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DIGESTIVE SYSTEM The Digestive System  Peritoneum  Oral Cavity  Tongue  Teeth  Salivary glands  Pharynx  Esophagus  Stomach  Pancreas  Liver  Small Intestine  Large Intestine   -       Functions Ingestion Secretion Mixing and propulsion Digestion (mechanical and chemical) Absorpt...

Description

DIGESTIVE SYSTEM The Digestive System  Peritoneum  Oral Cavity  Tongue  Teeth  Salivary glands  Pharynx  Esophagus  Stomach  Pancreas  Liver  Small Intestine  Large Intestine

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Functions Ingestion Secretion Mixing and propulsion Digestion (mechanical and chemical) Absorption Defecation

Digestive System Gastrointestinal tract (alimentary canal): Oral Cavity - Pharynx - Esophagus - Stomach Small intestine - Large intestine. Accessory digestive organs: Teeth - Tongue - Salivary glands Liver - Gallbladder - Pancreas

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Digesting Food Bolus: soft, rounded mass food that is swallowed. Chyme: semifluid mixture of partly digested food and digestive secretions found in stomach & small intestine. Feces: material discharged from rectum, made of bacteria, excretions and food residue.

Alimentary Canal  Hollow tube with a lumen of variable diameter surrounded by a wall made of 4 layers: mucosa, submucosa, muscularis, serosa (adventitia). Mucosa Submucosa  Mucous membrane:  Made of areolar connective tissue; blood and lymph vessels. - Epithelial lining = enteroendocrine cells.  Submucosal (Meisnner’s) Nerve plexus. - Lamina propria (areolar connective tissue); MALT - Enteric Nervous System (ENS). - Muscularis mucosae (inner circular, outer longitudinal layer of smooth muscle). Muscularis Serosa  Skeletal muscle in the mouth; pharynx superior and middle  Serous membrane – covered by serous fluid. parts of esophagus – external anal sphincter.  Areolar connective tissue.  Smooth muscle in other regions.  Contains blood and lymph vessels.  Internal (circular) external (longitudinal).  Simple squamous epithelium (mesothelium).  Myenteric (Auerbach’s) nerve plexus (ENS).  NO serosa in esophagus (single layer of connective tissue – adventitia)

Parts of the Digestive System Peritoneum  Largest serous membrane: abdominopelvic cavity – parietal (covers the internal surface of thoracic cavity) and visceral (covers the lungs) layers = peritoneal cavity (lubrication, serous fluid).  Retroperitoneal organs: duodenum, pancreas, ascending and descending colon, kidneys, adrenal glands.

Oral Cavity & Tongue Oral (buccal cavity) Cheeks (buccinators); tongue; hard and soft palate (muscle, uvula). Palatoglossa and palatopharyngeal arches. Lips – labia (orbicularis oris); labial frenulum. Oral vestibule (cheeks – teeth, gingiva) and oral cavity proper (teeth, gums – fauces). Covered by mucous membrane = stratified squamous epithelium (keratinised at places of high mechanical impact.

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Tongue  2 symmetrical halves divided by median septum.  Covered by mucous membrane: lingual frenulum. Skeletal muscle:  Extrinsic: originate outside the tongue, form the floor of the oral cavity, move the tongue side to side, in and out.  Intrinsic: originate and insert into connective tissue of the tongue, changes its shape and size. Papillae  Fungiform: tip of the tongue, along the margins, taste buds.  Vallate: V shaped distribution, posterior surface, taste buds.  Foliate: lateral margins, taste buds mostly degenerate in the childhood, scarce.  Filiform: anterior 2/3 – no taste buds, receptors for touch.

Structures that make the Nasal Septum  Perpendicular plate of ethmoid bone.  Vomer bone.  Septal nasal cartilage.  Crest of the maxillary bone.  Crest of the palatine bone.

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Teeth In sockets of alveolar processes – covered by the gingiva = gum – attached by periodontal ligament. 3 external regions: Crown (above gingiva). Neck Root (within the socket). Tissue – 3 mineralised substances (high content of calcium salts): Dentin, enamel, cementum. Pulp cavity (connective tissue, nerves, blood vessels and lymphatics), root canals, apical foramen. 2 dentitions: Deciduous (milk): 20 Permanent: 32 Types Incisors, canines, premolars, molars.

Salivary Glands Exocrine glands – surrounded by capsule of connective tissue with septa dividing gland into lobes and lobules.  Connective tissue contains blood vessels and lymphocytes.  Major glands (90% of saliva) paired: - Parotid, Submandibular, Sublingual.  Minor glands - Lingual, Labial, Buccal, Molar, Palatine.  Secretory part – acini bind sacs composed of secretory cells = produces secretions which drains to intercalated ducts then striated ducts and finally excretory ducts; mucous, serous, mixed.  99.5% water, 1-1.5L per day.  Keeps mucous membrane moist, cleanse teeth and, mouth = lubricates, dissolves, begins chemical breakdown of food (salivary amylase, lingual lipase).  Salivation: parasympathetic and sympathetic control.

Pharynx

Esophagus

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13 cm long tube – connects oral (nasal) cavity and the esophagus (larynx). Wall of skeletal muscle – lined by mucous membrane = passageway for food and air: Nasopharynx Oropharynx: Double passage. Palatine and lingual tonsils Stratified squamous epithelium. Laryngopharynx Double passage Begins at level of hyoid bone = opens antroinferiorly to the larynx, posteroinferiorly to the esophagus. Simples squamous epithelium.

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Muscular tube – 25cm. Passes through the mediastinum = posterior to trachea, anterior to vertebral column – through the esophageal hiatus in the in the diaphragm ends at the stomach. Upper and lower sphincters. 4 layers Mucosa (stratified non-keratinised squamous epithelium). Submucosa Muscularis Adventitia Submucosa – has mucous glands – esophageal glands.

Deglutition 3 stages  

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Major salivary glands Parotid Anterior – inferior to the ears, between the skin and masseter; parotid duct opens into the vestibule upper M2 via parotid duct (serous only).

Stomach  J shaped: lesser and greater curvatures. *Four main regions:  Cardia: superior opening – esophagus.  Fundus: superior to the cardia.  Body: large central portion.  Pylorus: connects to the duodenum - Pyloric antrum. - Pyloric canal (sphincter).

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Voluntary – the bolus passed into the oropharynx. Pharyngeal – the involuntary passage of the bolus through the pharynx into the esophagus. Esophageal – the involuntary passage of the bolus through the esophagus into the stomach.

Submandibular Beneath the base of the tongue, submandibular ducts (open lateral to the lingual frenulum (mixed).

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Sublingual Superior to submandibular, lesser sublingual ducts open into the flood of the mouth (mostly mucous).

Typical 4 layers of alimentary canal with modifications:  Inner surface thrown into numerous longitudinal folds – rugae.  Epithelium: simple columnar epithelium cells secrete alkaline mucus (surface mucus cells); invaginates into the lamina propria forming gastric pits; gastric glands.  Muscularis: an extra oblique muscle layer (pyloric sphincter).

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Stomach Secretions from mucous neck cells, chief cells and parietal cells = called gastric juices. Secretions = mucous, HCL, intrinsic factor, gastric lipase, pepsinogen (gastric juice – 2-3 L per day), gastrin. Mechanical digestion: peristaltic movements = mixing waves; chemical digestion and some absorption (H2O, fatty acids, some drugs). Bolus –> chyme. Rich blood supply from celiac trunk and its branches R and L gastric, R and L gastroepiploic (gastro-omental) and the short and posterior gastric arteries. Veins: accompanying, into IVC (inferior vena cava).

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Fundus = storage. Peristaltic wave = moves gastric contents from body of stomach to pyloric antrum (propulsion). Pyloric sphincter slightly open; most food particles too big to pass through pyloric sphincter (forced back into stomach = retropulsion). Continues until all food particles are small enough to pass through and enter small intestine (duodenum). Innervation: vagus – parasympathetic fibres; sympathetic nerve from the celiac ganglia.

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Gastric Gland Cells  Mucous neck cells secrete soluble mucous. Chief cells (zygomatic cells) Parietal cells (oxyntic cells) Secrete hormones e.g. serotonin,  HCL and helps convert pepsinogen to gastrin (from G-cells), somatostatin. pepsin; intrinsic factor for absorption vitamin B12.

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Enteroendocrine cells Secrete hormones e.g. serotonin, gastrin (from G-cells), somatostatin.

Pancreas & Liver Pancreas Compound acinar gland. Secretory portion = pancreatic acinar cells. Make a wide range of digestive enzymes: proteases, lipases (lipids), amylase (starch), phospholipase A2, nucleases.  Secretion is alkaline  neutralises acidic stomach contents. HORMONES  Secretin, cholecystokinin.  Released by enteroendocrine cells of the mucosa in response to acidic chyme into the duodenum = control the release of the pancreatic enzymes. BLOOD SUPPLY  Splenic artery  Pancreaticoduodenal arteries  Superior mesenteric arteries  Accompanying veins. INNERVATION  Autonomic nerves – branches from the celiac and superior mesenteric plexuses.   

Liver

Liver 1.5 Kg; inferior to diaphragm, R hypochondriac, epigastric, extends into L hypochondriac region = covered by peritoneum; capsule of dense irregular connective tissue.  Diaphragmatic (convex) and visceral surface (concave).  Lobes – Right, left, quadrate (inferior), caudate (posterior).  Hilum - (porta hepatis) on the visceral surface: vessels, hepatic, nerve plexus, lymphatics and hepatic ducts. FUNCTIONS  Metabolism (carbohydrates, lipids, proteins).  Detoxification.  Bile (yellow-brown or green fluid: water, salts, cholesterol, lecithin, pigments, ions – emulsifies fat) production.  Storage.  Phagocytosis.  Vitamin D activation. DUAL BLOOD SUPPLY  Hepatic portal vein – deoxygenated, nutrients and other materials from the Gastrointestinal Tract.  Hepatic artery – oxygenated blood. INNVERVATION  The vagus – parasympathetic and sympathetic from greater splanchnic nerve – through the celiac ganglia. 

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The liver (hepatic) lobule Polygonal mass of tissue with portal spaces at the periphery and vein (central) in the centre. Hepatocytes Cuboidal/ polyhedral shaped, large cells (1 or 2 large central nuclei) Lifespan 5 month. Form liver plates (laminae). Hepatic sinusoids Lined with a tine discontinuous epithelium (deliver blood to central vein). Kupffer cells (stellate sinusoidal macrophage) = detect and engulf bacteria and breakdown aged erythrocytes. Portal space contains portal triads and lymph vessels. Blood blows from portal triad through sinusoidal channels into a central vein  sublobar veins  inferior vena cava. Bile Systems of ducts from the liver and gallbladder to the duodenum. Contains water, electrolytes, bile acids, cholesterol, phospholipids, and bilirubin (breakdown product of haemoglobin). Bile acids = essential for digestion and absorption of fats. Waste products = eliminated in the bile (esp. cholesterol and converted to bile acids. 90% of bile acids = delivered into duodenum and reabsorbed in the liver then re-used.

Small Intestine Primary site of absorption = extends from pylorus to the ileocecal junction (central and inferior parts of the abdominal cavity).  Parts - Duodenum: C shaped (25cm), extraperitoneal. - Jejunum. - Ileum: ileocecal, sphincter. 

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Bile canaliculi and ductules Located between hepatocytes. Drain into bile ductules (bile ducts, R and L hepatic duct, cystic duct, common bile duct, duodenum. Ductules merge with bile ducts in portal spaces. Bile: made in hepatocytes, collects in bile canaliculi, used or stored in gall bladder = exocrine product of liver. Flows in opposite direction to blood.

Gallbladder Visceral surface of liver (pear shaped). Parts: fundus, body, neck. Function: stores and concentrates bile. Blood supply: cystic artery (from Right hepatic artery). Innervation: branches from the celiac plexus and vagus nerve.

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Blood Supply Duodenum. Jejunum and ileum: jejunal and ileal arteries. Venous drainage: through the superior mesenteric vein. Innervation: the superior mesenteric plexus. 4 layered wall Folds – increase of absorptive surface: Plicae circulares = folds of mucosa and submucosa. Villi = finger-like projections of the mucosa; projections of mucosa; capillaries, lacteals (lymphatic capillaries). Microvilli = projections of the plasma membrane.

Mucosa Submucosa Numerous crevices lined by glandular epithelium = intestinal  Duodenal (Brunner’s glands) – produce alkaline mucous to glands, cryptics of Lieberkuhn. neutralise acidic chyme entering the small intestine from the stomach. Cells: Absorptive: most common cell, tall columnar, brush border.  MALT – prominent in the small intestine; lymphocytes loose Main function-> release enzymes to digest cells. in the lamina propria and between epithelial cells. - large aggregations of lymphocytes = located in mucosa and Goblet: less abundant in duodenum. function-> secrete submucosa = especially in ileum. mucous. Enteroendocrine: function-> secrete hormones. (Peyers patches: 10-200 lymph nodules)  creates an immune barrier. M cells (microfold cells): modified absorptive, located over enlarged lymph nodes, carry antigens to MALT. Paneth: secrete anti-microbial lysozymes. Specialisations of Small intestine Duodenum Jejunum Ileum Villi are often leaf or ridge shaped  The main absorptive site; most  Greatest development of gut-

- Brunner’s glands.

complex finger like villi.

associated lymphoid tissue =Peyer’s patches; may have leaf like villi.

Large Intestines Description 1.5M long; large internal diameter (6.5cm). Functions: - Absorption of water and inorganic ions. - Formation of the fecal mass.  Portions of longitudinal muscles thickened to form 3 bands: teniae coli.  Haustra: pouches of colon.  Omental appendices: pouches of visceral peritoneum (filled with fat).  Surface of mucosa is relatively smooth (simple columnar epithelium) = no plicae circulares or intestinal villi.  Feces - Water, inorganic salts, epithelial cells, bacteria, unabsorbed digested materials, indigestible food.  Absorptive cells: columnar with short microvilli.  Many goblet cells (account for more of epithelial cells than in the small intestine.  Crypts of lieberkuhn present + longer and straighter than those of small intestine.  Presence of bacteria.  

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Blood supply - Caecum and colon: branches of the superior and inferior mesenteric arteries. - Rectum and anal canal: branches from superior, middle and inferior rectal arteries  Venous drainage – corresponding veins

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Parts Caecum inferior to ileocecal valve. (vermiform) appendix. Colon: Ascending (retrop): right side, right colic (hepatic) flexure. Transverse: left colic (splenic) flexure. Descending (retrol). Sigmoid: S-shaped. Rectum: the last 20cm anterior to sacrum. Anal canal: the last 2-3 cm= anal columns; ends in opening. Closed by internal and sphincter (smooth muscle), external anal sphincter.

Innervation - Sympathetic = from the celiac, superior and inferior mesenteric ganglia and inferior mesenteric plexuses. - Parasympathetic = the vagus and pelvic splanchnic nerves.

Peritoneum & Appendix      

Peritoneum Folds (contain fat, blood vessels, lymphatics). Greater omentum: attached to stomach and duodenum – transverse colon, covers small intestine. Lesser omentum: suspends the stomach and duodenum from the liver. Falciform ligament: the liver to anterior abdominal wall and diaphragm. Mesentery: the post abdominal to jejunum and ileum. Mesocolon: post abdominal wall to the transverse and sigmoid colon.

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Appendix Evagination of cecum – small narrow, irregular lumen; abundant lymph follicles. Fewer and shorter intestinal glands and NO teniae coli. Appendicitis...