Human digestive system
Human digestive system | |
---|---|
Details | |
Identifiers | |
Latin | systema digestorium |
MeSH | D004064 |
TA98 | A05.0.00.000 |
TA2 | 2773 |
FMA | 7152 |
Anatomical terminology |
The human digestive system consists of the
The first stage, the cephalic phase of digestion, begins with secretions from
The second stage, the gastric phase, happens in the stomach. Here, the food is further broken down by mixing with gastric acid until it passes into the duodenum, the first part of the small intestine.
The third stage, the intestinal phase, begins in the duodenum. Here, the partially digested food is mixed with a number of enzymes produced by the pancreas.
Digestion is helped by the chewing of food carried out by the muscles of mastication, the tongue, and the teeth, and also by the contractions of peristalsis, and segmentation. Gastric acid, and the production of mucus in the stomach, are essential for the continuation of digestion.
Peristalsis is the rhythmic contraction of
Components
There are several organs and other components involved in the digestion of food. The organs known as the accessory digestive organs are the
The largest structure of the digestive system is the gastrointestinal tract (GI tract). This starts at the mouth and ends at the anus, covering a distance of about nine metres.[1]
A major digestive organ is the stomach. Within its mucosa are millions of embedded gastric glands. Their secretions are vital to the functioning of the organ.
Most of the digestion of food takes place in the small intestine which is the longest part of the GI tract.
The largest part of the GI tract is the colon or large intestine. Water is absorbed here and the remaining waste matter is stored prior to defecation.[2]
There are many specialised
Some parts of the digestive system are also part of the excretory system, including the large intestine.[2]
Mouth
The
The roof of the mouth is termed the
At either side of the soft palate are the palatoglossus muscles which 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.[10]: 1208 Mucus helps in the mastication of food in its ability to soften and collect the food in the formation of the bolus.
Salivary glands
There are three pairs of main
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
Saliva
As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth.[15] It also has an immunological role in supplying antibodies to the system, such as immunoglobulin A.[16] This is seen to be key in preventing infections of the salivary glands, importantly that of parotitis.
Saliva also contains a glycoprotein called haptocorrin which is a binding protein to vitamin B12.[17] It 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 intrinsic factor.
Tongue
The tongue is attached to the floor of the mouth by a ligamentous band called the frenum[5] and this gives it great mobility for the manipulation of food (and speech); 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 intrinsic muscles that originate in the tongue and are involved with its shaping, and four extrinsic muscles originating in bone that are involved with its movement.
Taste
Teeth
Epiglottis
The
Pharynx
The
Esophagus
The
At rest the esophagus is closed at both ends, by the
Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known as peristalsis. The lower esophageal sphincter is a muscular sphincter surrounding the lower part of the esophagus. The gastroesophageal junction between 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 heartburn.
Diaphragm
The
Stomach
The stomach is 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.
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. Gastric lipase secreted 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.
The
The parietal cells in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption of vitamin B12. Vitamin B12 (cobalamin), is carried to, and through the stomach, bound to a glycoprotein secreted by the salivary glands – transcobalamin I also 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.
The stomach is a distensible organ and can normally expand to hold about one litre of food.[22] This expansion is enabled by a series of gastric folds in the inner walls of the stomach. The stomach of a newborn baby will only be able to expand to retain about 30 ml.
Spleen
The
Liver
The
Bile
Gallbladder
The gallbladder is a hollow part of the biliary tract that sits just beneath the liver, with the gallbladder body resting in a small depression.[26] It 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 bile ducts and into the gall bladder for storage. The bile is released in response to cholecystokinin (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.[27]
It is divided into three sections, a fundus, body and neck. The neck tapers and connects to the biliary tract via the
Pancreas
The
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
Lower gastrointestinal tract
The lower gastrointestinal tract (GI), includes the small intestine and all of the large intestine.[29] The intestine is also called the bowel or the gut. The lower GI starts at the pyloric sphincter of the stomach and finishes at the anus. The small intestine is subdivided into the duodenum, the jejunum and the ileum. The cecum marks the division between the small and large intestine. The large intestine includes the rectum and anal canal.[2]
Small intestine
Partially digested food starts to arrive in the small intestine as semi-liquid chyme, one hour after it is eaten.[citation needed] The stomach is half empty after an average of 1.2 hours.[30] After four or five hours the stomach has emptied.[31]
In the small intestine, the
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 duodenal bulb. From here it passes into the first of the three sections of the small intestine, the duodenum (the next section is the jejunum and the third is the ileum). 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 suspensory muscle of duodenum. 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.
Most food digestion takes place in the small intestine.
The chylomicrons are small enough to pass through the enterocyte villi and into their lymph capillaries called lacteals. A milky fluid called chyle, consisting mainly of the emulsified fats of the chylomicrons, results from the absorbed mix with the lymph in the lacteals.[clarification needed] Chyle is then transported through the lymphatic system to the rest of the body.
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 circular folds, flaps of doubled mucosal membrane which partially encircle and sometimes completely encircle the lumen of 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.
The last part of the small intestine is the ileum. This also contains villi and
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.[30]
Cecum
The
Large intestine
In the
Blood supply
The digestive system is supplied by the celiac artery. The celiac artery is the first major branch from the abdominal aorta, and is the only major artery that nourishes the digestive organs.
There are three main divisions – the left gastric artery, the common hepatic artery and the splenic artery.
The celiac artery supplies the liver, stomach, spleen and the upper 1/3 of the duodenum (to the sphincter of Oddi) and the pancreas with oxygenated blood. Most of the blood is returned to the liver via the portal venous system for further processing and detoxification before returning to the systemic circulation via the hepatic veins.
The next branch from the abdominal aorta is the superior mesenteric artery, 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.
The final branch which is important for the digestive system is the inferior mesenteric artery, 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 pectinate line.
Blood flow to the digestive tract reaches its maximum 20–40 minutes after a meal and lasts for 1.5–2 hours.[35]
Nerve supply
The
Parasympathetic innervation to the ascending colon is supplied by the vagus nerve. Sympathetic innervation is supplied by the splanchnic nerves that join the celiac ganglia. Most of the digestive tract is innervated by the two large celiac ganglia, with the upper part of each ganglion joined by the greater splanchnic nerve and the lower parts joined by the lesser splanchnic nerve. It is from these ganglia that many of the gastric plexuses arise.
Development
Early in
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
Clinical significance
Each part of the digestive system is subject to a wide range of disorders many of which can be
There are a number of esophageal diseases such as the development of Schatzki rings that can restrict the passageway, causing difficulties in swallowing. They can also completely block the esophagus.[42]
A number of problems including
A common disorder of the bowel is
Ulcerative colitis an ulcerative form of colitis, 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 colorectal cancer. Ulcerative colitis is the most common of the IBDs[45]
There are diagnostic tools mostly involving the ingestion of
In pregnancy
Gestation can predispose for certain digestive disorders. Gestational diabetes can develop in the mother as a result of pregnancy and while this often presents with few symptoms it can lead to pre-eclampsia.[51]
History
In the early 11th century, the Islamic medical philosopher Avicenna wrote extensively on many subjects including medicine. Forty of these treatises on medicine survive, and in the most famous one titled the Canon of Medicine he 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.[52]
In 1497, Alessandro Benedetti viewed 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.[53]
In the Renaissance of the 16th century, Leonardo da Vinci produced some early drawings of the stomach and intestines. He thought that the digestive system aided the respiratory system.[53] Andreas Vesalius provided some early anatomical drawings of the abdominal organs in the 16th century.
In the middle of the 17th century, a Flemish physician Jan Baptist van Helmont offered the first chemical account of digestion which was later described as being very close to the later conceptualised enzyme.[53]
In 1653, William Harvey described the intestines in terms of their length, their blood supply, the mesenteries, and fat (adenylyl cyclase).[53]
In 1823, William Prout discovered hydrochloric acid in the gastric juice.[54] In 1895, Ivan Pavlov described its secretion as being stimulated by a neurologic reflex with the vagus nerve having 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.
William Beaumont was an army surgeon who in 1825, was able to observe digestion as it took place in the stomach.[55] This 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.[53]
In the 19th century, it was accepted that chemical processes were involved in the process of digestion. Physiological research into secretion and the gastrointestinal tract was pursued with experiments undertaken by Claude Bernard, Rudolph Heidenhain and Ivan Pavlov.
The rest of the 20th century was dominated by research into enzymes. The first to be discovered was
Art historians have often noted that banqueters on 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.[57]
See also
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