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Gastrointestinal System Histology Dr. Twana A. Mustafa GI: Overview: Organ systems Gastrointestinal (GI) tract [Alimentary canal] a continuous muscular digestive tube Digests: Absorbs: breaks food into smaller fragments digested material is moved through mucosa into the blood Eliminates: unabsorbed & secreted wastes. Organ systems Includes: Mouth, pharynx & esophagus Stomach Small intestine Large intestine Accessory digestive organs: teeth, tongue, gall bladder, salivary glands, liver & pancreas Figure 23.1 Processes Ingestion Propulsion Mechanical digestion Chemical digestion Absorption Defecation Processes Ingestion: obtaining food Propulsion: moves food along the GI tract by peristalsis (wave-like muscular contraction) Mechanical digestion : chewing & mixing with saliva mixing in stomach segmentation (local constriction in intestine to mix food & digestive juices) Processes Chemical digestion: breaks down food to molecular fragments (monomers) (Hydrolysis). Begins in the mouth with saliva & continues into the small intestine. Absorption: movement of nutrients across the mucosal membrane into blood/lymph Defecation: eliminates unused/indigestible & secreted substances from the body Functional Considerations : Substances in the GI tract lumen are outside of the body. Multiple sensors & receptors line the GI tract to monitor contents & respond to conditions. Controls: intrinsic (local control) & extrinsic (CNS) Peritoneum : serous membrane Visceral peritoneum: covers the external surfaces of most digestive organs Parietal Peritoneum: lines the body wall Peritoneal Space: potential space containing fluid that separates the visceral & parietal peritoneum Figure 23.5a Histology GI tract wall has 4 layers: Mucosa Submucosa Muscularis Externa Serosa or Adventitia Histology of the Alimentary Canal Figure 23.6 Histology Mucosa: The epithelial membrane that lines the GI tract from mouth g anus. Secretes mucous, digestive enzymes & hormones Absorbs nutrients Protects from disease & from the GI contents Mucosa; 3 layers: Epidermis Lamina propria (loose ct : contain capillaries & some elements of MALT) Muscularis mucosa Histology Submucosa: moderately dense CT with blood, nerve, lymph vessels & lymphoid follicles; rich in elastic fibers Muscularis externa: smooth muscle Responsible for peristalsis & segmentation Circular layer Longitudinal layer Sphincters: in some areas the circular layer thickens; act as valves Functional Anatomy: Mouth Mouth: lips, palate, & tongue Mouth cavity = Buccal cavity Functional Anatomy: Mouth Lips: extend from inferior margin of the nose to the superior margin of the chin. Red area = red margin, is poorly keratinized & lacks sweat or sebaceous glands. Palate: Hard palate: rigid surface against which food is forced in chewing Soft palate: muscular structure that rises & blocks off the nasopharynx during swallowing Functional Anatomy: Mouth Tongue: muscular tentacle composed of interlaced muscle fibers that grips & repositions food, mixes food with saliva & compresses food to form a food bolus, prior to swallowing. Functional Anatomy: Mouth Filiform papillae: rough surface Fungiform papillae: house taste buds Circumvallate papillae: house taste buds, Foliate papillae: posterolateral; taste buds Functional Anatomy: Mouth Salivary Glands: intrinsic & extrinsic Intrinsic glands: scattered throughout the buccal cavity mucosa Extrinsic glands: supply most of the saliva; outside buccal cavity & supply secretions via ducts: Parotid Submandibular Sublingual Functional Anatomy: Mouth Composition of saliva: 97-99.5% H2O Electrolytes: pH 6.75-7.0 Amylase: (digestive enzyme) Proteins: mucin, lysozyme, & IgA Protection from microbes by saliva: IgA: immunglobulins in secretions Lysozyme: bacteriostatic (inhibits bacterial growth) Cyanide Defensins: local antibiotic activity & when activated promote chemotaxis by WBCs Normal flora: convert salivary components to nitrates then to NO. NO is toxic & bacteriocidal Figure 23.07 Figure 23.11 Teeth: Primary: 2I 1C 2M x 2 = 20 2I 1C 2M Permanent: 2I 1C 2PM 3M x 2 = 32 2I 1C 2PM 3M Structures Crown: exposed above gingiva (gum) Root: anchored by periodontal ligament to the bone by a fibrous joint (gomphosis)