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Digestive System Anatomy and Physiology

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Children are fascinated by the workings of the digestive system: they relish crunching a potato chip, delight in making “mustaches” with milk, and giggle when their stomach growls. As adults, we know that a healthy digestive system is essential for good health because it converts food into raw materials that build and fuel our body cells.

Functions of the Digestive System

The functions of the digestive system are:

  1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion.
  2. Propulsion. If foods are to be processed by more than one digestive organ, they must be propelled from one organ to the next; swallowing is one example of food movement that depends largely on the propulsive process called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).
  3. Food breakdown: mechanical digestion. Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and examples of mechanical digestion are: mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine.
  4. Food breakdown: chemical digestion. The sequence of steps in which the large food molecules are broken down into their building blocks by enzymes is called chemical digestion.
  5. Absorption. Transport of digested end products from the lumen of the GI tract to the blood or lymph is absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or passive transport processes.
  6. Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces.

Anatomy of the Digestive System

The organs of the digestive system can be separated into two main groups: those forming the alimentary canal and the accessory digestive organs.

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Organs of the Alimentary Canal

The alimentary canal, also called the gastrointestinal tract, is a continuous, hollow muscular tube that winds through the ventral body cavity and is open at both ends. Its organs include the following:

Mouth

Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity.

  • Lips. The lips (labia) protect its anterior opening.
  • Cheeks. The cheeks form its lateral walls.
  • Palate. The hard palate forms its anterior roof, and the soft palate forms its posterior roof.
  • Uvula.

Digestive system

Author: Lorenzo Crumbie MBBS, BSc

Reviewer: Francesca Salvador MSc

Last reviewed: March 09, 2022

Reading time: 30 minutes

Extending from the mouth to the anus, the digestive tract is one of the largest systems in the human body.

It contains organs that regulate food intake, its digestion and absorbtion of the useful materia that it contains.

In addition to this, the digestive system also eliminates the waste products from food and products from various endogenous metabolic processes.

In a nutshell, the digestive tract has the tumultuous responsibility of converting large chunks of food into their constituent micro-molecules that will subsequently be used to build and repair the body.

Key facts

Functions Regulation of satiety and hunger, mechanical digestion (mastication), swallowing and chemical digestion of food; absorption of necessary nutrients, elimination of unnecessary matter and waste
Components Primary organs: mouth, pharynx, esophagus, stomach, small intestines (duodenum, jejunum, ileum), large intestine (colon), rectum and anal canal Accessory organs: salivary glands, liver, gall bladder, pancreas
Mouth Contains structures that start digestion: teeth (choping the food), salivary glands (secrete saliva that contain enzymes that start chemical digestion of sugar and fats), tongue (detects taste, pushes bolus towards the pharynx)
Pharynx Conducts the food to the esophagus
Esophagus Muscular tube that conduct the bolus to the stomach; It has upper sphincter (opens with swallow reflex and allows the bolus to enter the esophagus) and lower sphincter (controls emptying of the esophagus content to the stomach) 
Stomach Function — secretion of gastric acid (hydrochloric acid + sodium chloride + pepsine) that digests proteins and converts bolus to chyme Parts — cardia (where content of the esophagus empties into stomach), fundus (upper curved part), body (main, central region), pylorus (empties the chyme into the duodenum)
Spleen Breaks down spent erythrocytes -> production of bilirubin -> bilirubin sent to the liver -> secreted in the bile
Liver Main functions: detoxication of metabolytes, synthesis of proteins, production of biochemicals needed for digestion -> regulation of body's metabolism and energy storage
Gallbladder Stores the bile and empties it into the duodenum from where it partially eliminates via defecation
Pancreas Secretes insulin when sugar levels are high, secretes glucagon when sugar levels are low, secretes pancreatic juice (tripsinogen, chymotripsinogen, elastase, amilase etc.) into the duodenum where it digests the chyme
Small intestines Duodenum — mixes chyme with bile, secretes bicarbonates to rise pH in order to activate pancreatic enzymes which digest the chyme Jejunum — absorbs small nutrients that have been previously digested in duodenum Ileum — absorbs vitamin B12, bile salts and all necessary materia that were not absorbed in jejunum Cecum — a pouch that marks division between small and large intestines -> connects the ileum with ascending colon
Large intestine Ascending colon — absorbs water from content and moves in to the transverse colon by peristalsis Transverse colon — extends from hepatic flexure to the splenic flexure; absorbs water and salts Descending colon — extends from splenic flexure to the sigmoid colon; stores feces that will be emptied into the sigmoid colon Sigmoid colon — contracts to increase pressure inside the colon, causing the stool to move into the rectum
Rectum Holds the formed feces awaiting elimination via defecation
Bowel mnemonic Dow Jones Industrial Average Closing Stock Report (stands for duodenum, jejunum, ileum, appendix, colon, sigmoid, rectum)
Anal canal Passage through which undigested food and exfoliated mucosa exit the body
Vascularization Supplied by the branches of abdominal aorta:Celiac trunk — supplies the liver, stomach, spleen, upper 1/3 of duodenum, pancreas — Superior mesenteric artery — supplies distal 2/3 of duodenum, jejunum, ileum, cecum, appendix, ascending colon, proximal 1/3 of transverse colon — Inferior mesenteric artery — supplies distal 1/2 of transverse colon, descending colon, sigmoid colon, rectum, anus
Innervation Parasympathetic supplyvagus nerve and pelvic splanchnic nervesSympathetic supply thoracic and lumbar splanchnic nerves
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This article will discuss the anatomy of the digestive system.

The digestive system can be broken down into two major components:

  • There is the primary digestive tract, which functions mainly as a conduit and storage pathway. This portion is needed in order to move food contents along the tract (peristalsis) so that absorption of nutrients and excretion of undigested substances can occur. The tract also allows for segmentation of food at different stages of digestion. This is important because some enzymes produced in one part of the tract (e.g. pepsin in the stomach) will not function optimally in another part of the tract (e.g. the jejunum).
  • The other component is the accessory digestive tract. This group of organs are responsible for the synthesis and secretion enzymes to facilitate chemical digestion.

This article aims to give an overview of the anatomy and function of the digestive tract. Details regarding neurovascular supply, histology, and clinically significant points regarding each segment of the digestive tract can be found in the respective articles. It will also briefly mention a few elements of the abdominal examination.

The function of the digestive system truly begins within the brain. Whenever the body’s energy stores (i.e. blood glucose, protein, or fat stores) fall below a set point, the hunger centres of the hypothalamus are activated.

These centres regulate satiety (fullness) and appetite in order to maintain energy homeostasis. This signals to the brain that there is a need to obtain food.

Keep in mind, however, that this is not the only source of hunger stimulation, as there are elements of pleasure and comfort that can be derived from ingesting food.

23.2 Digestive System Processes and Regulation — Anatomy and Physiology

By the end of this section, you will be able to:

  • Discuss six fundamental activities of the digestive system, giving an example of each
  • Compare and contrast the neural and hormonal controls involved in digestion

The digestive system uses mechanical and chemical activities to break food down into absorbable substances during its journey through the digestive system. Table 23.3 provides an overview of the basic functions of the digestive organs.

Visit this site for an overview of digestion of food in different regions of the digestive tract. Note the route of non-fat nutrients from the small intestine to their release as nutrients to the body.

The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.

The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.

Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis.

Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along (Figure 23.5). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.

Figure 23.5 Peristalsis Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva.

Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme.

Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents.

By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.

In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine.

Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption, which takes place primarily within the small intestine.

There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream (the subclavian veins near the heart).

The details of these processes will be discussed later.

In defecation, the final step in digestion, undigested materials are removed from the body as feces.

Age-related changes in the digestive system begin in the mouth and can affect virtually every aspect of the digestive system. Taste buds become less sensitive, so food isn’t as appetizing as it once was.

A slice of pizza is a challenge, not a treat, when you have lost teeth, your gums are diseased, and your salivary glands aren’t producing enough saliva. Swallowing can be difficult, and ingested food moves slowly through the alimentary canal because of reduced strength and tone of muscular tissue.

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Neurosensory feedback is also dampened, slowing the transmission of messages that stimulate the release of enzymes and hormones.

Pathologies that affect the digestive organs—such as hiatal hernia, gastritis, and peptic ulcer disease—can occur at greater frequencies as you age. Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption.

Problems in the large intestine include hemorrhoids, diverticular disease, and constipation.

Conditions that affect the function of accessory organs—and their abilities to deliver pancreatic enzymes and bile to the small intestine—include jaundice, acute pancreatitis, cirrhosis, and gallstones.

In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal (Figure 23.6).

Figure 23.6 Digestive Processes The digestive processes are ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.

Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.

Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption. These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically.

The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions. These include mechanoreceptors, chemoreceptors, and osmoreceptors, which are capable of detecting mechanical, chemical, and osmotic stimuli, respectively.

For example, these receptors can sense when the presence of food has caused the stomach to expand, whether food particles have been sufficiently broken down, how much liquid is present, and the type of nutrients in the food (lipids, carbohydrates, and/or proteins).

Stimulation of these receptors provokes an appropriate reflex that furthers the process of digestion.

This may entail sending a message that activates the glands that secrete digestive juices into the lumen, or it may mean the stimulation of muscles within the alimentary canal, thereby activating peristalsis and segmentation that move food along the intestinal tract.

The walls of the entire alimentary canal are embedded with nerve plexuses that interact with the central nervous system and other nerve plexuses—either within the same digestive organ or in different ones. These interactions prompt several types of reflexes.

Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli from outside the digestive system. Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal wall.

These two plexuses and their connections were introduced earlier as the enteric nervous system. Short reflexes regulate activities in one area of the digestive tract and may coordinate local peristaltic movements and stimulate digestive secretions.

For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata. The response to the signal is to stimulate cells in the stomach to begin secreting digestive juices in preparation for incoming food.

In contrast, food that distends the stomach initiates short reflexes that cause cells in the stomach wall to increase their secretion of digestive juices.

A variety of hormones are involved in the digestive process. The main digestive hormone of the stomach is gastrin, which is secreted in response to the presence of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa.

Other GI hormones are produced and act upon the gut and its accessory organs.

Hormones produced by the duodenum include secretin, which stimulates a watery secretion of bicarbonate by the pancreas; cholecystokinin (CCK), which stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; and gastric inhibitory peptide, which inhibits gastric secretion and slows gastric emptying and motility. These GI hormones are secreted by specialized epithelial cells, called endocrinocytes, located in the mucosal epithelium of the stomach and small intestine. These hormones then enter the bloodstream, through which they can reach their target organs.

Human digestive system

Digestive system in humans
See also gastrointestinal tract.
«Digestive system» and «alimentary system» redirect here. For digestive systems of non-human animals, see Digestion.

Human digestive systemHuman digestive systemDetailsIdentifiersLatinSystema digestoriumMeSHD004064TA98A05.0.00.000TA22773FMA7152Anatomical terminology[edit on Wikidata]

The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.

The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food. This stage includes the mechanical breakdown of food by chewing, and the chemical breakdown by digestive enzymes, that takes place in the mouth.

Saliva contains the digestive enzymes amylase, and lingual lipase, secreted by the salivary and serous glands on the tongue. Chewing, in which the food is mixed with saliva, begins the mechanical process of digestion.

This produces a bolus which is swallowed down the esophagus to enter the stomach.

The second stage of digestion begins in the stomach with the gastric phase. 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 begins in the duodenum with the intestinal phase, where 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 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 rectum via the anus.

Components

Adult digestive system

There are several organs and other components involved in the digestion of food. The organs known as the accessory digestive organs 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 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 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.[2]

Mouth

3D Medical illustration explaining the oral digestive system

The mouth is the first part of the upper gastrointestinal tract and is equipped with several structures that begin the first processes of digestion.[3] These include salivary glands, teeth and the tongue. The mouth consists of two regions; the vestibule and the oral cavity proper. The vestibule is the area between the teeth, lips and cheeks,[4] 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.[5] It covers the cheeks, inner surfaces of the lips, and floor of the mouth, and the mucin produced is highly protective against tooth decay.[6]

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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.

[7] The surface of the hard palate allows for the pressure needed in eating food, to leave the nasal passage clear.[8] The opening between the lips is termed the oral fissure, and the opening into the throat is called the fauces.[9]

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

The main salivary glands

There are three pairs of main salivary 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.[11] The main glands are all exocrine glands, secreting via ducts. 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. They produce about 70% of the oral cavity saliva. 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).

[12] The glands also secrete amylase 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 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.[11][13] 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

Main article: Saliva

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.

Saliva in the mouth can account for 30% of this initial starch digestion. 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.[14]

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

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 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[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

Main article: Taste
Cross section of circumvallate papilla showing arrangement of nerves and taste buds

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 1991, 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.[18]

Teeth

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