The Digestive System Anatomy Notes PDF

The Digestive System Alimentary canal (gastrointestinal tract) • mouth • pharynx • esophagus • stomach • small intestine • large intestine • rectum Accessory organs • teeth • tongue • salivary glands • pancreas • liver • gallbladder Functions: 1) Ingestion – moving food into the oral cavity 2) Pr...

The Digestive System Alimentary canal (gastrointestinal tract) • mouth • pharynx • esophagus • stomach • small intestine • large intestine • rectum Accessory organs • teeth • tongue • salivary glands • pancreas • liver • gallbladder Functions: 1) Ingestion – moving food into the oral cavity 2) Propulsion (motility) – swallowing, peristalsis, segmentation 3) Digestion – conversion of large molecules into smaller, absorbable subunits i) mechanical – chewing, churning ii) chemical secretions – enzymes, acid, bile salts 4) Absorption – intracellular passage of digestion products from the gut lumen into the blood and lymph 5) Defecation – elimination of undigested material and wastes lymph blood Need to know: Abdominopelvic quadrants Right upper quadrant Left upper quadrant Right lower quadrant Left lower quadrant Epigastric region Right hypochondriac region Abdominopelvic regions Right lumbar region Right iliac (inguinal) region Hypogastric (public) region Left hypochondriac region Umbilical region Left lumbar region Left iliac (inguinal) region Histology Four tissue layers: 1. Mucosa – deep (innermost) layer a) epithelium with many goblet cells  stratified squamous: mouth, pharynx, esophagus, anal canal stomach, small and large intestine, rectum  simple columnar: b) lamina propria (areolar CT)  houses blood and lymph vessels, nerve fibers, and lymphoid tissue c) muscularis mucosae  smooth muscle layer – produces folds and local movements of the mucosa Mucosa epithelium lamina propria muscularis mucosae Histology 2. Submucosa – areolar CT layer containing blood and lymphatic vessels and a network of nerve cells (submucosal plexus) 3. Muscularis externa – two layers of smooth muscle separated by a 2nd network of nerve cells (myenteric plexus) a) thick inner circular layer  contraction constricts the lumen Submucosa b) thin outer longitudinal layer  contraction shortens gut length Submucosal + myenteric plexus = enteric nervous system  regulates gut secretions and mobility (propulsion and mixing) circular layer myenteric plexus longitudinal layer Muscularis externa Histology 4. Serosa (or adventitia*) – superficial layer parietal peritoneum  simple squamous epithelium and areolar CT Peritoneum Largest serous membrane of the body peritoneal cavity serosa (=visceral peritoneum)  thin continuous membrane that lines the abdominopelvic cavity and most of the organs a) visceral peritoneum: lines the external surface of digestive organs (=serosa) b) parietal peritoneum: lines the abdominal peritoneal cavity wall visceral peritoneum cavity Forms a serous-fluid containing peritoneal cavity that lubricates mobile organs parietal peritoneum Peritoneum single layer thick single layer thick mesentery Mesentery – a fused, double-layer sheet of (double layer) parietal peritoneum that binds organs to the abdominal cavity wall and helps hold them in place  also a route for blood/lymph vessels and nerves to enter and exit the digestive organs Omenta are mesentery formed of visceral peritoneum a) greater omentum – ‘fatty apron’  descends from the greater curvature of the stomach and drapes over the small intestine and transverse colon mesentery b) lesser omentum  extends between the liver and the lesser curvature of the stomach lesser omentum greater omentum Peritoneum Some abdominal organs are retroperitoneal, in that they lie behind the peritoneum, i.e., the peritoneum covers only their anterior side E.g., pancreas, duodenum  anterior serosa – parietal peritoneum  posterior serosa – sheath of fibrous CT with no epithelium (=adventitia) Peritonitis: inflammation of the peritoneum  e.g., following rupture of appendix pancreas duodenum Anatomy 1. Oral cavity (mouth)  lined by mucus membrane (stratified squamous epithelium and lamina propria); cheeks have underlying skeletal muscle a) lips hard palate b) cheeks uvula soft palate hard palate c) palate i) hard palate (anterior)  formed by maxilla + palatine bones ii) soft palate (posterior)  arch of skeletal muscle that rises to close the nasopharynx when swallowing  uvula = hanging posterior projection soft palate uvula 1. Oral cavity d) tongue – skeletal muscle  attached to hyoid bone  dorsal surface covered with: i) nodular lingual tonsil (posterior) papillae ii) papillae – projections of mucosa  contain taste buds (nervous tissue) taste bud iii) lingual glands (anterior)  secrete lingual lipase 1. Oral cavity e) teeth – in maxillae and mandible Children: primary dentition – deciduous (milk) teeth Adults: secondary dentition – permanent teeth Classification Child Adult central incisor 4 4 lateral incisor 4 4 canine 4 4 premolars 0 8 molars 8 12 total 20 32 e) Teeth enamel Structure i) crown – above gum  dentin (internal) – dentin crown calcified CT acellular Ca2+ neck phosphate salts ii) neck (crown/root boundary)  enamel (cover) – iii) root – dentin with overlay of cementum (calcified CT) root iv) periodontal ligaments – attach the tooth root to the jaw bones v) pulp cavity – contains CT, blood and lymph vessels, nerves vi) root canal – internal extension of the pulp cavity pulp cavity cementum periodontal ligaments root canal Anatomy 2. Paired salivary glands a) parotid – inferior and anterior to the ears  secrete a  mumps thin saliva rich in amylase – inflammation of the parotid gland b) submandibular – on the floor of mouth  secrete saliva of mucus and amylase c) sublingual – below the tongue  saliva mostly mucus Saliva (99.5% water)  0.5% solutes (enzymes, ions) sublingual parotid submandibular Anatomy 3. Pharynx (oropharynx + laryngopharynx) Similar histology to the oral cavity  mucus membrane connected to skeletal muscle 4. Esophagus – posterior to trachea  passes through the diaphragm pharynx Histology similar to the gut, except: esophagus a) muscularis externa upper ⅓ middle ⅓ lower ⅓ skeletal muscle trachea smooth muscle b) no serosa above diaphragm; fibrous areolar CT (adventitia) here Anatomy 5. Stomach Stores, partially digests and, regulates emptying of chyme (food + gastric juices) into the small intestine Regions: a) cardial (cardiac) – site where esophagus enters the stomach b) fundus – superior to the esophageal entrance esophagus c) body – midportion d) pyloric – inferior; funnel shaped  has pyloric sphincter pyloric sphincter Curvatures of the lateral and medial surface connect to the greater and lesser omenta, respectively cardial fundus body lesser curvature greater curvature pyloric region 5. Stomach Unique histology: a) the muscularis externa has an additional layer i) inner oblique ii) middle circular rugae iii) outer longitudinal longitudinal When empty, the mucosa forms many folds (rugae) stomach expansion without tearing the mucosa circular  allows oblique 5. Stomach b) the mucosa possesses millions of invaginations (gastric pits), that form tubular (exocrine) gastric glands that secrete gastric juices into the stomach lumen i) chief cells  secrete pepsinogen and gastric lipase ii) parietal cells  secrete HCl and intrinsic factor iii) mucus neck (goblet) cells  secrete acidic mucus (stomach pH = ~2) Additionally, enteroendocrine (G) cells located located deep within the gastric pits secrete the hormone gastrin into the blood Anatomy 6. Small intestine From the pyloric sphincter to the ileocaecal valve a) duodenum – short (~25 cm)  curves stomach duodenum around the pancreas; retroperitoneal  possesses Brunner’s glands that secrete an alkaline mucous (neutralizes acidic chyme, thereby increasing its pH)  ducts of accessory organs (liver, gallbladder, pancreas) enter the duodenum here 6. Small intestine b) jejunum duodenum ~2.5 m long (6 m in cadaver) c) ileum jejunum  b) and c) contain many lymphatic nodules (Peyer’s patches; protect against bacteria and infections) ileum 6. Small intestine Specializations to increase absorptive surface area: i) submucosa thrown into many circular folds  plicae circulares ii) villi – finger-like projections of mucosa containing:  blood capillaries – provide O2 to, and absorb nutrients/secretions from, the villus epithelium  lacteals (lymph capillary) – absorb fats; drain into the cisterna chyli iii) microvilli – ‘brush border’  on absorptive intestinal cells (enterocytes) Villi Villus epithelium: a) enterocytes (with many microvilli) b) goblet cells – secrete mucus c) enteroendocrine cells  located within the intestinal glands (crypts of Lieberkühn)  three different types: secrete hormones that inhibit gastric secretions and stomach emptying; e.g., i) secretin – when chyme pH too low ii) cholecystokinin (CCK) – when chyme [fat] too high 6. Small intestine Accessory organs: A) pancreas – retroperitoneal  divided into head, body, and tail body i) exocrine glands – 99% head a) clusters of acinar cells that secrete digestive enzymes destined for the duodenum ‘pancreatic juice’ b) duct cells that secrete an alkaline fluid into this mix acinar cells duct cells ii) endocrine glands – Islets of Langerhans  secretions from β tail cells (insulin) and α cells (glucagon) into the blood regulate blood sugar levels Accessory organs B) liver – composed of hepatocyte cells Divided into four lobes: right, left, quadrate, anterior view posterior view caudate caudate left right right quadrate The liver has many metabolic and digestive roles; e.g., i) secretes bile (aids fat digestion) into tiny canals (bile canaliculi) that drain into the common hepatic duct ii) filters and processes material absorbed from the GI tract (e.g., nutrients, toxins) before they are distributed to the rest of body Accessory organs C) gallbladder – muscular sac on the inferior surface of the liver  simple columnar epithelium; has many folds (rugae) when empty  no submucosal layer  concentrates and stores bile between meals 6. Small intestine Accessory organ secretions enter the duodenum via a series of ducts: pancreas gall bladder liver cystic duct common hepatic duct pancreatic duct common bile duct hepatopancreatic ampulla accessory pancreatic duct duodenum Anatomy 7. Large intestine From the ileocaecal valve to the anus – has no folds or villi  simple columnar epithelium with many goblet cells a) caecum and appendix – inferior to the ileocaecal valve b) colon i) ascending (right side) hepatic flexure transverse colon splenic flexure ii) hepatic flexure iii) transverse descending colon ascending colon iv) splenic flexure v) descending (left side) vi) sigmoid appendix caecum sigmoid colon 7. Large intestine Unique features of the colon: 1) longitudinal layer of muscularis is reduced to three bands  teniae coli 2) haustra – pouch-forming folds arising from contraction of the teniae epiploic appendages 3) epiploic appendages  fat-filled sacs of visceral peritoneum haustra teniae coli 7. Large intestine c) rectum – no teniae coli  anal canal (inferior 3 cm of rectum) d) anus – stratified squamous epithelium  possesses internal (smooth) and external (skeletal) sphincter muscles rectum external sphincter anal canal Anatomy superior mesenteric 8. Blood supply abdominal aorta inferior mesenteric artery inferior mesenteric superior mesenteric artery large intestine small and large intestine inferior mesenteric veins superior mesenteric hepatic veins portal splenic vein system hepatic portal vein splenic superior mesenteric inferior mesenteric liver hepatic veins inferior vena cava Portal system: blood vessel with capillary beds on both ends