoid organ in the body but has other functions.[<\/span>28]<\/span><\/a><\/sup>\u00a0It breaks down both red and white\u00a0blood cells<\/a>\u00a0that are\u00a0spent<\/i>. This is why it is sometimes known as the ‘graveyard of red blood cells’.[<\/span>28]<\/span><\/a><\/sup>\u00a0A product of this\u00a0digestion<\/i>\u00a0is the pigment\u00a0bilirubin<\/a>, which is sent to the\u00a0liver<\/a>\u00a0and secreted in the\u00a0bile<\/a>. Another product is\u00a0iron<\/a>, which is used in the formation of new blood cells in the\u00a0bone marrow<\/a>.[<\/span>10]<\/span><\/a><\/sup>\u00a0Medicine<\/a>\u00a0treats the spleen solely as belonging to the\u00a0lymphatic system<\/a>, though it is acknowledged that the full range of its important functions is not yet understood.[<\/span>15]<\/span><\/a><\/sup>:\u200a1751\u200a<\/span><\/sup><\/p>\n

\n

Liver<\/h3>\n<\/div>\n
Main article:\u00a0Liver<\/a><\/div>\n
<\/a>
Liver and gall bladder<\/figcaption><\/figure>\n

The\u00a0liver<\/a>\u00a0is the second largest organ (after the\u00a0skin<\/a>) and is an accessory digestive gland which plays a role in the body’s\u00a0metabolism<\/a>. The liver has many functions some of which are important to digestion. The liver can detoxify various\u00a0metabolites<\/a>; synthesise proteins and produce\u00a0biochemicals<\/a>\u00a0needed for digestion. It regulates the storage of\u00a0glycogen<\/a>\u00a0which it can form from\u00a0glucose<\/a>\u00a0(glycogenesis<\/a>). The liver can also synthesise glucose from certain\u00a0amino acids<\/a>. Its digestive functions are largely involved with the breaking down of carbohydrates. It also maintains protein metabolism in its synthesis and degradation. In lipid metabolism it synthesises\u00a0cholesterol<\/a>. Fats are also produced in the process of\u00a0lipogenesis<\/a>. The liver synthesises the bulk of lipoproteins. The liver is located in the upper right quadrant of the abdomen and below the diaphragm to which it is attached at one part, the bare area of the liver. This is to the right of the stomach and it overlies the gall bladder. The liver synthesises\u00a0bile acids<\/a>\u00a0and\u00a0lecithin<\/a>\u00a0to promote the digestion of fat.[<\/span>29]<\/span><\/a><\/sup><\/p>\n

\n

Bile<\/h4>\n<\/div>\n

Bile<\/a>\u00a0produced by the liver is made up of water (97%),\u00a0bile salts<\/a>, mucus and\u00a0pigments<\/a>, 1% fats and inorganic salts.[<\/span>30]<\/span><\/a><\/sup>\u00a0Bilirubin<\/a>\u00a0is its major pigment. Bile acts partly as a\u00a0surfactant<\/a>\u00a0which lowers the surface tension between either two liquids or a solid and a liquid and helps to\u00a0emulsify<\/a>\u00a0the fats in the chyme. Food fat is dispersed by the action of bile into smaller units called\u00a0micelles<\/a>. The breaking down into micelles creates a much larger surface area for the pancreatic enzyme, lipase to work on. Lipase digests the\u00a0triglycerides<\/a>\u00a0which are broken down into two\u00a0fatty acids<\/a>\u00a0and a\u00a0monoglyceride<\/a>. These are then absorbed by\u00a0villi<\/a>\u00a0on the intestinal wall. If fats are not absorbed in this way in the small intestine problems can arise later in the large intestine which is not equipped to absorb fats. Bile also helps in the absorption of\u00a0vitamin K<\/a>\u00a0from the diet. Bile is collected and delivered through the\u00a0common hepatic duct<\/a>. This duct joins with the\u00a0cystic duct<\/a>\u00a0to connect in a\u00a0common bile duct<\/a>\u00a0with the gallbladder. Bile is stored in the gallbladder for release when food is discharged into the duodenum and also after a few hours.[<\/span>10]<\/span><\/a><\/sup><\/p>\n

\n

Gallbladder<\/h4>\n<\/div>\n
<\/a>
Gallbladder shown in green below the liver<\/figcaption><\/figure>\n

The\u00a0gallbladder<\/a>\u00a0is a hollow part of the\u00a0biliary tract<\/a>\u00a0that sits just beneath the liver, with the gallbladder body resting in a small depression.[<\/span>31]<\/span><\/a><\/sup>\u00a0It is a small organ where the bile produced by the liver is stored, before being released into the small intestine. Bile flows from the liver through the\u00a0bile ducts<\/a>\u00a0and into the gall bladder for storage. The bile is released in response to\u00a0cholecystokinin<\/a>\u00a0(CCK), a peptide hormone released from the duodenum. The production of CCK (by endocrine cells of the duodenum) is stimulated by the presence of fat in the duodenum.[<\/span>32]<\/span><\/a><\/sup><\/p>\n

It is divided into three sections, a fundus, body and neck. The neck tapers and connects to the biliary tract via the\u00a0cystic duct<\/a>, which then joins the common hepatic duct to form the common bile duct. At this junction is a mucosal fold called\u00a0Hartmann’s pouch<\/i>, where\u00a0gallstones<\/a>\u00a0commonly get stuck. The\u00a0muscular layer<\/a>\u00a0of the body is of smooth muscle tissue that helps the gallbladder contract, so that it can discharge its bile into the bile duct. The gallbladder needs to store bile in a natural, semi-liquid form at all times.\u00a0Hydrogen ions<\/a>\u00a0secreted from the inner lining of the gallbladder keep the bile acidic enough to prevent hardening. To dilute the bile, water and\u00a0electrolytes<\/a>\u00a0from the digestion system are added. Also, salts attach themselves to cholesterol molecules in the bile to keep them from\u00a0crystallising<\/a>. If there is too much cholesterol or bilirubin in the bile, or if the gallbladder does not empty properly the systems can fail. This is how gallstones form when a small piece of calcium gets coated with either cholesterol or bilirubin and the bile crystallises and forms a gallstone. The main purpose of the gallbladder is to store and release bile, or\u00a0gall<\/i>. Bile is released into the small intestine in order to help in the digestion of fats by breaking down larger molecules into smaller ones. After the fat is absorbed, the bile is also absorbed and transported back to the liver for reuse.<\/p>\n

\n

Pancreas<\/h3>\n<\/div>\n
Main article:\u00a0Pancreas<\/a><\/div>\n
<\/a>
Pancreas, duodenum and bile duct<\/figcaption><\/figure>\n
<\/a>
Action of digestive hormones<\/figcaption><\/figure>\n

The\u00a0pancreas<\/a>\u00a0is a major organ functioning as an accessory digestive gland in the digestive system. It is both an\u00a0endocrine gland<\/a>\u00a0and an\u00a0exocrine gland<\/a>.[<\/span>33]<\/span><\/a><\/sup>\u00a0The endocrine part secretes\u00a0insulin<\/a>\u00a0when the\u00a0blood sugar<\/a>\u00a0becomes high; insulin moves glucose from the blood into the muscles and other tissues for use as energy. The endocrine part releases\u00a0glucagon<\/a>\u00a0when the blood sugar is low; glucagon allows stored sugar to be broken down into glucose by the liver in order to re-balance the sugar levels. The pancreas produces and releases important digestive enzymes in the\u00a0pancreatic juice<\/a>\u00a0that it delivers to the duodenum.[<\/span>29]<\/span><\/a><\/sup>\u00a0The pancreas lies below and at the back of the stomach. It connects to the duodenum via the\u00a0pancreatic duct<\/a>\u00a0which it joins near to the bile duct’s connection where both the bile and pancreatic juice can act on the chyme that is released from the stomach into the duodenum. Aqueous pancreatic secretions from\u00a0pancreatic duct cells<\/a>\u00a0contain\u00a0bicarbonate<\/a>\u00a0ions which are alkaline and help with the bile to neutralise the acidic chyme that is churned out by the stomach.<\/p>\n

The pancreas is also the main source of enzymes for the digestion of fats and proteins. Some of these are released in response to the production of cholecystokinin in the duodenum. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the intestines.) The cells are filled with secretory granules containing the precursor digestive enzymes. The major\u00a0proteases<\/a>, the pancreatic enzymes which work on proteins, are\u00a0trypsinogen<\/a>\u00a0and\u00a0chymotrypsinogen<\/a>.\u00a0Elastase<\/a>\u00a0is also produced. Smaller amounts of lipase and amylase are secreted. The pancreas also secretes\u00a0phospholipase A2<\/a>,\u00a0lysophospholipase<\/a>, and cholesterol\u00a0esterase<\/a>. The precursor\u00a0zymogens<\/a>, are inactive variants of the enzymes; which avoids the onset of\u00a0pancreatitis<\/a>\u00a0caused by autodegradation. Once released in the intestine, the enzyme\u00a0enteropeptidase<\/a>\u00a0present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin; further cleavage results in chymotripsin.<\/p>\n

\n

Lower gastrointestinal tract<\/h3>\n<\/div>\n
Main article:\u00a0Gastrointestinal tract<\/a><\/div>\n
Further information:\u00a0Gastrointestinal physiology<\/a><\/div>\n

The lower gastrointestinal tract (GI), includes the\u00a0small intestine<\/a>\u00a0and all of the\u00a0large intestine<\/a>.[<\/span>34]<\/span><\/a><\/sup>\u00a0The intestine is also called the bowel or the gut. The lower GI tract starts at the pyloric sphincter of the stomach and finishes at the anus. The small intestine is subdivided into the\u00a0duodenum<\/a>, the\u00a0jejunum<\/a>\u00a0and the\u00a0ileum<\/a>. The\u00a0cecum<\/a>\u00a0marks the division between the small and large intestine. The large intestine includes the rectum and\u00a0anal canal<\/a>.[<\/span>6]<\/span><\/a><\/sup><\/p>\n

\n

Small intestine<\/h4>\n<\/div>\n
<\/a>
Illustration of small intestine<\/figcaption><\/figure>\n

The\u00a0small intestine<\/a>\u00a0starts at the\u00a0pyloric sphincter<\/a>, and finishes at the\u00a0ileocecal valve<\/a>.[<\/span>4]<\/span><\/a><\/sup>\u00a0Partially digested food starts to arrive in the small intestine as semi-liquid\u00a0chyme<\/a>, one hour after it is eaten.[citation needed<\/span><\/a><\/i>]<\/sup>\u00a0The stomach is half empty after an average of 1.2 hours.[<\/span>35]<\/span><\/a><\/sup>\u00a0After four or five hours the stomach has emptied.[<\/span>36]<\/span><\/a><\/sup><\/p>\n

In the small intestine, the\u00a0pH<\/a>\u00a0becomes crucial; it needs to be finely balanced in order to activate digestive enzymes. The chyme is very acidic, with a low pH, having been released from the stomach and needs to be made much more alkaline. This is achieved in the\u00a0duodenum<\/a><\/b>\u00a0by the addition of bile from the gall bladder combined with the\u00a0bicarbonate<\/a>\u00a0secretions from the pancreatic duct and also from secretions of bicarbonate-rich mucus from duodenal glands known as\u00a0Brunner’s glands<\/a>. The chyme arrives in the intestines having been released from the stomach through the opening of the pyloric sphincter. The resulting alkaline fluid mix neutralises the gastric acid which would damage the lining of the intestine. The mucus component lubricates the walls of the intestine.<\/p>\n

<\/a>
Layers of the small intestine<\/figcaption><\/figure>\n

When the digested food particles are reduced enough in size and composition, they can be absorbed by the intestinal wall and carried to the bloodstream. The first receptacle for this chyme is the\u00a0duodenal bulb<\/a>. From here it passes into the first of the three sections of the small intestine, the duodenum (the next section is the\u00a0jejunum<\/a><\/b>\u00a0and the third is the\u00a0ileum<\/a><\/b>). The duodenum is the first and shortest section of the small intestine. It is a hollow, jointed C-shaped tube connecting the stomach to the jejunum. It starts at the duodenal bulb and ends at the\u00a0suspensory muscle of duodenum<\/a>. The attachment of the suspensory muscle to the diaphragm is thought to help the passage of food by making a wider angle at its attachment.<\/p>\n

Most food digestion takes place in the small intestine.\u00a0Segmentation contractions<\/a>\u00a0act to mix and move the chyme more slowly in the small intestine allowing more time for absorption (and these continue in the large intestine). In the duodenum, pancreatic lipase is secreted together with a\u00a0co-enzyme<\/a>,\u00a0colipase<\/a>\u00a0to further digest the fat content of the chyme. From this breakdown, smaller particles of emulsified fats called\u00a0chylomicrons<\/a>\u00a0are produced. There are also digestive cells called\u00a0enterocytes<\/a>\u00a0lining the intestines (the majority being in the small intestine). They are unusual cells in that they have\u00a0villi<\/a>\u00a0on their surface which in turn have innumerable\u00a0microvilli<\/a>\u00a0on their surface. All these villi make for a greater surface area, not only for the absorption of chyme but also for its further digestion by large numbers of digestive enzymes present on the microvilli.<\/p>\n

The chylomicrons are small enough to pass through the enterocyte villi and into their\u00a0lymph<\/a>\u00a0capillaries called\u00a0lacteals<\/a>. A milky fluid called\u00a0chyle<\/a>, consisting mainly of the emulsified fats of the chylomicrons, results from the absorbed mix with the lymph in the lacteals.[clarification needed<\/span><\/a><\/i>]<\/sup>\u00a0Chyle is then transported through the\u00a0lymphatic system<\/a>\u00a0to the rest of the body.<\/p>\n

The suspensory muscle marks the end of the duodenum and the division between the upper gastrointestinal tract and the lower GI tract. The digestive tract continues as the jejunum which continues as the ileum. The jejunum, the midsection of the small intestine contains\u00a0circular folds<\/a>, flaps of doubled mucosal membrane which partially encircle and sometimes completely encircle the\u00a0lumen<\/a>\u00a0of the intestine. These folds together with villi serve to increase the surface area of the jejunum enabling an increased absorption of digested sugars, amino acids and fatty acids into the bloodstream. The circular folds also slow the passage of food giving more time for nutrients to be absorbed.<\/p>\n

The last part of the small intestine is the ileum. This also contains villi and\u00a0vitamin B12<\/a>; bile acids and any residue nutrients are absorbed here. When the chyme is exhausted of its nutrients the remaining waste material changes into the semi-solids called\u00a0feces<\/a>, which pass to the large intestine, where bacteria in the\u00a0gut flora<\/a>\u00a0further break down residual proteins and starches.[<\/span>37]<\/span><\/a><\/sup><\/p>\n

Transit time through the small intestine is an average of 4 hours. Half of the food residues of a meal have emptied from the small intestine by an average of 5.4 hours after ingestion. Emptying of the small intestine is complete after an average of 8.6 hours.[<\/span>35]<\/span><\/a><\/sup><\/p>\n

\n

Cecum<\/h4>\n<\/div>\n
<\/a>
Cecum and beginning of ascending colon<\/figcaption><\/figure>\n

The\u00a0cecum<\/a>\u00a0is a pouch marking the division between the small intestine and the large intestine. It lies below the\u00a0ileocecal valve<\/a>\u00a0in the\u00a0lower right quadrant<\/a>\u00a0of the abdomen.[<\/span>38]<\/span><\/a><\/sup>\u00a0The cecum receives chyme from the last part of the small intestine, the\u00a0ileum<\/a>, and connects to the\u00a0ascending colon<\/a>\u00a0of the large intestine. At this junction there is a sphincter or valve, the ileocecal valve which slows the passage of chyme from the ileum, allowing further digestion. It is also the site of the\u00a0appendix<\/a>\u00a0attachment.[<\/span>38]<\/span><\/a><\/sup><\/p>\n

\n

Large intestine<\/h4>\n<\/div>\n
<\/a>
Lower GI tract – 3) Small intestine; 5) Cecum; 6) Large intestine<\/figcaption><\/figure>\n
Further information:\u00a0Excretory system<\/a><\/div>\n

In the\u00a0large intestine<\/a>\u00a0the passage of the digesting food in the\u00a0colon<\/a>\u00a0is a lot slower, taking from 30 to 40 hours until it is removed by\u00a0defecation<\/a>.[<\/span>36]<\/span><\/a><\/sup>\u00a0The colon mainly serves as a site for the fermentation of digestible matter by the\u00a0gut flora<\/a>. The time taken varies considerably between individuals. The remaining semi-solid waste is termed\u00a0feces<\/a>\u00a0and is removed by the coordinated contractions of the intestinal walls, termed\u00a0peristalsis<\/a>, which propels the\u00a0excreta<\/a>\u00a0forward to reach the\u00a0rectum<\/a>\u00a0and exit through the\u00a0anus<\/a>\u00a0via defecation. The wall has an outer layer of longitudinal muscles, the\u00a0taeniae coli<\/a>, and an inner layer of circular muscles. The circular muscle keeps the material moving forward and also prevents any back flow of waste. Also of help in the action of peristalsis is the\u00a0basal electrical rhythm<\/a>\u00a0that determines the frequency of contractions.[<\/span>39]<\/span><\/a><\/sup>\u00a0The taeniae coli can be seen and are responsible for the bulges (haustra<\/a>) present in the colon. Most parts of the GI tract are covered with\u00a0serous membranes<\/a>\u00a0and have a\u00a0mesentery<\/a>. Other more muscular parts are lined with\u00a0adventitia<\/a>.<\/p>\n

\n

Blood supply<\/h2>\n<\/div>\n
<\/a>
Arteries and veins around the pancreas and spleen<\/figcaption><\/figure>\n

The digestive system is supplied by the\u00a0celiac artery<\/a>. The celiac artery is the first major branch from the\u00a0abdominal aorta<\/a>, and is the only major artery that nourishes the digestive organs.<\/p>\n

There are three main divisions \u2013 the\u00a0left gastric artery<\/a>, the\u00a0common hepatic artery<\/a>\u00a0and the\u00a0splenic artery<\/a>.<\/p>\n

The celiac artery supplies the liver, stomach, spleen and the upper 1\/3 of the duodenum (to the\u00a0sphincter of Oddi<\/a>) and the pancreas with oxygenated blood. Most of the blood is returned to the liver via the\u00a0portal venous system<\/a>\u00a0for further processing and detoxification before returning to the\u00a0systemic circulation<\/a>\u00a0via the\u00a0hepatic veins<\/a>.<\/p>\n

The next branch from the abdominal aorta is the\u00a0superior mesenteric artery<\/a>, which supplies the regions of the digestive tract derived from the midgut, which includes the distal 2\/3 of the duodenum, jejunum, ileum, cecum, appendix, ascending colon, and the proximal 2\/3 of the transverse colon.<\/p>\n

The final branch which is important for the digestive system is the\u00a0inferior mesenteric artery<\/a>, which supplies the regions of the digestive tract derived from the hindgut, which includes the distal 1\/3 of the transverse colon, descending colon, sigmoid colon, rectum, and the anus above the\u00a0pectinate line<\/a>.<\/p>\n

Blood flow to the digestive tract reaches its maximum 20\u201340 minutes after a meal and lasts for 1.5\u20132 hours.[<\/span>40]<\/span><\/a><\/sup><\/p>\n

\n

Nerve supply<\/h2>\n<\/div>\n
Further information:\u00a0Neurogastroenterology<\/a>\u00a0and\u00a0Gut\u2013brain axis<\/a><\/div>\n

The\u00a0enteric nervous system<\/a>\u00a0consists of some one hundred million\u00a0neurons<\/a>[<\/span>41]<\/span><\/a><\/sup>\u00a0that are embedded in the\u00a0peritoneum<\/a>, the lining of the\u00a0gastrointestinal tract<\/a>\u00a0extending from the esophagus to the anus.[<\/span>42]<\/span><\/a><\/sup>\u00a0These neurons are collected into two\u00a0plexuses<\/a>\u00a0\u2013 the\u00a0myenteric (or Auerbach’s) plexus<\/a>\u00a0that lies between the longitudinal and the smooth muscle layers, and the\u00a0submucosal (or Meissner’s) plexus<\/a>\u00a0that lies between the circular smooth muscle layer and the mucosa.[<\/span>43]<\/span><\/a><\/sup>[<\/span>44]<\/span><\/a><\/sup>[<\/span>45]<\/span><\/a><\/sup><\/p>\n

Parasympathetic innervation<\/a>\u00a0to the\u00a0ascending colon<\/a>\u00a0is supplied by the\u00a0vagus nerve<\/a>.\u00a0Sympathetic innervation<\/a>\u00a0is supplied by the\u00a0splanchnic nerves<\/a>\u00a0that join the\u00a0celiac ganglia<\/a>. Most of the digestive tract is innervated by the two large celiac ganglia, with the upper part of each ganglion joined by the\u00a0greater splanchnic nerve<\/a>\u00a0and the lower parts joined by the\u00a0lesser splanchnic nerve<\/a>. It is from these ganglia that many of the\u00a0gastric plexuses<\/a>\u00a0arise.<\/p>\n

\n

Development<\/h2>\n<\/div>\n
Main article:\u00a0Development of the human digestive system<\/a><\/div>\n

Early in\u00a0embryonic development<\/a>, the\u00a0embryo<\/a>\u00a0has three\u00a0germ layers<\/a>\u00a0and abuts a\u00a0yolk sac<\/a>. During the second week of development, the embryo grows and begins to surround and envelop portions of this sac. The enveloped portions form the basis for the adult gastrointestinal tract. Sections of this\u00a0foregut<\/a>\u00a0begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines.[<\/span>46]<\/span><\/a><\/sup><\/p>\n

During the fourth week of development, the stomach rotates. The stomach, originally lying in the midline of the embryo, rotates so that its body is on the left. This rotation also affects the part of the gastrointestinal tube immediately below the stomach, which will go on to become the duodenum. By the end of the fourth week, the developing duodenum begins to spout a small outpouching on its right side, the\u00a0hepatic diverticulum<\/a>, which will go on to become the\u00a0biliary tree<\/a>. Just below this is a second outpouching, known as the\u00a0cystic diverticulum<\/i>, that will eventually develop into the gallbladder.[<\/span>46]<\/span><\/a><\/sup><\/p>\n

\n

Clinical significance<\/h2>\n<\/div>\n
Main article:\u00a0Gastrointestinal disease<\/a><\/div>\n

Each part of the digestive system is subject to a wide range of disorders many of which can be\u00a0congenital<\/a>.\u00a0Mouth diseases<\/a>\u00a0can also be caused by\u00a0pathogenic<\/a>\u00a0bacteria<\/a>,\u00a0viruses<\/a>,\u00a0fungi<\/a>\u00a0and as a side effect of some\u00a0medications<\/a>. Mouth diseases include\u00a0tongue diseases<\/a>\u00a0and\u00a0salivary gland diseases<\/a>. A common\u00a0gum<\/a>\u00a0disease in the mouth is\u00a0gingivitis<\/a>\u00a0which is caused by bacteria in\u00a0plaque<\/a>. The most common viral infection of the mouth is\u00a0gingivostomatitis<\/a>\u00a0caused by\u00a0herpes simplex<\/a>. A common\u00a0fungal<\/a>\u00a0infection is\u00a0candidiasis<\/a>\u00a0commonly known as\u00a0thrush<\/i>\u00a0which affects the\u00a0mucous membranes<\/a>\u00a0of the mouth.<\/p>\n

There are a number of\u00a0esophageal diseases<\/a>\u00a0such as the development of\u00a0Schatzki rings<\/a>\u00a0that can restrict the passageway, causing difficulties in swallowing. They can also completely block the esophagus.[<\/span>47]<\/span><\/a><\/sup><\/p>\n

Stomach diseases<\/a>\u00a0are often chronic conditions and include\u00a0gastroparesis<\/a>,\u00a0gastritis<\/a>, and\u00a0peptic ulcers<\/a>.<\/p>\n

A number of problems including\u00a0malnutrition<\/a>\u00a0and\u00a0anemia<\/a>\u00a0can arise from\u00a0malabsorption<\/a>, the abnormal absorption of nutrients in the GI tract. Malabsorption can have many causes ranging from\u00a0infection<\/a>, to enzyme deficiencies such as\u00a0exocrine pancreatic insufficiency<\/a>. It can also arise as a result of other gastrointestinal diseases such as\u00a0coeliac disease<\/a>. Coeliac disease is an\u00a0autoimmune<\/a>\u00a0disorder of the small intestine. This can cause\u00a0vitamin deficiencies<\/a>\u00a0due to the improper absorption of nutrients in the small intestine. The small intestine can also be\u00a0obstructed<\/a>\u00a0by a\u00a0volvulus<\/a>, a loop of intestine that becomes twisted enclosing its attached\u00a0mesentery<\/a>. This can cause\u00a0mesenteric ischemia<\/a>\u00a0if severe enough.<\/p>\n

A common disorder of the bowel is\u00a0diverticulitis<\/a>.\u00a0Diverticula<\/a>\u00a0are small pouches that can form inside the bowel wall, which can become inflamed to give diverticulitis. This disease can have complications if an inflamed diverticulum bursts and infection sets in. Any infection can spread further to the lining of the abdomen (peritoneum<\/a>) and cause potentially fatal\u00a0peritonitis<\/a>.[<\/span>48]<\/span><\/a><\/sup><\/p>\n

Crohn’s disease<\/a>\u00a0is a common chronic\u00a0inflammatory bowel disease<\/a>\u00a0(IBD), which can affect any part of the GI tract,[<\/span>49]<\/span><\/a><\/sup>\u00a0but it mostly starts in the\u00a0terminal ileum<\/a>.<\/p>\n

Ulcerative colitis<\/a>, an ulcerative form of\u00a0colitis<\/a>, is the other major inflammatory bowel disease which is restricted to the colon and rectum. Both of these IBDs can give an increased risk of the development of\u00a0colorectal cancer<\/a>. Ulcerative colitis is the most common of the IBDs[<\/span>50]<\/span><\/a><\/sup><\/p>\n

Irritable bowel syndrome<\/a>\u00a0(IBS) is the most common of the\u00a0functional gastrointestinal disorders<\/a>. These are\u00a0idiopathic<\/a>\u00a0disorders that the\u00a0Rome process<\/a>\u00a0has helped to define.[<\/span>51]<\/span><\/a><\/sup><\/p>\n

Giardiasis<\/a>\u00a0is a disease of the small intestine caused by a\u00a0protist<\/a>\u00a0parasite<\/a>\u00a0Giardia lamblia<\/a><\/i>. This does not spread but remains confined to the lumen of the small intestine.[<\/span>52]<\/span><\/a><\/sup>\u00a0It can often be\u00a0asymptomatic<\/a>, but as often can be indicated by a variety of symptoms. Giardiasis is the most common\u00a0pathogenic<\/a>\u00a0parasitic infection<\/a>\u00a0in humans.[<\/span>53]<\/span><\/a><\/sup><\/p>\n

There are diagnostic tools mostly involving the ingestion of\u00a0barium sulphate<\/a>\u00a0to investigate disorders of the GI tract.[<\/span>54]<\/span><\/a><\/sup>\u00a0These are known as\u00a0upper gastrointestinal series<\/a>\u00a0that enable\u00a0imaging<\/a>\u00a0of the pharynx, larynx, oesophagus, stomach and small intestine[<\/span>55]<\/span><\/a><\/sup>\u00a0and\u00a0lower gastrointestinal series<\/a>\u00a0for imaging of the colon.<\/p>\n

\n

Cancer<\/h3>\n<\/div>\n

The estimated new cases of digestive system cancer in the Unitied States in 2023 was 348,840 and the estimated number of deaths from digestive system cancers was 172,010.[<\/span>56]<\/span><\/a><\/sup>\u00a0The major specific causes of digestive system cancer related death in 2023 were cancers of the\u00a0colon and rectum<\/a>\u00a0(52,550),\u00a0pancreas<\/a>\u00a0(50,550),\u00a0liver<\/a>\u00a0and intrahepatic bile duct (29,380),\u00a0esophagus<\/a>\u00a0(16,120) and\u00a0stomach<\/a>\u00a0(11,130).[<\/span>56]<\/span><\/a><\/sup>\u00a0Bile acids<\/a>, when present in specific regions of the digestive system at abnormally high levels, have been implicated as important carcinogens.[<\/span>57]<\/span><\/a><\/sup><\/p>\n

\n

In pregnancy<\/h3>\n<\/div>\n

Gestation<\/a>\u00a0can predispose for certain digestive disorders.\u00a0Gestational diabetes<\/a>\u00a0can develop in the mother as a result of\u00a0pregnancy<\/a>\u00a0and while this often presents with few symptoms it can lead to\u00a0pre-eclampsia<\/a>.[<\/span>58]<\/span><\/a><\/sup><\/p>\n

\n

History<\/h2>\n<\/div>\n
<\/a>
Dietary life rules, Japan,\u00a0Edo period<\/a>. Illustrating the ill effects of drinking alcohol on the digestive system.<\/figcaption><\/figure>\n
\"\"<\/a>
Historical depiction of the digestive system, 17th century Persia<\/figcaption><\/figure>\n

In the early 11th century, the Islamic medical philosopher\u00a0Avicenna<\/a>\u00a0wrote extensively on many subjects including medicine. Forty of these treatises on medicine survive, and in the most famous one titled the\u00a0Canon of Medicine<\/i>\u00a0he discusses “rising gas”. Avicenna believed that digestive system dysfunction was responsible for the overproduction of gas in the gastrointestinal tract. He suggested lifestyle changes and a compound of herbal drugs for its treatment.[<\/span>59]<\/span><\/a><\/sup><\/p>\n

In 1497,\u00a0Alessandro Benedetti<\/a>\u00a0viewed the stomach as an unclean organ separated off by the diaphragm. This view of the stomach and intestines as being base organs was generally held until the mid-17th century.[<\/span>60]<\/span><\/a><\/sup><\/p>\n

In the\u00a0Renaissance<\/a>\u00a0of the 16th century,\u00a0Leonardo da Vinci<\/a>\u00a0produced some early drawings of the stomach and intestines. He thought that the digestive system aided the respiratory system.[<\/span>60]<\/span><\/a><\/sup>\u00a0Andreas Vesalius<\/a>\u00a0provided some early anatomical drawings of the abdominal organs in the 16th century.<\/p>\n

In the middle of the 17th century, a Flemish physician\u00a0Jan Baptist van Helmont<\/a>\u00a0offered the first\u00a0chemical account of digestion<\/a>\u00a0which was later described as being very close to the later conceptualised enzyme.[<\/span>60]<\/span><\/a><\/sup><\/p>\n

In 1653,\u00a0William Harvey<\/a>\u00a0described the intestines in terms of their length, their blood supply, the mesenteries, and fat (adenylyl cyclase).[<\/span>60]<\/span><\/a><\/sup><\/p>\n

In 1823,\u00a0William Prout<\/a>\u00a0discovered\u00a0hydrochloric acid<\/a>\u00a0in the gastric juice.[<\/span>61]<\/span><\/a><\/sup>\u00a0In 1895,\u00a0Ivan Pavlov<\/a>\u00a0described its secretion as being stimulated by a neurologic reflex with the\u00a0vagus nerve<\/a>\u00a0having a crucial role. Black in the 19th century suggested an association of histamine with this secretion. In 1916, Popielski described histamine as a gastric secretagogue of hydrochloric acid.<\/p>\n

William Beaumont<\/a>\u00a0was an army surgeon who in 1825, was able to observe digestion as it took place in the stomach.[<\/span>62]<\/span><\/a><\/sup>\u00a0This was made possible by experiments on a man with a stomach wound that did not fully heal leaving an opening into the stomach. The churning motion of the stomach was described among other findings.[<\/span>60]<\/span><\/a><\/sup><\/p>\n

In the 19th century, it was accepted that chemical processes were involved in the process of digestion.\u00a0Physiological research<\/a>\u00a0into secretion and the gastrointestinal tract was pursued with experiments undertaken by Claude Bernard, Rudolph Heidenhain and Ivan Pavlov.<\/p>\n

The rest of the 20th century was dominated by research into enzymes. The first to be discovered was\u00a0secretin<\/a>\u00a0by\u00a0Ernest Starling<\/a>\u00a0in 1902, with ensuing results from John Edkins in 1905 who first suggested\u00a0gastrin<\/a>\u00a0with its structure being determined in 1964.[<\/span>61]<\/span><\/a><\/sup>\u00a0Andre Latarjet and Lester Dragstedt found a role for\u00a0acetylcholine<\/a>\u00a0in the digestive system.[<\/span>61]<\/span><\/a><\/sup>\u00a0In 1972,\u00a0H2 receptor agonists<\/a>\u00a0were described by J. Black, that block the action of histamine and decrease the production of hydrochloric acid. In 1980,\u00a0proton pump inhibitors<\/a>\u00a0were described by Sachs. In 1983, the role of\u00a0Helicobacter pylori<\/a><\/i>\u00a0in the formation of ulcers was described by\u00a0Barry Marshall<\/a>, and\u00a0Robin Warren<\/a>.[<\/span>63]<\/span><\/a><\/sup><\/p>\n

Art historians have often noted that\u00a0banqueters<\/a>\u00a0on iconographic records of ancient Mediterranean societies almost always appear to be lying down on their left sides. One possible explanation could lie in the anatomy of the stomach and in the digestive mechanism. When lying on the left, the food has room to expand because the curvature of the stomach is enhanced in that position.[<\/span>64]<\/span><\/a><\/sup><\/p>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

“Digestive system” and “alimentary system” redirect here. For digestive systems of non-human animals, see\u00a0Digestion. Human digestive system Human digestive system Details Identifiers Latin systema digestorium MeSH D004064 TA98 A05.0.00.000 TA2 … <\/p>\n","protected":false},"author":1,"featured_media":2554,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-2553","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/posts\/2553","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/edmpackz.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2553"}],"version-history":[{"count":1,"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/posts\/2553\/revisions"}],"predecessor-version":[{"id":2555,"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/posts\/2553\/revisions\/2555"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/edmpackz.com\/index.php?rest_route=\/wp\/v2\/media\/2554"}],"wp:attachment":[{"href":"https:\/\/edmpackz.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2553"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/edmpackz.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2553"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/edmpackz.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2553"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}