Lecture 23 – Human Digestive System

Lecture 23 – Human Digestive System Overview • Digestive system = Gastrointestinal (GI) tract + Accessory digestive organs • GI tract: Long muscular tube from mouth to anus (~9 meters) • Accessory organs: Help in digestion but do not directly carry food o Include: Teeth, tongue, salivary glands, pancreas, liver, gallbladder Functions of the Digestive System • Ingestion: Taking in food • Secretion: Enzymes, mucus, acids • Propulsion: Peristalsis – muscular waves move food • Digestion: o Mechanical – chewing, churning o Chemical – enzymes break down nutrients • Absorption: Nutrients enter blood/lymph • Elimination: Removal of undigested material (feces) Layers of the GI Tract (from outer to inner) 1. Serosa: Outer covering; in abdomen, called peritoneum 2. Muscle Layer: o Outer longitudinal + Inner circular muscles o Contains Myenteric plexus (Auerbach’s) – controls movement 3. Submucosa: o Connective tissue, blood vessels, Meissner’s plexus – controls secretion 4. Mucosa: o Inner lining with 3 layers: mucous membrane (absorption), lamina propria (support), muscularis mucosa (local movement) Nervous Control (Enteric Nervous System) • Myenteric plexus: Movement (motility) • Submucosal plexus: Secretions & blood flow • Parasympathetic (Vagus/Sacral): Increases secretion & motility • Sympathetic: Decreases them Splanchnic Circulation • Blood supply to stomach, intestines, liver, pancreas, spleen • Major arteries: Celiac, SMA, IMA • Blood from GIT drains via hepatic portal vein to liver Organs • Mouth, pharynx, esophagus, stomach, small & large intestines • Each has unique structures and functions for digestion and movement Lecture 24 – Salivary Secretion Saliva • Daily volume: 800–1500 mL • pH: 6.0 – 7.0 • Secreted by extrinsic (90%) and intrinsic (10%) salivary glands Types of Salivary Glands • Extrinsic: o • Parotid (serous), Submandibular (mixed), Sublingual (mixed) Intrinsic: Buccal, labial, palatine (mostly mucous) Composition of Saliva • Serous part: Contains ptyalin (amylase) – breaks starch • Mucous part: Contains mucin – lubricates food • Rich in K⁺, HCO₃⁻, low in Na⁺ and Cl⁻ Regulation • Parasympathetic (CN VII, IX): ↑ watery secretion • Sympathetic (T1-T4): Thick, mucous-rich saliva • Stimuli: Chewing, taste, smell, thought of food, GI irritation, nausea Functions • Starts starch digestion • Lubricates and softens food for swallowing • Maintains oral hygiene and neutral pH • Antimicrobial: Lysozyme, IgA, thiocyanate • Enhances taste (dissolves food for taste buds) • Prevents dental caries with proline-rich proteins Lecture 26 – Gastrointestinal Motility What is Motility? • Involuntary contraction/relaxation of GI muscles to move food • Types: o Swallowing o Peristalsis: Forward propulsion o Segmentation: Mixing o Mass movement (colon) o Defecation Muscle Types • Outer longitudinal: Moves food along • Inner circular: Mixes, controls lumen • Controlled by ANS: o Parasympathetic: Stimulates motility o Sympathetic: Inhibits Electrical Activity • Slow waves: Baseline rhythm (not AP) • Spike potentials: True action potentials, cause contraction • Controlled by Interstitial Cells of Cajal (ICC) – pacemakers Types of Contractions • Phasic: Rhythmic, e.g., peristalsis • Tonic: Sustained, e.g., sphincters Motility in Different Organs • Esophagus: Peristalsis pushes bolus to stomach • Stomach: • • o Orad region stores food (relaxes with VIP & NO) o Caudad region mixes & empties (retropulsion) o Pyloric pump pushes chyme to duodenum Small Intestine: o Segmentation: Chops chyme o Peristalsis: Moves it slowly (~1 cm/min) Large Intestine: o Haustrations: Mixing o Mass Movements: Push feces to rectum (3–4/day) Lecture 27 – Gastric & Intestinal Secretions Gastric Glands & Cells • Oxyntic glands (fundus/body): o Chief cells (pepsinogen), Parietal cells (HCl, IF), ECL (histamine) • Pyloric glands (antrum): Mucus, pepsinogen, gastrin • Surface mucous cells: Line stomach, protect with mucus Phases of Secretion • Cephalic: Vagus, triggered by thought/smell • Gastric: Stretch + protein digestion → Gastrin • Intestinal: Minor; chyme in intestine gives feedback inhibition Regulation • Stimulators: ACh, Gastrin, Histamine • Inhibitors: Low pH (↓ gastrin), Somatostatin, Enterogastric reflex Intestinal Secretions • • Small intestine: o From Brunner’s glands: Alkaline mucus o From Crypts of Lieberkühn: Water, electrolytes, enzymes (e.g. maltase, sucrase, lipase) Large intestine: o Only mucus (no enzymes) o Protects wall, lubricates feces, prevents bacterial damage Lecture 28 – Pancreas, Liver & Biliary Tract Pancreas • Mixed gland (endocrine & exocrine) • Exocrine part: Acinar cells → pancreatic juice (via duct → duodenum) • Enzymes: o Carbs: Amylase o Proteins: Trypsin, chymotrypsin (inactive forms) o Fats: Lipase, cholesterol esterase, phospholipase • HCO₃⁻ neutralizes acid (pH 8 for enzyme activity) • Regulation: ACh, CCK (enzymes), Secretin (HCO₃⁻) Liver • Largest gland (~1–2.3 kg) • Made of lobules with hepatocytes (arranged around central vein) • Kupffer cells: Remove old RBCs, bacteria • Bile canaliculi: Drain bile from hepatocytes to ducts • Functions: o Metabolism (CHO, proteins, fats) o Detox, hormone breakdown o Stores glycogen, iron, vitamins o Bile production Bile & Biliary Tract • Produced by hepatocytes, stored in gallbladder • Daily volume: 600–1000 mL • Composition: Water, bile salts, cholesterol, bilirubin, phospholipids • Function: • • o Emulsifies fats for digestion o Waste removal (bilirubin, excess cholesterol) Secretion: o 1st stage by hepatocytes o 2nd stage (adds HCO₃⁻) by cholangiocytes → stimulated by secretin Gallbladder concentrates bile; empties in response to CCK