What is the digestive system?
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 most common viral infection of the mouth is gingivostomatitis caused by herpes simplex. It is divided into cervical, thoracic and abdominal parts. The detection of bitterness warns of poisons—many of a plant's defences are of poisonous compounds that are bitter. Ti and Ri transfer between bacteria uses an independent system the tra , or transfer, operon from that for inter-kingdom transfer the vir , or virulence , operon. Bile produced by the liver is also used to mechanically break fats into smaller globules.
Why is digestion important?
The stomach is a sac-like organ with strong muscular walls. In addition to holding the food, it's also a mixer and grinder. The stomach secretes acid and powerful enzymes that continue the process of breaking down the food. When it leaves the stomach, food is the consistency of a liquid or paste. From there the food moves to the small intestine. Made up of three segments, the duodenum, jejunum, and ileum, the small intestine is a long tube loosely coiled in the abdomen spread out, it would be more than 20 feet long.
The small intestine continues the process of breaking down food by using enzymes released by the pancreas and bile from the liver. Bile is a compound that aids in the digestion of fat and eliminates waste products from the blood. Peristalsis contractions is also at work in this organ, moving food through and mixing it up with digestive secretions.
The duodenum is largely responsible for continuing the process of breaking down food, with the jejunum and ileum being mainly responsible for the absorption of nutrients into the bloodstream.
Three organs play a pivotal role in helping the stomach and small intestine digest food:. Among other functions, the oblong pancreas secretes enzymes into the small intestine. These enzymes break down protein, fat, and carbohydrates from the food we eat. The liver has many functions, but two of its main functions within the digestive system are to make and secrete bile, and to cleanse and purify the blood coming from the small intestine containing the nutrients just absorbed.
The gallbladder is a pear-shaped reservoir that sits just under the liver and stores bile. Bile is made in the liver then if it needs to be stored travels to the gallbladder through a channel called the cystic duct. During a meal, the gallbladder contracts, sending bile to the small intestine. Once the nutrients have been absorbed and the leftover liquid has passed through the small intestine, what is left of the food you ate is handed over to the large intestine, or colon.
The colon is a 5- to 6-foot-long muscular tube that connects the cecum the first part of the large intestine to the rectum the last part of the large intestine. Gastric acid , and the production of mucus in the stomach, are essential for the continuation of digestion. Peristalsis is the rhythmic contraction of muscles 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 of chyme which when fully broken down in the small intestine is absorbed as chyle into the lymphatic system. Most of the digestion of food takes place in the small intestine. Water and some minerals are reabsorbed back into the blood in the colon of the large intestine. The waste products of digestion feces are defecated from the anus via the rectum.
There are several organs and other components involved in the digestion of food. The organs known as the accessory digestive glands are the liver , gall bladder and pancreas. Other components include the mouth , salivary glands , tongue , teeth and epiglottis. 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.
Organ transplants can be performed on the liver, pancreas, and small intestine. Many health care providers can help diagnose and treat digestive problems. A gastroenterologist is a physician specialist who has received extra training in the diagnosis and treatment of the digestive disorders. Other providers involved in the treatment of digestive diseases include:. Conn's Current Therapy Maheshwari A, Carlo WA.
Nelson Textbook of Pediatrics. Goldman L, Schafer AI, eds.