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 the
accessory digestive organs and include the
mouth, and
tongue, and glandular organs – the
salivary glands, the
liver,
gall bladder and
pancreas. Other components considered are the
teeth and
epiglottis. A number of
sphincters in the GI tract are also involved in digestion, including those of the esophagus (
esophageal sphincters) and stomach (
pyloric sphincter). The largest structure of the digestive
system is the GI tract. This starts at the mouth and ends at the
anus, covering a distance of about . A major digestive organ is the
stomach. Within its
mucosa are millions of embedded
gastric glands. Their secretions as
gastric juice 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 but has a smaller diameter than the large intestine. There are many specialised
cells of the GI tract. These include the
taste receptors, various cells of the gastric glands,
centroacinar cells of the
pancreas,
enterocytes lining the intestinal epithelium, and
microfold cells also known as mucosal cells, mainly found in
gut-associated lymphoid tissue of the small intestine. Some parts of the digestive system are also part of the
excretory system, including the large intestine.
Mouth 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 vestibule is the area between the teeth, lips and cheeks, and the rest is the oral cavity proper. It covers the cheeks, inner surfaces of the
lips, and floor of the mouth, and the mucin produced is highly protective against
tooth decay. 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. The surface of the
hard palate allows for the pressure needed in eating food, to leave the nasal passage clear. The opening between the lips is termed the oral fissure, and the opening into the throat is called the
fauces. 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. Mucus helps in the mastication of food in its ability to soften and collect the food in the formation of the bolus.
Salivary glands There are three pairs of main
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. 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). 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. 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 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. As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth. It also has an
immunological role in supplying antibodies to the system, such as
immunoglobulin A. 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. 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 Teeth 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. The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles. Its laryngeal surface faces into the larynx. 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.
Pharynx 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. This expansion is enabled by a series of
gastric folds in the inner walls of the stomach. The stomach of a newborn baby will only be able to expand to retain about 30 ml.
Spleen The
spleen is the largest lymphoid organ in the body but has other functions. It 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'.
Bile Bile produced by the liver is made up of water (97%),
bile salts, mucus and
pigments, 1% fats and inorganic salts.
Bilirubin is its major pigment. 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. It is a small organ where the bile produced by the liver is stored, before being released into the small intestine. Bile flows from the liver through the
bile ducts and into the gall bladder for storage. The bile is released in response to
cholecystokinin (CCK), a peptide hormone released from the duodenum. The production of CCK (by endocrine cells of the duodenum) is stimulated by the presence of fat in the duodenum. 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 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, 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.
Pancreas 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 secretes
insulin when the
blood sugar becomes high; insulin moves glucose from the blood into the muscles and other tissues for use as energy. 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 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 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. After four or five hours the stomach has emptied. In the small intestine, the
pH 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 the
duodenum by the addition of bile from the gall bladder combined with the
bicarbonate secretions from the pancreatic duct and also from secretions of bicarbonate-rich mucus from duodenal glands known as
Brunner'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. When the digested food particles are reduced enough in size and composition, they can be absorbed by the intestinal wall and carried to the bloodstream. The first receptacle for this chyme is the
duodenal bulb. From here it passes into the first of the three sections of the small intestine, the duodenum (the next section is the
jejunum and the third is the
ileum). The duodenum is the first and shortest section of the small intestine. It is a hollow, jointed C-shaped tube connecting the stomach to the jejunum. It starts at the duodenal bulb and ends at the
suspensory muscle of duodenum. The attachment of the suspensory muscle to the diaphragm is thought to help the passage of food by making a wider angle at its attachment. Most food digestion takes place in the small intestine.
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 a
co-enzyme,
colipase to further digest the fat content of the chyme. From this breakdown, smaller particles of emulsified fats called
chylomicrons are produced. There are also digestive cells called
enterocytes lining the intestines (the majority being in the small intestine). They are unusual cells in that they have
villi on their surface which in turn have innumerable
microvilli 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 their
lymph capillaries called
lacteals. A milky fluid called
chyle, consisting mainly of the emulsified fats of the chylomicrons, results from the absorbed mix with the lymph in the lacteals. Chyle is then transported through the
lymphatic system to the rest of the body. The suspensory muscle marks the end of the duodenum and the division between the upper gastrointestinal tract and the lower GI tract. The digestive tract continues as the jejunum which continues as the ileum. The jejunum, the midsection of the small intestine contains
circular folds, flaps of doubled mucosal membrane which partially encircle and sometimes completely encircle the
lumen of the intestine. These folds together with villi serve to increase the surface area of the jejunum enabling an increased absorption of digested sugars, amino acids and fatty acids into the bloodstream. The circular folds also slow the passage of food giving more time for nutrients to be absorbed. The last part of the small intestine is the ileum. This also contains villi and
vitamin 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 called
feces, which pass to the large intestine, where bacteria in the
gut flora further break down residual proteins and starches. 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. The cecum receives chyme from the last part of the small intestine, the
ileum, and connects to the
ascending 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 the
appendix attachment. 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 with
serous membranes and have a
mesentery. Other more muscular parts are lined with
adventitia. ==Blood supply==