Thursday July 31st, 2014The Digestive System: OverviewChapter 24

Learning ObjectivesIdentify the organs of the digestive systemDescribe the basic processes performed by the digestive system Describe the structure and function of the layers of the gastrointestinal (GI) tractDescribe the nerve supply to the GI tractDescribe the peritoneum and its foldsBriefly describe the major functions of the mouth and tongueIdentify locations of salivary glands and their major functionsDescribe the three phases of deglutition

The Digestive System: OverviewComposed of two groups of organs:GASTROINTESTINAL (GI) TRACT (alimentary canal)ACCESSORY DIGESTIVE ORGANS

The Digestive System: OverviewBasic digestive system processes:IngestionSecretionDigestion (chemical and mechanical) Mixing and propulsion Absorption DefecationDIGESTION: mechanical and chemical breakdown of food into smaller components - absorbed more easily into the body

The Digestive System: FunctionGI tract and accessory organs (i.e. liver and pancreas) are responsible for facilitating the bodys metabolic processes:CATABOLISM:

ANABOLISM:Hydrolysis: Large carbohydrate (CHO), lipid, protein, nucleic acid moleculesAbsorption without chemical digestion: vitamins, ions, cholesterol, water

Digestive System Anatomy: OverviewGI tract wall (from lower esophagus to anal canal): four-layered arrangement of tissuesFrom deep to superficial: MUCOSASUBMUCOSA MUSCULARISSEROSA (adventitia) LUMEN: inside of the tubular structure

MUCOSA: mucous membrane, forming inner layer of GI tract Composed of multiple layers:

GI Tract Layers: MucosaEpithelium Enteroendocrine glands: Among epithelial cells. Exocrine: Secrete mucus and fluid into the lumen of the tract Endocrine: Secrete hormones

Lymphoid tissue (mucosa-associated lymphatic tissue or MALT)2. Lamina Propria: Connective tissue containing blood and lymphatic vessels

3. Muscularis mucosae: thin layer of smooth muscle Lining of stomach and small intestines forms small foldsGI Tract Layers: MucosaMovement of muscularis mucosae ensures all absorptive cells are exposed to GI tract contents

GI Tract LayersSmooth muscle: rest of GI tract. Food breakdown, mixing with digestive juices and propelling food down GI tractMUSCULARIS: muscular layerSkeletal muscle: mouth, pharynx, esophagus, anal sphincter. SUBMUCOSA: areolar connective tissue binding mucosa to muscularis Contains: blood and lymphatic vessels, glands and lymphatic tissue, submucosal plexus

GI Tract LayersSEROSA: superficial layer of the GI tractadventitia - fibrous connective tissue arranged around the organ which it supportsReproduced from siumed.edu

Neural Innervation of the GI TractENTERIC NERVOUS SYSTEMIntrinsic set of nerves arranged in 2 plexuses: Myenteric plexus

Between longitudinal and circular muscle of muscularis. Controls GI tract motility. Submucosal plexusWithin the submucosa. Supply secretory cells of mucosal epithelium, controlling secretions ChemoreceptorsStretch receptors

Neural Innervation of the GI TractAUTONOMIC NERVOUS SYSTEMNeurons of the ENS can function independently, but are subject to regulation by the ANS Parasympathetic Innervation: Increased GI motility and secretionVagus (X) NervePelvic Splanchnic nervesSacral spinal cord

Sympathetic Innervation: Decreased GI secretion and motilityThoracic and lumbar spinal cord

Digestive System Anatomy: OverviewPeritoneum: the bodys largest serous membrane. It wraps around most abdominopelvic organs Visceral peritoneum: serosa of the alimentary canal and covers other intra-abdominal organs parietal peritoneum: continuation of visceral peritoneum around abdominal wall

There are five major peritoneal folds: Greater omentumFalciform ligamentLesser omentumMesenteryMesocolon

Falciform ligament: attaches the liver to the anterior abdominal wall and diaphragmAnatomy Overview: PeritoneumGreater omentum: largest peritoneal foldDrapes over transverse colon and small intestine like a fatty apron Contains lymph nodes (GI immunity) Lesser omentum: suspends stomach & duodenum from inferior edge of liverPathway for blood vessels (hepatic portal vein; common hepatic artery) to enter liver Contains the common bile duct

Digestive System Anatomy: OverviewRetroperitoneal: Abdominopelvic organs are covered by visceral peritoneum only on their anterior surfaces. Organs in the retroperitoneal space include:Kidneys and uretersMajority of the pancreas Adrenal glandsAorta and inferior vena cava

The Digestive System

The MouthOral (buccal) cavity: formed by the cheeks, hard and soft palates, and the tongueMechanical digestion of food through mastication (chewing) Mixing with saliva to form a soft flexible bolusSaliva: starts process of chemical digestion 99.5% water, small amounts of dissolved ions, IgA, lysozyme (a bacteriolytic enzyme), and salivary amylase (a digestive enzyme that acts on starch).

The tongue: composed of skeletal muscle under voluntary somatic motor controlForces the moistened food bolus into position for swallowing (deglutition) and places the bolus into contact with the teeth for chewing

Three large salivary glands secrete most of the saliva:ParotidsubmandibularSublingual

The MouthThe smaller glands are found on the: lips (labial)cheeks (buccal)palate (palatal)tongue (lingual)

Esophagus: AnatomySphincters: Muscularis forms a sphincter at either end of the esophagusUpper esophageal sphincter regulates food from larynx to esophagusLower esophageal sphincter regulates food from esophagus to stomach

Esophagus: collapsible muscular tube, posterior to tracheaBegins at inferior end of laryngopharynx, entering the mediastinum anterior to vertebral columnPierces diaphragm through esophageal hiatus (opening)

Deglutition (swallowing)Stage of deglutition:Voluntary stage: Bolus forced to back of oral cavity and into oropharynx by movement of the tongue Pharyngeal stage: Soft palate raised and epiglottis closes off opening to larynx (respiratory tract) Esophageal stage:Contraction of pharyngeal musclesOpening of esophagusPeristalsis

NOTE: Deglutination center located in medulla oblongata and lower pons of the brain stem. Simulated by receptors in oropharynx

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