Functional Anatomy and Histology of the Gastrointestinal Tract

The Functional Anatomy and Histology of the Gastrointestinal Tract Including the Liver and the Pancreas MBS 232 Dr KB Makhathini 2020  Digestive System Provides fuel for body cells’ functioning Two gro...

The Functional Anatomy and Histology of the Gastrointestinal Tract Including the Liver and the Pancreas MBS 232 Dr KB Makhathini 2020  Digestive System Provides fuel for body cells’ functioning Two groups of organs 1. Alimentary canal (gastrointestinal or GI tract) Digests and absorbs food Mouth, pharynx, esophagus, stomach, small intestine, and large intestine 2. Accessory digestive organs Teeth, tongue, gallbladder Digestive glands – Salivary glands – Liver – pancreas 2  Parotid gland Mouth (oral cavity) Sublingual gland Salivary Tongue Submandibular glands gland Esophagus Pharynx Stomach Pancreas Liver (Spleen) Gallbladder Transverse colon Duodenum Descending colon Small Jejunum Ascending colon intestine Ileum Cecum Large Sigmoid colon intestine Rectum Vermiform appendix Anus Anal canal 3 Six Functions of the Digestive System 1. Ingestion – Occurs when food and drink enter mouth 2. Mechanical processing – Crushing of solid food to make it easier to move along the digestive tract – Increases surface area for enzymes to work – Process begins in oral cavity with teeth and tongue 3. Chemical processing – Chemical breakdown of food to absorbable size 4  Six Functions of the Digestive System 4. ​Secretion – Release of water, acids, enzymes, and buffers into lumen of digestive tract 5. ​Absorption – Movement of small organic molecules, electrolytes, and water across digestive epithelium and into interstitial fluid of digestive tract 6. ​Excretion (defecation) – Elimination of waste products from digestive tract – Products are ejected as feces in process called defecation 5 Ingestion Food Mechanical digestion Pharynx Chewing (mouth) Esophagus Churning (stomach) Propulsion Segmentation Swallowing (small intestine) (oropharynx) Chemical Peristalsis digestion Stomach (esophagus, stomach, small intestine, large intestine) Absorption Lymph vessel Small intestine Large Blood intestine vessel Mainly H2O Feces Defecation Anus 6  Digestive Tract Lining Plays defensive role for the body Protects surrounding tissues from: – Corrosive effects of digestive acids and enzymes – Physical abrasion – Bacteria that are ingested or live in digestive tract 7 Four Histological Layers of the GI Tract 1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa 8  Mucosa Mucous membrane that forms the inner lining of the digestive tract Consists of: – Mucosal epithelium – Underlying layer of areolar tissue called the lamina propria – Muscle layer called the muscularis mucosae Smooth muscle that helps move mucosa 9  Mucosal Epithelia Stratified squamous in high physical stress organs – Oral cavity, pharynx, esophagus, anus Rest is simple columnar with surface modifications Ducts of secretory glands open to surface of epithelium Circular folds and villi increase surface area for absorption 10  Submucosa Layer of dense irregular connective tissue Binds mucosa to muscularis externa Contains blood vessels and lymphatics Outer margin contains: – Parasympathetic neurons and sensory neurons – Submucosal plexus Neural network that can function without CNS Regulates secretion and motility 11  Muscularis Externa Band of smooth muscle arranged in: – Inner circular and outer longitudinal layer Function to mix and propel materials Myenteric plexus between layers of muscle – Contains parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers – Parasympathetic stimulation increases activity – Sympathetic stimulation decreases activity 12  Serosa and Adventitia Serosa – Serous membrane covering muscularis externa along GI tract enclosed by peritoneum – Also called the visceral peritoneum – Continuous with the parietal peritoneum, which lines inner surfaces of body wall Adventitia – Layer covering muscularis externa of regions where there is no serosa Examples: oral cavity, pharynx, esophagus, and rectum – Attaches GI tract to adjacent structures 13  Mesenteries Double sheets of serous membrane (parietal and visceral peritoneum) Suspend portions of digestive tract Provide pathways for blood vessels, lymphatics, and nerves Help organize and stabilize attached organs 14 Structure of the Digestive Tract 15 16  The Oral Cavity Also called the buccal cavity Part of digestive tract that receives food Lined by oral mucosa – Stratified squamous epithelium Contains tongue, teeth, and gingivae, or gums – Gingivae are ridges of oral mucosae surrounding base of teeth 17  Functions of the Oral Cavity Senses food before swallowing Mechanically processes food Lubricates food with saliva and mucus Begins enzymatic digestion of carbohydrates and lipids 18  Oral Cavity Boundaries Lateral walls formed by cheeks – Supported by pads of fat and buccinator muscle Anterior boundary – Labia, or lips, are continuous with cheek – Vestibule is space between cheeks (or lips) and teeth Roof formed by hard palate and soft palate Floor dominated by tongue – Free edge of tongue is attached to floor with lingual frenulum Imaginary line dividing oral cavity from oropharynx – Extends between base of tongue and uvula 19 The Oral Cavity 20  The Tongue Manipulates food within oral cavity – Mechanically compresses, abrades, distorts material – Assists in chewing and preparing food for swallowing – Provides sensory analysis of touch, temperature, and taste Lingual tonsils – Paired lymphoid nodules at base of tongue – Help resist infection 21  Tongue Functions include – Repositioning and mixing food during chewing – Formation of the bolus – Initiation of swallowing, speech, and taste Intrinsic muscles change the shape of the tongue Extrinsic muscles alter the tongue’s position Lingual frenulum: attachment to the floor of the mouth 22  Tongue Surface bears papillae 1. Filiform—whitish, give the tongue roughness and provide friction 2. Fungiform—reddish, scattered over the tongue 3. Circumvallate (vallate)—V-shaped row in back of tongue These three house taste buds 4. Foliate—on the lateral aspects of the posterior tongue Terminal sulcus marks the division between – Body: anterior 2/3 residing in the oral cavity – Root: posterior third residing in the oropharynx 23  Palate Hard palate: palatine bones and palatine processes of the maxillae – Slightly corrugated to help create friction against the tongue Soft palate: fold formed mostly of skeletal muscle – Closes off the nasopharynx during swallowing – Uvula projects downward from its free edge 24 Epiglottis Palatopharyngeal arch Palatine tonsil Lingual tonsil Palatoglossal arch Terminal sulcus Foliate papillae Circumvallate papilla Midline groove of tongue Dorsum of tongue Fungiform papilla Filiform papilla 25  The Salivary Glands Three pairs of glands that secrete into oral cavity 1. Parotid salivary glands On each side of oral cavity between mandible and skin Parotid duct empties into vestibule at level of second upper molar 2. Sublingual salivary glands Under mucous membrane on floor of mouth Numerous sublingual ducts open on either side of lingual frenulum 3. Submandibular salivary glands In floor of the mouth along inner surfaces of mandible Ducts open into mouth behind teeth on either side of lingual frenulum 26 The Salivary Glands 27  Saliva Amount and Contents About 1.0–1.5 liters produced each day – During eating, production increases to about 7 mL per minute – Regulated by autonomic nervous system Composition – 99.4 percent water – Mucins, ions, buffers, waste products, metabolites, and enzymes Mucins absorb water and form mucus 28  Functions of Saliva 1. Water lubricates mouth and dissolves chemicals 2. Mucus reduces friction and makes swallowing easier 3. Buffers keep pH near 7.0 and prevent buildup of acids produced by bacteria 4. Salivary antibodies (IgA) and lysozyme help control bacterial levels 5. Salivary amylase (an enzyme) begins chemical digestion of starches (complex carbohydrates) – Produced primarily by parotid salivary gland 29  The Teeth Perform chewing, or mastication Parts of a tooth – Neck is boundary between root and crown – Enamel layer covers crown Hardest biologically manufactured substance Requires Ca2+, phosphates, and vitamin D3 for formation – Dentin makes up most of tooth – Pulp cavity receives blood vessels and nerves through root canals – Periodontal ligament, with cementum, binds to bone in tooth socket, or alveolus 30 Teeth: Structural Components and Dental Succession 31  Tooth Decay Generally result of bacterial action Bacteria produce matrix, trapping food particles – Creates deposits called dental plaque – Plaque can calcify and form hard layer of tartar, or dental calculus 32  Types of Teeth Incisors – Blade-shaped teeth at front of mouth – Used for clipping or cutting Cuspids, or canines – Sharp ridgeline and pointed tip – Used for tearing or slashing Bicuspids, or premolars, and molars – Have flattened crowns with ridges – Used for crushing, mashing, and grinding 33 Incisors Incisors Central (6–8 mo) Central (7 yr) Lateral (8 yr) Lateral (8–10 mo) Canine (eyetooth) Canine (eyetooth) (11 yr) (16–20 mo) Premolars Molars (bicuspids) First molar First premolar (10–15 mo) Deciduous (11 yr) Second molar (milk) teeth Second premolar (about 2 yr) (12–13 yr) Molars First molar (6–7 yr) Second molar (12–13 yr) Third molar Permanent (wisdom tooth) teeth (17–25 yr) 34  Dental Formulas A shorthand way of indicating the number and relative position of teeth – Ratio of upper to lower teeth for one-half of the mouth – Primary: 2I,1C, 2M – Permanent: 2I,1C, 2PM, 3M 35  Dental Succession Two sets of teeth form during development Deciduous teeth are first to appear – Also called primary, baby, or milk teeth – Usually 20 in number – Periodontal ligaments and roots erode during eruption (emergence) of adult teeth Secondary dentition, or permanent dentition – Usually 32 in number – Third molars, or wisdom teeth, are last to come in 36  The Pharynx Commonly called the throat Serves as common passageway for food, liquid, and air Food passes through oropharynx and laryngopharynx to esophagus Mucosa is stratified squamous epithelium Lamina propria contains mucous glands and tonsils Pharyngeal muscles cooperate with oral cavity and esophageal muscles for swallowing 37  The Esophagus Muscular tube (25cm long and 2cm wide) that acts as passageway from pharynx to stomach Located posterior to trachea Enters abdominal cavity through esophageal hiatus in diaphragm – Diaphragmatic, or hiatal, hernia involves movement of abdominal organs upward through the esophageal hiatus Lined with stratified squamous epithelium Circular muscles at either end form upper and lower esophageal sphincters 38  Lips and Cheeks Contain orbicularis oris and buccinator muscles Vestibule: recess internal to lips and cheeks, external to teeth and gums Oral cavity lies within the teeth and gums Labial frenulum: median attachment of each lip to the gum 39  Upper lip Gingivae (gums) Superior labial Palatine raphe frenulum Palatoglossal arch Hard palate Soft palate Palatopharyngeal arch Uvula Posterior wall Palatine tonsil of oropharynx Tongue Sublingual fold Lingual frenulum with openings of sublingual ducts Opening of submandibular duct Gingivae (gums) Vestibule Inferior labial Lower lip frenulum Anterior view 40  The Stomach Four primary functions 1. Temporary storage of ingested food 2. Mechanical breakdown of ingested food 3. Chemical digestion by acids and enzymes 4. Production of intrinsic factor needed for vitamin B12 absorption Chyme is mixture of food and gastric secretions 41  Regions of the Stomach J-shaped organ with four main regions 1. Cardia Where the esophagus connects 2. Fundus Bulge of stomach superior to cardia 3. Body Large area between fundus and curve of the J 4. Pylorus Most distal portion Connects stomach to small intestine Pyloric sphincter regulates flow of chyme into small intestine 42  Internal Features of the Stomach Rugae – Folds of mucosa – Show prominently when stomach is empty – Flatten out with stomach distention – Stomach can expand to accommodate up to 1.0–1.5 liters Muscularis externa – Has circular, longitudinal, and third oblique layer – Extra layer strengthens stomach wall and assists in mixing and churning chyme 43 The Anatomy of the Stomach 44  Mesenteries Associated with the Stomach Greater omentum – Very large peritoneal pouch – Extends from greater curvature of stomach down over abdominal viscera Lesser omentum – Smaller peritoneal pouch – Extends from lesser curvature of stomach to liver 45  The Gastric Wall Lined by simple columnar epithelium with numerous mucous cells Mucous epithelium secretes alkaline mucus that protects epithelium Gastric pits – Shallow depressions that open to gastric surface – Mucous cells at base, or neck, undergo active mitosis, replacing mucosal cells every three to seven days Gastric glands – Located in fundus, body, and pylorus – Connected to gastric pits – Cells produce 1.5 liters/day of gastric juice 46  The Gastric Gland Cells Parietal cells secrete: – Intrinsic factor for vitamin B12 absorption – Hydrochloric acid (HCl) Lowers pH of gastric juice to 1.5–2.0 Kills microorganisms and activates enzymes Chief cells secrete: – Pepsinogen, activated by HCl, which is converted into proteolytic enzyme, pepsin – In infants, also secrete rennin and gastric lipase that are important in digestion of milk 47 The Anatomy of the Stomach 48 Stomach Histology Rugae: Folds in stomach when empty Gastric pits: Openings for gastric glands – Contain cells Surface mucous cells: Mucus Mucous neck cells: Mucus Parietal cells: Hydrochloric acid and intrinsic factor Chief cells: Pepsinogen Endocrine cells: Regulatory hormones 24-49 Small Intestine Site of greatest amount of digestion and absorption Divisions – Duodenum – Jejunum – Ileum: Peyer’s patches or lymph nodules Modifications – Circular folds or plicae circulares, villi, lacteal, microvilli Cells of mucosa – Absorptive, goblet, granular, endocrine 24-50  Small Intestine Secretions Mucus – Protects against digestive enzymes and stomach acids Digestive enzymes – Disaccharidases: Break down disaccharides to monosaccharides – Peptidases: Hydrolyze peptide bonds – Nucleases: Break down nucleic acids Duodenal glands – Stimulated by vagus nerve, secretin, chemical or tactile irritation of duodenal mucosa 51 Duodenum and Pancreas 52 Duodenum Anatomy and Histology 53  Functions of the Liver Bile production – Salts emulsify fats, contain pigments as bilirubin Storage – Glycogen, fat, vitamins, copper and iron Nutrient interconversion Detoxification – Hepatocytes remove ammonia and convert to urea Phagocytosis – Kupffer cells phagocytize worn-out and dying red and white blood cells, some bacteria Synthesis – Albumins, fibrinogen, globulins, heparin, clotting factors 54 Liver Lobes – Major: Left and right – Minor: Caudate and quadrate Ducts – Common hepatic – Cystic From gallbladder – Common bile Joins pancreatic duct at hepatopancreatic ampulla 24-55 Duct System 56  Gallbladder Bile is stored and concentrated Stimulated by cholecystokinin and vegal stimulation Dumps into small intestine Production of gallstones possible – Drastic dieting with rapid weight loss 57  Pancreas Anatomy Secretions – Endocrine – Pancreatic juice (exocrine) Pancreatic islets produce Trypsin insulin and glucagon Chymotrypsin – Exocrine Carboxypeptidase Acini produce digestive Pancreatic amylase enzymes Pancreatic lipases – Regions: Head, body, tail Enzymes that reduce DNA and ribonucleic acid 58 The Pancreas 59  Large Intestine Extends from ileocecal junction to anus Consists of cecum, colon, rectum, anal canal Movements sluggish (18-24 hours) 24-60  Large Intestine Cecum – Blind sac, vermiform appendix attached Colon – Ascending, transverse, descending, sigmoid Rectum – Straight muscular tube Anal canal – Internal anal sphincter (smooth muscle) – External anal sphincter (skeletal muscle) – Hemorrhoids: Vein enlargement or inflammation 61 Histology of Large Intestines 62