Regulation of acid secretion
Atropine: |
Muscarinic antagonist |
NSAID's & PG's: |
PGE decreases acid |
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Misoprostol=PGE2 analogue |
Proglumide: |
Gastrin receptor antagonist |
H2 receptor antagonists: |
Cimetidine |
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Ranitidine |
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Famotidine |
Proton Pump Inhibitors: |
Omeprazole |
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Pantoprazole |
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Rabeprazole |
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Esomeprazole |
Digestive Processes
6 basic processes in digestion: |
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1.Ingestion |
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2.Secretion |
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3.Motility |
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4.Digestion |
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5.Absorption |
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6.Defecation |
Saliva
Constituents: |
Mostly water 99.5% |
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0.5% solutes: |
Ions, dissolved gases, urea, uric acid, mucus, immunoglobulin A, lysozyme and salivary amylase (acts on starch) & muramidase (anti-bacterial) |
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Not all salivary glands produce the same saliva |
Submandibular and sublingual: mucin rich saliva |
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Parotid: salivary amylase |
Control of salivation: |
Controlled by autonomic nervous system |
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Parasympathetic stimulation promotes secretion of moderate amount of saliva |
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Sympathetic stimulation decrease salivation |
Hormones in GI tract
Hormone: |
Gastrin: |
Released by: G cells (in the stomach) |
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Triggered by: Food in the stomach (especially proteins), stomach distension |
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Main actions: |
Increases HCl (acid) secretion from parietal cells. Stimulates gastric motility. Promotes growth of stomach lining. |
Secretin: |
Released by: Duodenum (when acidic chyme enters) |
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Triggered by: Low pH (acid from the stomach) |
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Main actions: |
Stimulates bicarbonate secretion from pancreas (to neutralize acid). Inhibits gastric acid secretion. Slows gastric emptying. |
Cholecystokinin (CCK): |
Released by: Duodenum and jejunum |
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Triggered by: Fats and proteins in the chyme |
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Main actions: |
Stimulates bile release from gallbladder. Stimulates enzyme secretion from pancreas. Slows gastric emptying (so digestion has time to occur). |
Motilin: |
Released by: Small intestine (cyclically, between meals) |
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Triggered by: Fasting state |
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Main actions: |
Stimulates migrating motor complexes (waves of contraction that clear the gut). Keeps the gut “clean” between meals. |
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Overview of GI tract functions
Mouth: |
Bite, chew, swallow |
Pharynx and oesophagus: |
Transport |
Stomach: |
Mechanical disruption; absorption of water & alcohol |
Small intestine: |
Chemical & mechanical digestion & absorption |
Large intestine: |
Absorb electrolytes & vitamins (B and K) |
Rectum and anus: |
Defecation |
Histology of the Small Intestine
2 muscle layers: |
Serosa not adventitia |
Microvilli are microscopic folds in the apical surface of the plasma membrane on each simple columnar cell (about 200 million/mm2). The plicae circulares, villi, and microvilli all contribute to increase the surface area of the small intestine, allowing for maximum absorption of nutrients.
Digestive enzymes attached to the microvilli complete the digestive process of carbohydrates and proteins, called a brush border.
Pancreas
Lies posterior to greater curvature of stomach |
Pancreatic juice secreted into pancreatic duct and accessory duct and to small intestine |
Pancreatic duct joins common bile duct and enters duodenum at hepatopancreatic ampulla |
Histology: |
99% of cells are acini: |
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Exocrine |
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Secrete pancreatic juice -mixture of fluid and digestive enzymes |
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1% of cells are pancreatic islets (islets of Langerhans) |
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Endocrine |
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Secrete hormones glucagon, insulin somatostatin, and pancreatic polypeptide |
Pancreas
Lies posterior to greater curvature of stomach |
Pancreatic juice secreted into pancreatic duct and accessory duct and to small intestine |
Pancreatic duct joins common bile duct and enters duodenum at hepatopancreatic ampulla |
Histology: |
99% of cells are acini: |
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Exocrine |
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Secrete pancreatic juice -mixture of fluid and digestive enzymes |
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1% of cells are pancreatic islets (islets of Langerhans) |
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Endocrine |
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Secrete hormones glucagon, insulin somatostatin, and pancreatic polypeptide |
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The Large Intestine
The large Intestine has 4 parts: |
The cecum |
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The colon: |
Ascending |
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Transverse |
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Descending |
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Sigmoid |
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The rectum |
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The anal canal |
There are no circular folds or villi in the large intestine.
The mucosa is mostly an absorptive epithelium (mainly for water), and microvilli are plentiful.
Interspersed goblet cells produce mucous, but no digestive enzymes are secreted
Phases of Digestion
Cephalic phase: |
Stimulates salivary and gastric secretions and motility |
Sight, smell and thought of food. Prepares mouth and stomach for food |
Gastric phase: |
Neural and hormonal mechanisms |
Activates gastric secretions |
Intestinal Phase: |
Neural and hormonal mechanisms |
Activates the small intestine and inhibits gastric emptying and secretions |
Visceral Muscle Contractions Summary
Oesophagus: |
Peristaltic |
Stomach: |
Peristaltic |
Small intestine: |
Segmental, MMC |
Colon: |
Segmentation, Mass movement |
Pancreatic Juice
Volume: |
1200-1500ml daily |
Constituents: |
Mostly water |
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Sodium bicarbonate - buffers acidic stomach chyme |
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Enzyme: |
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Pancreatic amylase |
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Proteolytic enzymes - trypsin (secreted as trypsinogen), chymotrypsin (chymotrypsinogen), carboxypeptidase (procarboxypeptidase), elastase (proelastase) |
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Pancreatic lipase |
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Ribonuclease and deoxyribonuclease |
Major Valves (Sphincters)
Oesophagus: |
Upper oesophageal sphincter |
Pharynx and oesophagus |
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Lower oesophageal sphincter |
Oesophagus and stomach |
Stomach: |
Cardiac sphincter (LOS) |
Oesophagus and stomach |
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Pyloric sphincter |
Stomach and duodenum |
Large intestine valves: |
Ileocaecal sphincter |
Ileum and caecum |
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Internal anal sphincter |
Involuntary smooth muscle |
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External anal sphincter |
Voluntary skeletal muscle |
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Accessory Organs
Salivary Glands: |
Three sets: |
Parotid, sublingual and submandibular |
Pancreas: |
Endocrine: |
Insulin, glucagon |
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Exocrine: |
Digestive enzymes, bicarbonate |
Liver: |
Excretion of bile pigments: |
Bilirubin, Biliverdin |
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Bile salts: |
E.g Deoxycholic acid emulsification of fats |
4 layers Anatomy of the small intestine
Circular folds called the plicae circulares are permanent ridges of the mucosa and submucosa that encourage turbulent flow of chyme.
Control of Gastrointestinal Function
Enteric nervous system (ENS): |
Intrinsic set of nerves - "brain of gut" |
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Neurons extending from oesophagus to anus |
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2 plexuses |
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Myenteric plexus: |
GI tract motility |
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Submucosal plexus |
Controlling secretions |
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Autonomic nervous system |
Extrinsic set of nerves |
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Parasympathetic stimulation increases secretion and activity by stimulating ENS |
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Sympathetic stimulation decreases secretions and activity by inhibiting ENS |
Gastric glands and cell types in the stomach
Surface mucous cell: |
Secretes Mucus |
Mucous neck cells: |
Secretes Mucus |
Parietal cells: |
Secretes hydrochloric acid and intrinsic factor |
Chief cells: |
Secretes pepsinogen and gastric lipase |
G cell: |
Secretes the hormone gastrin |
Summary of Organs in the GI system
Tongue: |
Manoeuvres food for mastication, shapes food into a bolus, manoeuvres food for deglutition, detects sensations for taste, and initiates digestion of triglycerides |
Salivary glands: |
Saliva produced by these glands softens, moistens, and dissolves foods; cleanses mouth and teeth: initiates the digestion of starch |
Teeth: |
Cut, tear, and pulverise food to reduce solids into smaller particles for swallowing |
Pancreas: |
Pancreatic juice buffers acidic gastric juice in chyme, stops the action of pepsin from the stomach, creates the proper pH for digestion in the small intestine, and participates in the digestion of carbohydrates, proteins, triglycerides and nucleic acid. |
Liver: |
Produces bile, which is required for the emulsification and absorption of lipids in the small intestine |
Gallbladder: |
Stores and concentrates bile and releases it into the small intestine |
Mouth: |
Buccal glands lining the mouth produce saliva |
Pharynx: |
Receives a bolus from the oral cavity and passes it into the oesophagus |
oesophagus: |
Receives a bolus from the pharynx and moves it into the stomach: this requites relaxation of the upper oesophagus sphincter and secretion of mucus |
Stomach: |
Mixing waves combine saliva, food, and gastric juice, which activates pepsin, initiates protein digestion, kills microbes in food, helps absorb vitamin B12, contracts the lower oesophageal sphincter, increase stomach motility, relaxes the pyloric sphincter and moves chyme into the small intestine. |
Small intestine: |
Segementation mixes chyme with digestive juices: peristalsis propels chyme toward the ileocecal sphincter: digestive secretions from the small intestine, pancreas, and liver complete the digestion of carbohydrates, proteins, lipids and nucleic acids: circular folds, villi and microvilli help absorb about 90% of digest of nutrients |
Large intestine: |
Haustral churning, peristalsis, and mass peristalsis drive the colonic contents into the rectum: bacteria produce some B vitamins and vitamin K: absorption of some water, ions and vitamin occurs: defecation |
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