The first stage, the cephalic phase of digestion, begins with secretions fromgastric glands in response to the sight and smell of food, and continues in themouth with the mechanical breakdown of food bychewing, and the chemical breakdown bydigestive enzymes in thesaliva. Saliva containsamylase, andlingual lipase, secreted by thesalivary glands, andserous glands on the tongue. Chewing mixes the food with saliva to produce abolus to beswallowed down theesophagus to enter thestomach. The second stage, the gastric phase, takes place in the stomach, where the food is further broken down by mixing withgastric juice until it passes into theduodenum, the first part of thesmall intestine. The intestinal phase where the partially digested food is mixed withpancreatic digestive enzymes completes the process of digestion.
Peristalsis is the rhythmic contraction ofmuscles that 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 ofchyme which when fully broken down in the small intestine is absorbed aschyle into thelymphatic system. Most of the digestion of food takes place in the small intestine. Water and someminerals are reabsorbed back into the blood in thelarge intestine. The waste products of digestion (feces) areexcreted from therectum via theanus.
Components
Adult digestive system
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 theaccessory digestive organs and include themouth, andtongue, and glandular organs – thesalivary glands, theliver,gall bladder andpancreas.[1] Other components considered are theteeth andepiglottis.[2]
The largest structure of the digestivesystem is the GI tract. This starts at the mouth and ends at theanus, covering a distance of about nine metres (30 ft).[3]
A major digestive organ is thestomach. Within itsmucosa are millions of embeddedgastric glands. Their secretions are vital to the functioning of the organ.
Most of the digestion of food takes place in thesmall intestine which is the longest part of the GI tract but has a smaller diameter than the large intestine.[2]
The largest part of the GI tract is thelarge intestine, of which thecolon is the main part. Water is absorbed here and the remaining waste matter is stored prior todefecation.[4]
Some parts of the digestive system are also part of theexcretory system, including the large intestine.[6]
Mouth
3D medical illustration explaining the oral digestive system
Themouth is the first part of theupper gastrointestinal tract and is equipped with several structures that begin the first processes of digestion.[7][2] These include salivary glands, teeth and the tongue. The mouth consists of two regions; the vestibule and the oral cavity proper.[8] The vestibule is the area between the teeth, lips and cheeks, and the rest is the oral cavity proper.[9][8] Most of the oral cavity is lined withoral mucosa, amucous membrane that produces a lubricatingmucus, 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 aglycoprotein calledmucin and 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 ofsmooth muscle tissue and the loose connection to the membrane gives it its great elasticity.[10] It covers the cheeks, inner surfaces of thelips, and floor of the mouth, and the mucin produced is highly protective againsttooth decay.[11]
The roof of the mouth is termed thepalate and 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 ofbone; 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. Thesoft palate ends at theuvula.[12] The surface of thehard palate allows for the pressure needed in eating food, to leave the nasal passage clear.[13] The opening between the lips is termed the oral fissure, and the opening into the throat is called thefauces.[14]
At either side of the soft palate are thepalatoglossus 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.[15]: 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
The main salivary glands
There are three pairs of mainsalivary glands and 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.[16] The main glands are allexocrine glands, secreting via ducts. All of these glands terminate in the mouth. The largest of these are theparotid glands—their secretion is mainlyserous. The next pair are underneath the jaw, thesubmandibular glands, these produce both serous fluid and mucus. The serous fluid is produced byserous glands in these salivary glands which also producelingual lipase. They produce about 70% of the oral cavity saliva. The third pair are thesublingual glands located underneath the tongue and their secretion is mainly mucous with a small percentage of saliva.
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 thefacial nerve (CN7).[17] The glands also secreteamylase a 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 encircletaste buds on the back part of the tongue and these also produce linguallipase. Lipase is adigestive enzyme that catalyses thehydrolysis oflipids (fats). These glands are termedVon Ebner's glands which have also been shown to have another function in the secretion ofhistatins which offer an early defense (outside of the immune system) against microbes in food, when it makes contact with these glands on the tongue tissue.[16][18] Sensory 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.
Saliva moistens and softens food, and along with the chewing action of the teeth, transforms the food into a smoothbolus. 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 thestarch incarbohydrates, breaking it down into the simplesugars ofmaltose anddextrose that 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 downfats. 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.[19]
As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth.[20] It also has animmunological role in supplying antibodies to the system, such asimmunoglobulin A.[21] This is seen to be key in preventinginfections of the salivary glands, importantly that ofparotitis.
Saliva also contains a glycoprotein calledhaptocorrin which is a binding protein to vitamin B12.[22] 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 withintrinsic factor.
Tongue
Food enters the mouth where thefirst stage in the digestive process takes place, with the action of thetongue and the secretion of saliva. The tongue is a fleshy andmuscularsensory organ, and the first sensory information is received via the taste buds in thepapillae on 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).
The tongue is attached to the floor of the mouth by a ligamentous band called thefrenum[10] and this gives it great mobility for the manipulation of food (andspeech); 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 fourintrinsic muscles that originate in the tongue and are involved with its shaping, and fourextrinsic muscles originating in bone that are involved with its movement.
Taste is a form ofchemoreception that takes place in the specialisedtaste receptors, contained in structures calledtaste buds in 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 theepiglottis and upper part of theesophagus. The taste buds are innervated by a branch of the facial nerve thechorda tympani, and theglossopharyngeal nerve. Taste messages are sent via thesecranial nerves to thebrain. The brain can distinguish between the chemical qualities of the food. The fivebasic tastes are referred to as those ofsaltiness,sourness,bitterness,sweetness, andumami. The detection of saltiness and sourness enables the control of salt and acid balance. The detection of bitterness warns of poisons—many 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 firstolfactory receptors that helped to prompt the research into taste. The olfactory receptors are located on cell surfaces in thenose which 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.[23]
Teeth are complex structures made of materials specific to them. They are made of a bone-like material calleddentin, which is covered by the hardest tissue in the body—enamel.[13] Teeth have different shapes to deal with different aspects ofmastication employed 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—incisors are used for cutting or biting off pieces of food;canines, are used for tearing,premolars andmolars are 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 beswallowed to make its way down theupper gastrointestinal tract to the stomach.[24] The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles.[25][20]
Theepiglottis is a flap ofelastic cartilage attached to the entrance of thelarynx. It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth.[26] Its laryngeal surface faces into the larynx. The epiglottis functions to guard the entrance of theglottis, the opening between thevocal folds. 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 strongcough reflex in order to protect the lungs.
Thepharynx is a part of theconducting zone of therespiratory system and also a part of the digestive system. It is the part of the throat immediately behind thenasal cavity at the back of the mouth and above the esophagus andlarynx. The pharynx is made up of three parts. The lower two parts—theoropharynx and thelaryngopharynx are 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 thepharyngeal plexus of the vagus nerve.[15]: 1465 Muscles in the pharynx push the food into the esophagus. The pharynx joins the esophagus at the oesophageal inlet which is located behind thecricoid cartilage.
Esophagus shown in yellow passing behind the trachea and the heart
Theesophagus, 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 posteriormediastinum in thethorax and enters thestomach through a hole in thethoracic diaphragm—theesophageal hiatus, at the level of the tenththoracic vertebra (T10). Its length averages 25 cm, varying with an individual's height. It is divided into cervical,thoracic andabdominal parts. The pharynx joins the esophagus at the esophageal inlet which is behind thecricoid cartilage.
At rest the esophagus is closed at both ends, by theupper and lower esophageal sphincters. The opening of the upper sphincter is triggered by theswallowing reflex so 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 thetrachea.
Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known asperistalsis. The lower esophageal sphincter is a muscular sphincter surrounding the lower part of the esophagus. Thegastroesophageal 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
Thediaphragm is an important part of the body's digestive system. The muscular diaphragm separates thethoracic cavity from theabdominal cavity where most of the digestive organs are located. Thesuspensory muscle attaches 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 theduodenojejunal flexure for the easier passage of digesting material. The diaphragm also attaches to, and anchors the liver at itsbare area. The esophagus enters the abdomen through ahole in the diaphragm at the level ofT10.
Thestomach 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.Gastric acid (informallygastric juice), produced in the stomach plays a vital role in the digestive process, and mainly containshydrochloric acid andsodium chloride. Apeptide hormone,gastrin, produced byG cells in thegastric glands, stimulates the production of gastric juice which activates the digestive enzymes.Pepsinogen is a precursor enzyme (zymogen) produced by thegastric chief cells, and gastric acid activates this to the enzymepepsin which begins the digestion ofproteins. 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.
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.
Thepylorus, the lowest section of the stomach which attaches to theduodenum via thepyloric canal, contains countless glands which secrete digestive enzymes including gastrin. After an hour or two, a thick semi-liquid calledchyme is produced. When thepyloric sphincter, 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.
Theparietal cells in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption ofvitamin 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.[27] This expansion is enabled by a series ofgastric 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.
Thespleen is the largest lymphoid organ in the body but has other functions.[28] It breaks down both red and whiteblood cells that arespent. This is why it is sometimes known as the 'graveyard of red blood cells'.[28] A product of thisdigestion is the pigmentbilirubin, which is sent to theliver and secreted in thebile. Another product isiron, which is used in the formation of new blood cells in thebone marrow.[10]Medicine treats the spleen solely as belonging to thelymphatic system, though it is acknowledged that the full range of its important functions is not yet understood.[15]: 1751
Theliver is the second largest organ (after theskin) and is an accessory digestive gland which plays a role in the body'smetabolism. The liver has many functions some of which are important to digestion. The liver can detoxify variousmetabolites; synthesise proteins and producebiochemicals needed for digestion. It regulates the storage ofglycogen which it can form fromglucose (glycogenesis). The liver can also synthesise glucose from certainamino acids. 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 synthesisescholesterol. Fats are also produced in the process oflipogenesis. 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 synthesisesbile acids andlecithin to promote the digestion of fat.[29]
Bile
Bile produced by the liver is made up of water (97%),bile salts, mucus andpigments, 1% fats and inorganic salts.[30]Bilirubin is its major pigment. Bile acts partly as asurfactant which lowers the surface tension between either two liquids or a solid and a liquid and helps toemulsify the fats in the chyme. Food fat is dispersed by the action of bile into smaller units calledmicelles. The breaking down into micelles creates a much larger surface area for the pancreatic enzyme, lipase to work on. Lipase digests thetriglycerides which are broken down into twofatty acids and amonoglyceride. These are then absorbed byvilli on 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 ofvitamin K from the diet.Bile is collected and delivered through thecommon hepatic duct. This duct joins with thecystic duct to connect in acommon bile duct with the gallbladder.Bile is stored in the gallbladder for release when food is discharged into the duodenum and also after a few hours.[10]
Gallbladder
Gallbladder shown in green below the liver
Thegallbladder is a hollow part of thebiliary tract that sits just beneath the liver, with the gallbladder body resting in a small depression.[31] 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 thebile ducts and into the gall bladder for storage. The bile is released in response tocholecystokinin (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.[32]
It is divided into three sections, a fundus, body and neck. The neck tapers and connects to the biliary tract via thecystic duct, which then joins the common hepatic duct to form the common bile duct. At this junction is a mucosal fold calledHartmann's pouch, wheregallstones commonly get stuck. Themuscular layer of 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.Hydrogen ions secreted from the inner lining of the gallbladder keep the bile acidic enough to prevent hardening. To dilute the bile, water andelectrolytes from the digestion system are added. Also, salts attach themselves to cholesterol molecules in the bile to keep them fromcrystallising. 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, orgall. 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.
Pancreas, duodenum and bile ductAction of digestive hormones
Thepancreas is a major organ functioning as an accessory digestive gland in the digestive system. It is both anendocrine gland and anexocrine gland.[33] The endocrine part secretesinsulin when theblood sugar becomes high; insulin moves glucose from the blood into the muscles and other tissues for use as energy. The endocrine part releasesglucagon when 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 thepancreatic juice that it delivers to the duodenum.[29] The pancreas lies below and at the back of the stomach. It connects to the duodenum via thepancreatic duct which 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 frompancreatic duct cells containbicarbonate ions which are alkaline and help with the bile to neutralise the acidic chyme that is churned out by the stomach.
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 majorproteases, the pancreatic enzymes which work on proteins, aretrypsinogen andchymotrypsinogen.Elastase is also produced. Smaller amounts of lipase and amylase are secreted. The pancreas also secretesphospholipase A2,lysophospholipase, and cholesterolesterase. The precursorzymogens, are inactive variants of the enzymes; which avoids the onset ofpancreatitis caused by autodegradation. Once released in the intestine, the enzymeenteropeptidase present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin; further cleavage results in chymotripsin.
The lower gastrointestinal tract (GI), includes thesmall intestine and all of thelarge intestine.[34] The 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 theduodenum, thejejunum and theileum. The cecum marks the division between the small and large intestine. The large intestine includes the rectum andanal canal.[6]
Small intestine
Illustration of small intestine
The small intestine starts at thepyloric sphincter, and finishes at theileocecal valve.[4]Partially digested food starts to arrive in thesmall intestine as semi-liquidchyme, one hour after it is eaten.[citation needed] The stomach is half empty after an average of 1.2 hours.[35] After four or five hours the stomach has emptied.[36]
In the small intestine, thepH becomes 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 theduodenum by the addition of bile from the gall bladder combined with thebicarbonate secretions from the pancreatic duct and also from secretions of bicarbonate-rich mucus from duodenal glands known asBrunner's glands. 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.
Layers of the small intestine
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 theduodenal bulb. From here it passes into the first of the three sections of the small intestine, the duodenum (the next section is thejejunum and the third is theileum). 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 thesuspensory 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.Segmentation contractions act 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 aco-enzyme,colipase to further digest the fat content of the chyme. From this breakdown, smaller particles of emulsified fats calledchylomicrons are produced. There are also digestive cells calledenterocytes lining the intestines (the majority being in the small intestine). They are unusual cells in that they havevilli on their surface which in turn have innumerablemicrovilli on 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.
The chylomicrons are small enough to pass through the enterocyte villi and into theirlymph capillaries calledlacteals. A milky fluid calledchyle, 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 thelymphatic 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 containscircular folds, flaps of doubled mucosal membrane which partially encircle and sometimes completely encircle thelumen 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 andvitamin B12; 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 calledfeces, which pass to the large intestine, where bacteria in thegut flora further break down residual proteins and starches.[37]
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.[35]
Cecum
Cecum and beginning of ascending colon
Thececum is a pouch marking the division between the small intestine and the large intestine. It lies below theileocecal valve in thelower right quadrant of the abdomen.[38] The cecum receives chyme from the last part of the small intestine, theileum, and connects to theascending colon of 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 theappendix attachment.[38]
Large intestine
Lower GI tract - 3) Small intestine; 5) Cecum; 6) Large intestine
In thelarge intestine the passage of the digesting food in thecolon is a lot slower, taking from 30 to 40 hours until it is removed bydefecation.[36] The colon mainly serves as a site for the fermentation of digestible matter by thegut flora. The time taken varies considerably between individuals. The remaining semi-solid waste is termedfeces and is removed by the coordinated contractions of the intestinal walls, termedperistalsis, which propels theexcreta forward to reach therectum and exit through theanus via defecation. The wall has an outer layer of longitudinal muscles, thetaeniae coli, 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 thebasal electrical rhythm that determines the frequency of contractions.[39] The taeniae coli can be seen and are responsible for the bulges (haustra) present in the colon. Most parts of the GI tract are covered withserous membranes and have amesentery. Other more muscular parts are lined withadventitia.
Blood supply
Arteries and veins around the pancreas and spleen
The digestive system is supplied by theceliac artery. The celiac artery is the first major branch from theabdominal aorta, and is the only major artery that nourishes the digestive organs.
The celiac artery supplies the liver, stomach, spleen and the upper 1/3 of the duodenum (to thesphincter of Oddi) and the pancreas with oxygenated blood. Most of the blood is returned to the liver via theportal venous system for further processing and detoxification before returning to thesystemic circulation via thehepatic veins.
The next branch from the abdominal aorta is thesuperior 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 theinferior 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 thepectinate line.
Blood flow to the digestive tract reaches its maximum 20–40 minutes after a meal and lasts for 1.5–2 hours.[40]
Early inembryonic development, theembryo has threegerm layers and abuts ayolk sac. 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 thisforegut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines.[46]
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, thehepatic diverticulum, which will go on to become thebiliary tree. Just below this is a second outpouching, known as thecystic diverticulum, that will eventually develop into the gallbladder.[46]
There are a number ofesophageal diseases such as the development ofSchatzki rings that can restrict the passageway, causing difficulties in swallowing. They can also completely block the esophagus.[47]
A number of problems includingmalnutrition andanemia can arise frommalabsorption, the abnormal absorption of nutrients in the GI tract. Malabsorption can have many causes ranging frominfection, to enzyme deficiencies such asexocrine pancreatic insufficiency. It can also arise as a result of other gastrointestinal diseases such ascoeliac disease. Coeliac disease is anautoimmune disorder of the small intestine. This can causevitamin deficiencies due to the improper absorption of nutrients in the small intestine. The small intestine can also beobstructed by avolvulus, a loop of intestine that becomes twisted enclosing its attachedmesentery. This can causemesenteric ischemia if severe enough.
A common disorder of the bowel isdiverticulitis.Diverticula are 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) and cause potentially fatalperitonitis.[48]
Ulcerative colitis, an ulcerative form ofcolitis, 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 ofcolorectal cancer. Ulcerative colitis is the most common of the IBDs[50]
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.[56] The major specific causes of digestive system cancer related death in 2023 were cancers of thecolon and rectum (52,550),pancreas (50,550),liver and intrahepatic bile duct (29,380),esophagus (16,120) andstomach (11,130).[56]Bile acids, when present in specific regions of the digestive system at abnormally high levels, have been implicated as important carcinogens.[57]
Dietary life rules, Japan,Edo period Illustrating the ill effects of drinking alcohol on the digestive system.Historical depiction of the digestive system, 17th century Persia
In the early 11th century, the Islamic medical philosopherAvicenna wrote extensively on many subjects including medicine. Forty of these treatises on medicine survive, and in the most famous one titled theCanon 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.[59]
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.[60]
In theRenaissance 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.[60]Andreas Vesalius provided some early anatomical drawings of the abdominal organs in the 16th century.
In 1653,William Harvey described the intestines in terms of their length, their blood supply, the mesenteries, and fat (adenylyl cyclase).[60]
In 1823,William Prout discoveredhydrochloric acid in the gastric juice.[61] In 1895,Ivan Pavlov described its secretion as being stimulated by a neurologic reflex with thevagus 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.[62] 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.[60]
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 wassecretin byErnest Starling in 1902, with ensuing results from John Edkins in 1905 who first suggestedgastrin with its structure being determined in 1964.[61] Andre Latarjet and Lester Dragstedt found a role foracetylcholine in the digestive system.[61] In 1972,H2 receptor agonists were described by J. Black, that block the action of histamine and decrease the production of hydrochloric acid. In 1980,proton pump inhibitors were described by Sachs. In 1983, the role ofHelicobacter pylori in the formation of ulcers was described byBarry Marshall, andRobin Warren.[63]
Art historians have often noted thatbanqueters 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.[64]
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