10/15/2018 Digestive System Alimentary Tract and Accessory Organs Function of Digestive System 1. Take in food and water – ingestion or intake 2. Break food down into nutrient molecules 3. Absorb molecules into the bloodstream 4. Eliminate any indigestible remains – elimination or defecation 5. Dispose of some metabolic wastes Overview of Digestive System • Alimentary canal (gastrointestinal or GI tract or gut) • Continuous muscular tube that runs from the mouth to anus • Breaks down food into smaller fragments: • Mechanical: movement • Chemical: enzymes • Absorbs nutrient molecules through lining into blood • Organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus • Accessory digestive organs • Oral cavity: • Teeth • Tongue • Salivary glands: lubrication • Liver • Gallbladder • Pancreas • Digestive glands: produce secretions that help break down foodstuffs 1 10/15/2018 Digestive Processes Processing of food involves six essential activities: 1. Ingestion 2. Propulsion • Peristalsis • Swallowing 3. Mechanical breakdown • Chewing • Churning, mixing • Segmentation: local constriction of intestine that mixes food with digestive juices 4. Chemical Breakdown • series of catabolic steps supported by groups of enzymes 5. Absorption • blood or lymph 6. Defecation - Elimination • indigestible substances • Bile includes wastes produced as liver processes blood Peristalsis Segmentation Adjacent segments of the Nonadjacent segments of the alimentary canal organs alternately alimentary canal organs contract contract and relax. and relax. • Food is moved distally • Food is moved forward, along the tract then backward • Primarily propulsive; some • Primarily mixes food and breaks mixing may occur it down mechanically; some From propulsion may occur mouth Histology of the Alimentary Canal 1. Mucosa • Functions • Secretes mucus, digestive enzymes, and hormones • Absorbs end products of digestion • Protects against infectious disease • Capillaries, lymphoid follicles, localized movement, glands • Made up of three sublayers • Epithelium– stratified squamous or simple columnar, mucus producing • Lamina propria – areolar: ample space for follicles, capillaries, and mucosal glands • Muscularis mucosae 2. Submucosa • Consists of areolar connective tissue • abundant amount of elastic tissue • blood and lymphatic vessels • lymphoid follicles • submucosal nerve plexus 3. Muscularis externa • Circular and longitudinal muscle layer responsible for segmentation, peristalsis, and sphincters 4. Serosa (adventitia in some areas) • Aka the visceral peritoneum 2 10/15/2018 Figure 23.5 Basic structure of the alimentary canal. Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus Glands in submucosa Mucosa • Epithelium • Lamina propria • Muscularis mucosae Submucosa Muscularis externa • Circular layer • Longitudinal layer Serosa • Connective tissue • Epithelium (mesothelium) Nerve Gland in Artery mucosa Lumen Vein Mucosa-associated Duct of Lymphatic vessel lymphoid tissue Mesentery gland outside alimentary canal Nerve and Blood Supply to Walls • Blood Supply: • Splanchnic circulation includes • Arteries that branch off aorta to serve digestive organs • Hepatic, splenic, and left gastric arteries • Inferior and superior mesenteric arteries • Hepatic portal circulation • Drains nutrient-rich blood from digestive organs • Delivers blood to liver for processing • Enteric Nervous System • Submucosal nerve plexus • Myenteric nerve plexus • Semiautonomous • Controls patterns of segmentation and peristalsis • Linked to CNS: visceral sensory fibers (afferent), autonomic motor fibers synapse with internal plexi (called long reflex arcs) • 100 million neurons in plexi Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract. External stimuli (sight, smell, taste, Central nervous system thought of food) Long reflexes Visceral afferents Extrinsic visceral (autonomic) efferents Internal Chemoreceptors, Local (intrinsic) Effectors: (GI tract) osmoreceptors, or nerve plexus Smooth muscle stimuli mechanoreceptors (“gut brain”) or glands Short reflexes Gastrointestinal Response: wall (site of short Change in reflexes) contractile or secretory activity Lumen of the alimentary canal 3 10/15/2018 Path of Alimentary Tract Begins at Oral Cavity and Pharynx . Ingestion . Propulsion . Saliva Posterior nasal . Oral mucosa aperture Nasopharynx • Lubricants • Pharyngeal tonsil • Opening of • Antimicrobial pharyngotympanic proteins tube • Early enzyme Oropharynx Hard palate introduction • Palatine tonsil Soft palate • • Isthmus of the Particle size fauces Tongue reduction Lingual tonsil • Mechanical Laryngopharynx Hyoid bone Larynx mixing • Epiglottis • Vestibular fold • Thyroid cartilage o Taste cell Esophagus • Vocal fold • Cricoid cartilage activation Trachea o Olfactory Thyroid gland membrane (b) Structures of the pharynx and larynx involvement The salivary glands and tongue papillae Tongue Teeth Ducts of Parotid sublingual gland gland Parotid duct Frenulum of tongue Masseter muscle Sublingual Body of mandible gland (cut) Mylohyoid Posterior belly of muscle (cut) digastric muscle • Intrinsic glands Anterior belly of Submandibular • Extrinsic glands duct digastric muscle • Continuous production and more upon activation Submandibular • Average of 1500 ml/day gland • Mostly under parasympathetic control Deglutition (swallowing) Slide 6 Bolus of food Nasopharynx Tongue Uvula Oro- Bolus pharynx Epiglottis Upper Trachea Upper esophageal esophageal Bolus sphincter Esophagus sphincter 1 Buccal phase: 2 Pharyngeal-esophageal phase begins: 3 Pharyngeal-esophageal phase • The upper esophageal sphincter is • The tongue blocks the mouth. continues (steps 3 5 ): contracted (closed). • The soft palate and its uv ula rise, closing • The constrictor muscles of the • The tongue presses against the hard off the nasopharynx. pharynx contract, forcing food into the esophagus inferiorly. palate, forcing the food bolus into • The larynx rises so that the epiglottis the oropharynx. blocks the trachea. • The upper esophageal sphincter contracts after food enters. • The upper esophageal sphincter relaxes; Skeletal muscle used here food enters the esophagus. Relaxed muscles Relaxed Circular muscles muscles contract All 4 tunics present Bolus of food Longitudinal muscles Circular muscles contract All smooth contract muscle here Gastroesophageal Gastroesophageal sphincter opens sphincter closed 5 The gastroesophageal sphincter surrounding the 4 Stomach Peristalsis moves cardial orifice opens. After food through the food enters the stomach, esophagus to the the sphincter closes, stomach. preventing regurgitation. 4 10/15/2018 Stomach Organization • Storage • Expands from 50 ml to 4 l • Gastric juice • Food becomes chyme • Region of the pyloris squishes chyme and allows (‘filters’) small amount of chyme to pass to duodenum; remainder pushed back into fundus – prepares food for SI • Enteric pacemaker cells – 3 peristaltic waves per minute • Lesser and greater omentum • Stretch receptors • Environment best for protein digestion Slide 4 Peristaltic waves in the stomach. Pyloric Pyloric Pyloric valve valve valve closed slightly closed opened 1 Propulsion: Peristaltic 2 Grinding: The most 3 Retropulsion: The peristaltic waves move from the vigorous peristalsis and wave closes the pyloric valve, fundus toward the pylorus. mixing action occur close forcing most of the contents of to the pylorus. The pyloric the pylorus backward into the end of the stomach acts stomach. as a pump that delivers small amounts of chyme into the duodenum. Microscopic anatomy of the stomach. Gastric juice: • Mucus producing cells • Parietal cells: HCl + intrinsic factor (for SI absorption of B12 & RBC production) • Chief cells: pepsinogen, lipase Surface • Enteroendocrine cells: epithelium paracrine and endocrine substances influencing blood flow and stomach secretions Mucosa • Up to 3 l/day Lamina propria Muscularis mucosae Submucosa Mucosal barrier: (contains Stomach acid: submucosal Oblique • Tissue regeneration plexus) layer • Kills many bacteria • Tight junctions join Muscularis Circular • Chemically degrades externa layer (contains Longitudinal epithelial cells myenteric layer food • Bicarbonate-rich, plexus) • Activates enzymes Stomach wall thick mucus Serosa supplementing chemical Layers of the stomach wall breakdown 5 10/15/2018 Neural and hormonal mechanisms that regulate release of gastric juice. Stimulatory events Inhibitory events Cephalic 1a Sight and thought Cerebral cortex Para- Cerebral 1c Loss of phase of food sympathetic cortex appetite, activity depression 1b Stimulation of Hypothalamus Vagus taste and smell and medulla nerve receptors oblongata 2a Stomach Long reflexes (via medulla Gastrin G cells 2c Excessive distension and vagus nerve) release to acidity Gastric activates blood (pH < 2) stretch phase in stomach receptors Short Sympathetic 2d Emotional reflexes nervous stress system activation 2b Food chemicals G cells Gastrin (overrides (especially peptides and release parasympathetic caffeine) and rising pH to blood controls) activate chemoreceptors Stomach secretory activity Enterogastric reflex 3b Distension Intestinal (involves both short of duodenum; phase and long reflexes) presence of 3a Presence of Intestinal fatty, acidic, or (enteric) partially digested hypertonic gastrin foods in duodenum chyme or distension of the release Brief duodenum when to blood effect Release of stomach begins to enterogastrones empty (secretin, cholecystokinin)