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THE DIGESTIVE SYSTEM D. C. Mikulecky Faculty Mentoring Program Virginia Commonwealth University FUNCTIONS OF THE DIGESTIVE SYSTEM Motility Secretion Digestion Absorption Motility The digestive tract is surrounded by layers of smooth muscle These muscles enable mixing and propulsive movement to be carried out by the digestive tract Secretion Digestion requires that enzymes be secreted by the pancreas and other organs Mucous secretions protect the digestive tract Acid is secreted in the stomach There are other secretions of importance DIGESTION Breaking down complex foodstuffs into absorbable units by enzymes produced in the digestive system Involves the breakdown of carbohydrates, proteins fats, and other foods Absorption All other functions support this one This is how we obtain the necessary fuel for our cells THE DIGESTIVE TRACT Mouth and pharynx Esophagus Stomach Small Intestine Large Intestine Anus THE DIGESTIVE TRACT Mucosa Logitudinal Muscle Submucosal Plexus Circular Muscle Myenteric Plexus Lumen Extrinsic Nerves ACCESSORY DIGESTIVE ORGANS Salivary Glands Exocrine Pancreas Billiary System: (Liver and Gallbladder) CONTROL OF DIGESTIVE SYSTEM External Influences Local Influences Receptors in Digestive Tract Intrinsic Nerve Plexuses Extrinsic Autonomic Nerves Smooth muscle Exocrine Gland Cells Endocrine Gland Cells GI Hormones THE MOUTH Ingestion of food Chewing and swallowing Salivary secretion Chewing Chewing is the first step in the digestive process Food is mixed with saliva and broken up into smaller pieces Taste buds are stimulated Salivary secretion Salivary amylase: digests carbohydrate Moistening and lubricating agent Antibacterial action: lysozyme and rinsing Solvent for taste stimuli Speech facilitation Oral hygene Bicarbonate buffers neutralize acids Swallowing A programmed all-or-none reflex Chewing and moving the bolus of food back is manly voluntary (striated muscle) Pressure of bolus on pharynx triggers involuntary reflex (smooth muscle) Tongue prevents food from moving back Uvula elevated, sealing nasal passage Larynx elevates and closure of glottis Respiration briefly inhibited Pharyngeal muscles force bolus back Peristaltic waves move bolus through esophagus PERISTALSIS Ringlike contraction sweeps down the esophagus THE STOMACH Gastroesophageal Sphincter Esophagus Fundus Body Oxyntic Mucosa Pyloric Antrum Sphincter Pyloric Gland Area THE STOMACH Mixing and storage organ Has its own pacemaker: sets frequency of rhythmic peristaltic contractions Peristalsis becomes stronger in antrum Motility is highly controlled No absorption of food, only aspirin and alcohol FACTORS INFLUENCING GASTRIC MOTILITY Distension of stomach: increases Feedback from the small intestine: decreases Control from CNS Gastrin: increases MIXING MOTIONS IN THE STOMACH Pyloric Sphincter Closed Peristalsis Begins MIXING MOTIONS IN THE STOMACH Pyloric Sphincter Closed Peristaltic Wave Travels Downward STOMACH EMPTYING Pyloric Sphincter Opens Chyme Squirted into Duodenum Peristaltic Wave Travels Downward FACTORS CONTROLLING STOMAC EMPTYING Gastric Motility Enterogastric reflex: Via intrinsic and autonomic nerves Enterogastrones: secretin, cholecystokinin (CCK), gastric inhibitory peptide FACTORS IN SMALL INTESTINE CONTROLLING STOMAC EMPTYING Fat Acid Hypertonicity Distension GASTRIC SECRETIONS Mucous: Surface Cells Mucous: Mucous Neck Cells Hydrochloric Acid: Parietal cells Intrinsic Factor: Parietal Cells Pepsinogen: Chief Cells Gastrin: Only in the Pyloric Gland Area Activation of Pepsin Chief Cell PEPSIN-OGEN PEPSIN HCl Parietal Cell Digests Protein STIMULATION OF GASTRIC SECRETION: Cephalic Phase Seeing, Smelling, Tasting Food Vagus Intrinsic Nerves Pyloric Area Gastrin Parietal & Chief Cells Increased Gastric Secretion STIMULATION OF GASTRIC SECRETION: Gastric Phase Stimuli Vagus in Stomach: protein, distension, caffeine, alcohol Intrinsic Nerves Pyloric Area Gastrin Parietal & Chief Cells Increased Gastric Secretion THE GASTRIC MUCOSAL BARRIER Protects the cells from contents of stomach Luminal membranes of cells are impermeable to protons Cells are tightly adjoined Rapid turnover If broken, peptic ulcer may result: positive feedback involving histamine Gastrin secretion inhibition Acid in antrum removal of protein as stomach empties
