Almost everyone develops jowls as they age. It normally takes about 36 hours for stool to get through the colon. Temporary storage occurs in the crop where food and calcium carbonate are mixed. Generally the alimentary canal has layers of muscle that run lengthwise and around it and is lined with mucous membranes. Lymphoid tissue is found in most glands. The oesophagus is wide and is capable of being significantly stretched.
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 9 metres. 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. 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. There are many specialised cells of the GI tract.
These include the various cells of the gastric glands, taste cells , pancreatic duct cells , enterocytes and microfold cells. Some parts of the digestive system are also part of the excretory system , including the large intestine.
The mouth is the first part of the upper gastrointestinal tract and is equipped with several structures that begin the first processes of digestion. The mouth consists of two regions; the vestibule and the oral cavity proper. The vestibule is the area between the teeth, lips and cheeks,  and the rest is the oral cavity proper.
Most of the oral cavity is lined with oral mucosa , a mucous membrane that produces a lubricating mucus , 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 glycoprotein called mucin 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 of smooth muscle tissue and the loose connection to the membrane gives it its great elasticity. The roof of the mouth is termed the palate 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 of bone ; 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 soft palate ends at the uvula. 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. There are three pairs of main salivary glands and between and 1, 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.
All of these glands terminate in the mouth. The largest of these are the parotid glands —their secretion is mainly serous. The next pair are underneath the jaw, the submandibular glands , these produce both serous fluid and mucus. The serous fluid is produced by serous glands in these salivary glands which also produce lingual lipase.
The third pair are the sublingual 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 the facial nerve CN7.
There are other glands on the surface of the tongue that encircle taste buds on the back part of the tongue and these also produce lingual lipase. Lipase is a digestive enzyme that catalyses the hydrolysis of lipids fats.
These glands are termed Von Ebner's glands which have also been shown to have another function in the secretion of histatins 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.
Saliva moistens and softens food, and along with the chewing action of the teeth, transforms the food into a smooth bolus. 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 starch in carbohydrates , breaking it down into the simple sugars of maltose and dextrose that can be further broken down in the small intestine.
Lipase starts to work on breaking down fats. 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. As well as its role in supplying digestive enzymes , saliva has a cleansing action for the teeth and mouth.
Saliva also contains a glycoprotein called haptocorrin which is a binding protein to vitamin B When it reaches the duodenum, pancreatic enzymes break down the glycoprotein and free the vitamin which then binds with intrinsic factor.
Food enters the mouth where the first stage in the digestive process takes place, with the action of the tongue and the secretion of saliva. The tongue is a fleshy and muscular sensory organ , and the very first sensory information is received via the taste buds in the papillae 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 the frenum  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 is a form of chemoreception that takes place in the specialised taste receptors , contained in structures called taste 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 the epiglottis and upper part of the esophagus. The taste buds are innervated by a branch of the facial nerve the chorda tympani , and the glossopharyngeal nerve.
Taste messages are sent via these cranial nerves to the brain. The brain can distinguish between the chemical qualities of the food. The five basic tastes are referred to as those of saltiness , sourness , bitterness , sweetness , and umami.
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 , describing the first olfactory receptors that helped to prompt the research into taste.
The olfactory receptors are located on cell surfaces in the nose 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. Teeth are complex structures made of materials specific to them. They are made of a bone-like material called dentin , which is covered by the hardest tissue in the body— enamel.
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 and molars 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 be swallowed to make its way down the upper gastrointestinal tract to the stomach. The epiglottis is a flap of elastic cartilage attached to the entrance of the larynx. It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth.
The epiglottis functions to guard the entrance of the glottis , the opening between the vocal 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 strong cough reflex in order to protect the lungs. The pharynx is a part of the conducting zone of the respiratory system and also a part of the digestive system. It is the part of the throat immediately behind the nasal cavity at the back of the mouth and above the esophagus and larynx.
The pharynx is made up of three parts. The lower two parts—the oropharynx and the laryngopharynx 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 the pharyngeal plexus of the vagus nerve. The pharynx joins the esophagus at the oesophageal inlet which is located behind the cricoid cartilage. The esophagus , 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 mediastinum in the thorax and enters the stomach through a hole in the thoracic diaphragm —the esophageal hiatus , at the level of the tenth thoracic vertebra T It is divided into cervical, thoracic and abdominal parts. The pharynx joins the esophagus at the esophageal inlet which is behind the cricoid cartilage. At rest the esophagus is closed at both ends, by the upper and lower esophageal sphincters.
The opening of the upper sphincter is triggered by the swallowing 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 the trachea. 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 junction between the esophagus and the stomach the gastroesophageal junction 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. The esophagus has a mucous membrane of epithelium which has a protective function as well as providing a smooth surface for the passage of food.
Due to the high volume of food that is passed over time, this membrane is continuously renewed. The diaphragm is an important part of the body's digestive system. The muscular diaphragm separates the thoracic cavity from the abdominal cavity where most of the digestive organs are located. The suspensory 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 the duodenojejunal flexure for the easier passage of digesting material.
The diaphragm also attaches to, and anchors the liver at its bare area. The esophagus enters the abdomen through a hole in the diaphragm at the level of T 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. Gastric acid informally gastric juice , produced in the stomach plays a vital role in the digestive process, and mainly contains hydrochloric acid and sodium chloride.
A peptide hormone , gastrin , produced by G cells in the gastric glands , stimulates the production of gastric juice which activates the digestive enzymes. Pepsinogen is a precursor enzyme zymogen produced by the gastric chief cells , and gastric acid activates this to the enzyme pepsin which begins the digestion of proteins. 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. The pylorus , the lowest section of the stomach which attaches to the duodenum via the pyloric canal , contains countless glands which secrete digestive enzymes including gastrin.
After an hour or two, a thick semi-liquid called chyme is produced. When the pyloric 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.
When the chyme is fully digested, it is absorbed into the blood. Water and minerals are reabsorbed back into the blood in the colon of the large intestine, where the environment is slightly acidic. Some vitamins, such as biotin and vitamin K produced by bacteria in the gut flora of the colon are also absorbed.
The parietal cells in the fundus of the stomach, produce a glycoprotein called intrinsic factor which is essential for the absorption of vitamin B 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.
The stomach of a newborn baby will only be able to expand to retain about 30 ml. The spleen breaks down both red and white blood cells that are spent.
This is why it is sometimes known as the 'graveyard of red blood cells'. A product of this digestion is the pigment bilirubin , which is sent to the liver and secreted in the bile. Another product is iron , which is used in the formation of new blood cells in the bone marrow. The liver is the second largest organ after the skin and is an accessory digestive gland which plays a role in the body's metabolism. The liver has many functions some of which are important to digestion.
The liver can detoxify various metabolites ; synthesise proteins and produce biochemicals needed for digestion. It regulates the storage of glycogen which it can form from glucose glycogenesis. The liver can also synthesise glucose from certain amino 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 synthesises cholesterol. Fats are also produced in the process of lipogenesis. 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, This is to the right of the stomach and it overlies the gall bladder.
The liver produces bile , an important alkaline compound which aids digestion. Bile acts partly as a surfactant which lowers the surface tension between either two liquids or a solid and a liquid and helps to emulsify the fats in the chyme. Food fat is dispersed by the action of bile into smaller units called micelles. The breaking down into micelles creates a much larger surface area for the pancreatic enzyme, lipase to work on.
Lipase digests the triglycerides which are broken down into two fatty acids and a monoglyceride. These are then absorbed by villi 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 of vitamin K from the diet. Bile is collected and delivered through the common hepatic duct. This duct joins with the cystic duct to connect in a common 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.
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. 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. It is divided into three sections, a fundus, body and neck.
The neck tapers and connects to the biliary tract via the cystic duct , which then joins the common hepatic duct to form the common bile duct. At this junction is a mucosal fold called Hartmann's pouch , where gallstones commonly get stuck. The muscular 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 and electrolytes from the digestion system are added. Also, salts attach themselves to cholesterol molecules in the bile to keep them from crystallising.
If there is too much cholesterol or bilirubin in the bile, or if the gallbladder doesn't 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 gall. 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. The pancreas is a major organ functioning as an accessory digestive gland in the digestive system.
It is both an endocrine gland and an exocrine gland. The endocrine part releases glucagon 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 the pancreatic juice that it delivers to the duodenum.
The pancreas lies below and at the back of the stomach. It connects to the duodenum via the pancreatic 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 from pancreatic duct cells contain bicarbonate 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 CKK 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 proteases , the pancreatic enzymes which work on proteins, are trypsinogen and chymotrypsinogen. Elastase is also produced.
Smaller amounts of lipase and amylase are secreted. The pancreas also secretes phospholipase A2 , lysophospholipase , and cholesterol esterase. The precursor zymogens , are inactive variants of the enzymes; which avoids the onset of pancreatitis caused by autodegradation. Once released in the intestine, the enzyme enteropeptidase present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin; further cleavage results in chymotripsin.
The lower gastrointestinal tract GI , includes the small intestine and all of the large intestine. 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. The hard palate is formed by the horizontal portions of the two palatine bones and the palatine portions of the maxillae, or upper jaws. The hard palate is covered by a thick, somewhat pale mucous membrane that is continuous with that of the gums and is bound to the upper jaw and palate bones by firm fibrous tissue. The soft palate is continuous with the hard palate in front.
Posteriorly it is continuous with the mucous membrane covering the floor of the nasal cavity. The soft palate is composed of a strong, thin, fibrous sheet, the palatine aponeurosis, and the glossopalatine and pharyngopalatine muscles.
A small projection called the uvula hangs free from the posterior of the soft palate. The floor of the mouth can be seen only when the tongue is raised. In the midline is a prominent, elevated fold of mucous membrane frenulum linguae that binds each lip to the gums, and on each side of this is a slight fold called a sublingual papilla , from which the ducts of the submandibular salivary glands open.
Running outward and backward from each sublingual papilla is a ridge the plica sublingualis that marks the upper edge of the sublingual under the tongue salivary gland and onto which most of the ducts of that gland open.
The gums consist of mucous membranes connected by thick fibrous tissue to the membrane surrounding the bones of the jaw.
The gum membrane rises to form a collar around the base of the crown exposed portion of each tooth. Rich in blood vessels, the gum tissues receive branches from the alveolar arteries; these vessels, called alveolar because of their relationship to the alveoli dentales, or tooth sockets, also supply the teeth and the spongy bone of the upper and lower jaws, in which the teeth are lodged.
We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval. Unfortunately, our editorial approach may not be able to accommodate all contributions. Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article.
Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Read More on This Topic. Page 1 of Next page The teeth. Learn More in these related Britannica articles: As the embryo folds off, the endoderm is rolled in as the foregut and hindgut. Continued growth progressively closes both the midbody and the midgut.
The esophagus remains as a simple, straight tube. The stomach grows faster on its dorsal side, thereby forming…. Loss of teeth, which is often seen in elderly people, is more apt to be the result of long-term neglect than a result of aging itself. The loss of teeth and incidence of oral disease increase with age, but, as programs of water…. In either event, pregnancy complicates their problems because the gastrointestinal disturbances that often….
Some brain cells may also participate as hunger receptors. This is especially true of cells in the lower parts of the brain such as the hypothalamus where some cells have been found to…. More About Human digestive system 5 references found in Britannica articles Assorted References embryological development In prenatal development: Digestive tube sensory reception In human sensory reception effects of aging In human aging: Digestive system pregnancy In pregnancy: Gastrointestinal tract In pregnancy: Articles from Britannica Encyclopedias for elementary and high school students.
Help us improve this article! Contact our editors with your feedback.