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Organs of the digestive system fall into two groups: 1 Alimentary canal or GI tract Continuous muscular tube that runs from the mouth to anus Digests food: breaks down into smaller fragments Absorbs fragments through lining into blood Organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus 2 Accessory digestive organs Teeth Tongue Gallbladder Digestive glands: produce secretions that help break down foodstuffs Salivary glands Liver Pancreas Processing of food involves six essential activities: 1 Ingestion : eating 2 Propulsion : movement of food through the alimentary canal, which includes: Swallowing or Peristalsis : major means of propulsion of food that involves alternating waves of contraction and relaxation Peristalsis is a type of involuntary muscle movement that occurs in your digestive system. It begins in your throat when you swallow, and continues to propel food and fluids throughout your gastrointestinal tract. Segmentation is a muscular activity that divides and mixes the chyme by alternating between backward and forward movement of the gastrointestinal tract (GI) contents. 3 Mechanical breakdown : includes chewing, mixing food with saliva, churning food in stomach, and segmentation Segmentation: local constriction of intestine that mixes food with digestive juices 4 Digestion: series of catabolic steps that involves enzymes that break down complex food molecules into chemical building blocks 5 Absorption: passage of digested fragments from lumen of GI tract into blood or lymph 6 Defecation : elimination of indigestible substances via anus in form of feces Relationship of Digestive Organs to Peritoneum Peritoneum: serous membranes of abdominal cavity that consists of: Visceral peritoneum: membrane on external surface of most digestive organs Parietal peritoneum: membrane that lines body wall Peritoneal cavity Fluid-filled space between two peritoneums Fluid lubricates mobile organs Mesentery: double layer of peritoneum; layers are fused back to back Extends from body wall to digestive organs. Provides routes for blood vessels, lymphatics, and nerves Holds organs in place and also stores fat Intraperitoneal (peritoneal) organs : organs that are located within the peritoneum Retroperitoneal organs: located outside, or posterior to, the peritoneum Includes most of pancreas, duodenum, and parts of large intestine All digestive organs have the same four basic layers, or tunics Mucosa, Submucosa, Muscularis externa and Serosa 1 Mucosa Tunic layer that lines lumen Functions: different layers perform one or all three Secretes mucus, digestive enzymes, and hormones Absorbs end products of digestion Protects against infectious disease Made up of three sublayers Epithelium, Lamina propria and Muscularis mucosae Epithelium Simple columnar epithelium and mucus-secreting cells in most of tract Mouth, esophagus, and anus are made up of stratified squamous epithelium Secretes mucus to Protect digestive organs from enzymes Eases food passage May secrete enzymes and hormones (e.g., in stomach and small intestine) Lamina propria Made up of loose areolar connective tissue Rich supply of capillaries located here Needed for nourishment and absorption Also contains lymphoid follicles that help defend against microorganisms Follicles are part of MALT (mucosa-associated lymphoid tissue) Muscularis mucosae Smooth muscle that produces local movements of mucosa 2 Submucosa Consists of areolar connective tissue contains blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus that supply surrounding GI tract tissues Has abundant amount of elastic tissues that help organs to regain shape after storing large meal 3 Muscularis externa Muscle layer responsible for segmentation and peristalsis Contains inner circular muscle layer and outer longitudinal layers Circular layer thickens in some areas to form sphincters 4 Serosa Outermost layer, which is made up of the visceral peritoneum Formed from areolar connective tissue covered with mesothelium (single layer of squamous epithelium) in most organs Replaced by fibrous adventitia in esophagus Dense connective tissue that holds esophagus to surrounding structures Retroperitoneal organs have both an adventitia and a serosa 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 GI tract has its own nervous system, enteric nervous system Also called the gut brain Contains more neurons than spinal cord Gut brain is made up of enteric neurons that communicate extensively with each other Major nerve supply to GI tract wall that controls motility Enteric neurons make up bulk of two main reflexes: Submucosal nerve plexus that Regulates glands and smooth muscle in mucosa and Myenteric nerve plexus Controls GI tract motility Enteric nervous system participates in both short and long reflex arcs Short reflexes: mediated by enteric nerve plexuses (gut brain); respond to stimuli in GI tract Long reflexes: respond to stimuli arising inside or outside of gut, such as from autonomic nervous system Parasympathetic system enhances digestive process Sympathetic system inhibits digestion Three key concepts regulate GI activity 1 Digestive activity is provoked by a range of mechanical and chemical stimuli Receptors located in walls of GI tract organsRespond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion 2 Effectors of digestive activity are smooth muscle and glands When stimulated, receptors initiate reflexes that stimulate smooth muscle to mix and move lumen contents Reflexes can also activate or inhibit digestive glands that secrete digestive juices or hormones 3 Neurons (intrinsic and extrinsic) and hormones control digestive activity Nervous system control Intrinsic controls : involve short reflexes (enteric nervous system) Extrinsic controls: involve long reflexes (autonomic nervous system) Hormonal controls Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract Mouth is where food is chewed and mixed with enzyme-containing saliva that begins process of digestion, and swallowing process is initiated Associated organs include: Mouth, Tongue, Salivary glands andTeeth Also called the oral (buccal) cavity Bounded by lips anteriorly, cheeks laterally, palate superiorly, and tongue inferiorly Oral orifice is the anterior opening Walls of mouth lined with stratified squamous epithelium Tough cells that resist abrasion Cells of gums, hard palate, and part of tongue are keratinized for extra protection Lips and cheeks Cheeks buccinator muscles Oral vestibule: recess internal to lips and cheeks, external to teeth and gums Oral cavity proper: lies within teeth and gums Labial frenulum: median attachment of each lip to gum Palate forms the roof of the mouth and has two distinct parts Hard palate: midline ridge called raphe Mucosa is slightly corrugated to help create friction against tongue Soft palate: fold formed mostly of skeletal muscle Closes off nasopharynx during swallowing Area in between two arches is called fauces Palatine tonsils are located in fauces Tongue Formation of bolus, mixture of food and saliva Functions of saliva Cleanses mouth; Dissolves food chemicals for taste ;Moistens food; compacts into bolus Begins breakdown of starch with enzyme amylase Most saliva produced by major (extrinsic) salivary glands located outside oral cavity Minor salivary glands are scattered throughout oral cavity; augment slightly Salivary glands are composed of two types of secretory cells 1 Serous cells: produce watery secretion, enzymes, ions, bit of mucin and 2 Mucous cells: produce mucus Parotid and submandibular glands contain mostly serous cells, but sublingual gland consists mostly of mucous cells Composition of saliva Salivary amylase and lingual lipase Proteins: mucin, lysozyme, and IgA Metabolic wastes: urea and uric acid Lysozyme, IgA, defensins, and nitric oxide from nitrates in food protect against microorganisms Control of salivation 1500 ml/day can be produced Minor glands continuously keep mouth moist Major salivary glands are activated by parasympathetic nervous system Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia) Teeth are classified according to shape: Incisors: chisel shaped for cutting Canines: fanglike teeth that tear or pierce Premolars (bicuspids): broad crowns with rounded cusps used to grind or crush Tooth structure Each tooth has two major regions: Crown: exposed part above gingiva (gum) Heavily mineralized with calcium salts and hydroxyapatite crystals Enamel-producing cells degenerate when tooth erupts, so no healing if tooth decays or cracks; needs artificial repair by filling Root : portion embedded in jawbone Connected to crown by neck Cement: calcified connective tissue Covers root Forms fibrous joint called gomphosis Anchors tooth in bony socket (alveolus) Gingival sulcus: groove where gingiva borders tooth Dentin: bonelike material under enamel Maintained by odontoblast of pulp cavity Pulp cavity: surrounded by dentin Pulp: connective tissue, blood vessels, and nerves Root canal: as pulp cavity extends to root Apical foramen at proximal end of root Entry for blood vessels, nerves, etc.