ANATOMY AND PHYSIOLOGYThe gastrointestinal tract (GIT) consists of ahollow muscular tube starting from the oralcavity, where food enters the mouth, continuingthrough the pharynx, oesophagus, stomach andintestines to the rectum and anus, where food isexpelled. There are various accessory organs thatassist the tract by secreting enzymes to helpbreak down food into its component nutrients.Thus the salivary glands, liver, pancreas and gallbladder have important functions in the digestivesystem. Food is propelled along the length of theGIT by peristaltic movements of the muscularwalls.
ANATOMY AND PHYSIOLOGYThe primary purpose of the gastrointestinal tract is to break food down into nutrients, which canbe absorbed into the body to provide energy. First food must be ingested into the mouth to bemechanically processed and moistened. Secondly,digestion occurs mainly in the stomach and small intestine where proteins, fats and carbohydrates are chemically broken down into their basic building blocks. Smaller molecules are then absorbed across the epithelium of the small intestine and subsequently enter the circulation.The large intestine plays a key role in reabsorbing excess water. Finally, undigested material and secreted waste products are excreted from the body via defecation (passing of feces).
ANATOMY AND PHYSIOLOGY In the case of gastrointestinal disease ordisorders, these functions of the gastrointestinaltract are not achieved successfully. Patients maydevelop symptoms of nausea, vomiting, diarrhea, malabsorption, constipation or obstruction.Gastrointestinal problems are very common and most people will have experienced some of the above symptoms several times throughout their lives.
ANATOMY AND PHYSIOLOGY Four Layers of the WallMucosaThe innermost layer of the digestive tract has specialized epithelial cellssupported by an underlying connective tissue layer called the laminapropria. The lamina propria contains blood vessels, nerves, lymphoidtissue and glands that support the mucosa. Depending on its function,the epithelium may be simple (a single layer) or stratified (multiplelayers).Areas such as the mouth and esophagus are covered by a stratifiedsquamous (flat) epithelium so they can survive the wear and tear ofpassing food. Simple columnar (tall) or glandular epithelium lines thestomach and intestines to aid secretion and absorption. The inner liningis constantly shed and replaced, making it one of the most rapidlydividing areas of the body! Beneath the lamina propria is the muscularismucosa. This comprises layers of smooth muscle which can contract tochange the shape of the lumen.
ANATOMY AND PHYSIOLOGY Four Layers of the WallSubmucosaThe submucosa surrounds the muscularismucosa and consists of fat, fibrous connectivetissue and larger vessels and nerves. At itsouter margin there is a specialized nerveplexus called the submucosal plexus orMeissner plexus. This supplies the mucosa andsubmucosa.
ANATOMY AND PHYSIOLOGY Four Layers of the WallMuscularis externaThis smooth muscle layer has inner circularand outer longitudinal layers of muscle fibersseparated by the myenteric plexus or Auerbachplexus. Neural innervations control thecontraction of these muscles and hence themechanical breakdown and peristalsis of thefood within the lumen.
ANATOMY AND PHYSIOLOGY Four Layers of the WallSerosa/ MesenteryThe outer layer of the GIT is formedby fat and another layer of epithelialcells called mesothelium.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemOral cavityThe oral cavity or mouth is responsible for the intake of food. It is linedby a stratified squamous oral mucosa with keratin covering those areassubject to significant abrasion, such as the tongue, hard palate and roofof the mouth. Mastication refers to the mechanical breakdown of foodby chewing and chopping actions of the teeth. The tongue, a strongmuscular organ, manipulates the food bolus to come in contact with theteeth. It is also the sensing organ of the mouth for touch, temperatureand taste using its specialized sensors known as papillae.Insalivation refers to the mixing of the oral cavity contents with salivarygland secretions. The mucin (a glycoprotein) in saliva acts as alubricant. The oral cavity also plays a limited role in the digestion ofcarbohydrates. The enzyme serum amylase, a component of saliva,starts the process of digestion of complex carbohydrates. The finalfunction of the oral cavity is absorption of small molecules such asglucose and water, across the mucosa. From the mouth, food passesthrough the pharynx and esophagus via the action of swallowing.
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal SystemSalivary glandsThree pairs of salivary glands communicate with the oralcavity. Each is a complex gland with numerous acinilined by secretory epithelium. The acini secrete theircontents into specialized ducts. Each gland is dividedinto smaller segments called lobes. Salivation occurs inresponse to the taste, smell or even appearance of food.This occurs due to nerve signals that tell the salivaryglands to secrete saliva to prepare and moisten themouth. Each pair of salivary glands secretes saliva withslightly different compositions.
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal System
ANATOMY AND PHYSIOLOGY Salivary glandsParotidsThe parotid glands are large, irregular shaped glandslocated under the skin on the side of the face. Theysecrete 25% of saliva. They are situated below thezygomatic arch (cheekbone) and cover part of themandible (lower jaw bone). An enlarged parotid glandcan be easier felt when one clenches their teeth. Theparotids produce a watery secretion which is also rich inproteins. Immunoglobins are secreted help to fightmicroorganisms and a-amylase proteins start to breakdown complex carbohydrates.
ANATOMY AND PHYSIOLOGY Salivary glandsSubmandibularThe submandibular glands secrete 70% of the saliva inthe mouth. They are found in the floor of the mouth, in agroove along the inner surface of the mandible. Theseglands produce a more viscid (thick) secretion, rich inmucin and with a smaller amount of protein. Mucin is aglycoprotein that acts as a lubricant.
ANATOMY AND PHYSIOLOGY Salivary glandsSublingualThe sublinguals are the smallest salivary glands, coveredby a thin layer of tissue at the floor of the mouth. Theyproduce approximately 5% of the saliva and theirsecretions are very sticky due to the large concentrationof mucin. The main functions are to provide buffers andlubrication.
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal SystemEsophagusThe esophagus is a muscular tube of approximately25cm in length and 2cm in diameter. It extends from thepharynx to the stomach after passing through an openingin the diaphragm. The wall of the esophagus is made upof inner circular and outer longitudinal layers of musclethat are supplied by the esophageal nerve plexus. Thisnerve plexus surrounds the lower portion of theesophagus. The esophagus functions primarily as atransport medium between compartments.
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal SystemStomachThe stomach is a J shaped expanded bag, located justleft of the midline between the esophagus and smallintestine. It is divided into four main regions and has twoborders called the greater and lesser curvatures. The firstsection is the cardia which surrounds the cardial orificewhere the esophagus enters the stomach. The fundus isthe superior, dilated portion of the stomach that hascontact with the left dome of the diaphragm. The body isthe largest section between the fundus and the curvedportion of the J.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemThis is where most gastric glands are located and where mostmixing of the food occurs. Finally the pylorus is the curved baseof the stomach. Gastric contents are expelled into the proximalduodenum via the pyloric sphincter. The inner surface of thestomach is contracted into numerous longitudinal folds calledrugae. These allow the stomach to stretch and expand whenfood enters. The stomach can hold up to 1.5 Liters of materials.The functions of the stomach include:The short-term storage of ingested food.Mechanical breakdown of food by churning and mixing motions.Chemical digestion of proteins by acids and enzymes.Stomach acid kills bugs and germs.Some absorption of substances such as alcohol.Most of these functions are achieved by the secretion ofstomach juices by gastric glands in the body and fundus. Somecells are responsible for secreting acid and others secreteenzymes to break down proteins.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemSmall IntestineThe small intestine is composed of the duodenum, jejunum,and ileum. It averages approximately 6m in length, extendingfrom the pyloric sphincter of the stomach to the ileo-caecalvalve separating the ileum from the cecum. The small intestineis compressed into numerous folds and occupies a largeproportion of the abdominal cavity.The duodenum is the proximal C-shaped section that curvesaround the head of the pancreas. The duodenum serves amixing function as it combines digestive secretions from thepancreas and liver with the contents expelled from thestomach. The start of the jejunum is marked by a sharp bend,the duodenojejunal flexure. It is in the jejunum where themajority of digestion and absorption occurs. The final portion,the ileum, is the longest segment and empties into the cecumat the ileocaecal junction.
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal System
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemThe small intestine performs the majority of digestion andabsorption of nutrients. Partly digested food from the stomachis further broken down by enzymes from the pancreas and bilesalts from the liver and gallbladder. These secretions enter theduodenum at the Ampulla of Vater. After further digestion, foodconstituents such as proteins, fats, and carbohydrates arebroken down to small building blocks and absorbed into thebodys blood stream.The lining of the small intestine is made up of numerouspermanent folds called plicae circulares. Each plica hasnumerous villi (folds of mucosa) and each villus is covered byepithelium with projecting microvilli (brush border). Thisincreases the surface area for absorption by a factor of severalhundred. The mucosa of the small intestine contains severalspecialized cells. Some are responsible for absorption, whilstothers secrete digestive enzymes and mucous to protect theintestinal lining from digestive actions.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemLarge IntestineThe large intestine is horse-shoe shaped and extends aroundthe small intestine like a frame. It consists of the appendix,cecum, ascending, transverse, descending and sigmoid colon,and the rectum. It has a length of approximately 1.5m and awidth of 7.5cm.The cecum is the expanded pouch that receives material fromthe ileum and starts to compress food products into fecalmaterial. Food then travels along the colon. The wall of thecolon is made up of several pouches (haustra) that are heldunder tension by three thick bands of muscle (taenia coli).The rectum is the final 15cm of the large intestine. It expands tohold faecal matter before it passes through the anorectal canalto the anus. Thick bands of muscle, known as sphincters,control the passage of feces.
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal System
ANATOMY AND PHYSIOLOGYIndividual Components of the Gastrointestinal SystemThe mucosa of the large intestine lacks villi seen in the smallintestine. The mucosal surface is flat with several deepintestinal glands. Numerous goblet cells line the glands thatsecrete mucous to lubricate faecal matter as it solidifies.The functions of the large intestine can be summarized as:The accumulation of unabsorbed material to form feces.Some digestion by bacteria. The bacteria are responsible forthe formation of intestinal gas.Reabsorption of water, salts, sugar and vitamins.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemLiverThe liver is a large, reddish-brown organ situated in the rightupper quadrant of the abdomen. It is surrounded by a strongcapsule and divided into four lobes namely the right, left,caudate and quadrate lobes. The liver has several importantfunctions. It acts as a mechanical filter by filtering blood thattravels from the intestinal system. It detoxifies severalmetabolites including the breakdown of bilirubin and estrogen.In addition, the liver has synthetic functions, producing albuminand blood clotting factors. However, its main roles in digestionare in the production of bile and metabolism of nutrients. Allnutrients absorbed by the intestines pass through the liver andare processed before traveling to the rest of the body. The bileproduced by cells of the liver, enters the intestines at theduodenum. Here, bile salts break down lipids into smallerparticles so there is a greater surface area for digestiveenzymes to act.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemGall bladderThe gallbladder is a hollow, pear shaped organ that sits in adepression on the posterior surface of the livers right lobe. Itconsists of a fundus, body and neck. It empties via the cysticduct into the biliary duct system. The main functions of the gallbladder are storage and concentration of bile. Bile is a thickfluid that contains enzymes to help dissolve fat in the intestines.Bile is produced by the liver but stored in the gallbladder until itis needed. Bile is released from the gall bladder by contractionof its muscular walls in response to hormone signals from theduodenum in the presence of food.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemPancreasFinally, the pancreas is a lobular, pinkish-grey organ that liesbehind the stomach. Its head communicates with theduodenum and its tail extends to the spleen. The organ isapproximately 15cm in length with a long, slender bodyconnecting the head and tail segments. The pancreas has bothexocrine and endocrine functions. Endocrine refers toproduction of hormones which occurs in the Islets ofLangerhans. The Islets produce insulin, glucagon and othersubstances and these are the areas damaged in diabetesmellitus. The exocrine (secretrory) portion makes up 80-85% ofthe pancreas and is the area relevant to the gastrointestinaltract.
ANATOMY AND PHYSIOLOGY Individual Components of the Gastrointestinal SystemIt is made up of numerous acini (small glands) that secretecontents into ducts which eventually lead to the duodenum. Thepancreas secretes fluid rich in carbohydrates and inactiveenzymes. Secretion is triggered by the hormones released bythe duodenum in the presence of food. Pancreatic enzymesinclude carbohydrases, lipases, nucleases and proteolyticenzymes that can break down different components of food.These are secreted in an inactive form to prevent digestion ofthe pancreas itself. The enzymes become active once theyreach the duodenum.