BSC2086 A&P II Final Exam Study Guide – Lesson 16-18 1 Lesson 16-18: The Digestive System The Digestive Tract • Six (6) Functions of the Digestive System: o Ingestion: Occurs when materials enter digestive tract via the mouth o Mechanical Processing: Crushing & shearing; makes materials easier to propel along digestive tract o Digestion: Chemical breakdown of food into small organic fragments for absorption by digestive epithelium o Secretion: The release of water, acids, enzymes, buffers, and salts by epithelium of digestive tract or glandular organs o Absorption: Movement of organic substrates, electrolytes, vitamins, and water across the digestive epithelium & into interstitial fluid of digestive tract o Excretion: Removal of waste products from body fluids Defecation: Process that removes feces • Lining of the Digestive Tract o Protects surrounding tissues from: Corrosive effects of digestive acids & enzymes Mechanical stresses (abrasion) Bacteria ingested with food or residing in the digestive tract o Digestive epithelium & its secretions provide nonspecific defense • Digestive organs & peritoneum o Lined with serous membrane consisting of two (2) parts: Serosa (Visceral Peritoneum): Covers organs within peritoneal cavity Parietal Peritoneum: Lines inner surfaces of body wall o Peritoneal Fluid: Fluid produced by serous membrane lining Provides essential lubrication Reduces friction/irritation Separates parietal & visceral surfaces Ascites: Excess peritoneal fluid causing abdominal swelling • Caused by liver, kidney, heart disease • Can distort internal organs & cause heartburn, indigestion, lower back pain • Four (4) Major Layers of the Digestive Tract: o Mucosa: Inner mucosa membrane lining composed of epithelium & lamina propria Has blood vessels, sensory nerves, lymphatic vessels, smooth muscles Muscularis Mucosae: Narrow band of smooth muscle & elastic fibers in lamina propria • Smooth muscle cells arranged in two (2) concentric layers: o Inner circular layer o Outer longitudinal layer • Contractions alter shape of lumenBSC2086 A&P II Final Exam Study Guide – Lesson 16-18 2 o Move epithelial pleats & folds Enteroendocrine Cells: Secrete hormones that coordinate activities of the digestive tract & accessory glands Lining of digestive tract consists of permanent transverse folds (plicae circulares) to increase surface area for absorption • Longitudinal folds disappear as digestive tract fills o Submucosa: Layer of dense, irregular connective tissue Surrounds muscularis mucosae of mucosa & binds it to the muscularis externa Has large blood vessels & lymphatic vessels May contain exocrine glands that secrete buffers & enzymes into digestive tract lumen Submucosal Plexus: Intrinsic nerve fibers that innervate mucosa & submucosa • Contains sensory neurons & sympathetic/parasympathetic components o Muscularis Externa: Dominated by smooth muscle cells Two (2) muscle layers: • Inner circular layer • Outer longitudinal layer Moves substances along digestive tract Movement coordinated by enteric nervous system (ENS) Myenteric Plexus: Located between circular & longitudinal layers • Innervated primarily by parasympathetic division • Parasympathetic stimulation increases muscle tone/activity • Sympathetic stimulation causes muscular inhibition/relaxation o Serosa: Serous membrane covering muscularis externa NOT in oral cavity, pharynx, esophagus, & rectum • Those areas covered by adventitia – dense sheath of collagen fibers that firmly attaches the digestive tract to adjacent structuresBSC2086 A&P II Final Exam Study Guide – Lesson 16-18 3 • Movement of digestive materials – by muscular layers of digestive tract o Visceral smooth muscle tissue along digestive tract with rhythmic cycles of activity Controlled by pacesetter cells – located in muscularis mucosae & muscularis externa surrounding lumen of digestive tract o Cells undergo spontaneous depolarization Triggers wave of contraction through entire muscular sheet o Peristalsis: Waves of muscular contractions of the muscularis externa Moves a bolus along the length of the digestive tract Peristaltic Motion • Circular muscles contract behind bolus while circular muscles ahead of bolus relax • Longitudinal muscles ahead of bolus contract, shortening adjacent segments • Wave of contractions in circular muscles that forces bolus forwardBSC2086 A&P II Final Exam Study Guide – Lesson 16-18 4 o Segmentation: Occurs in most areas of small intestine & some areas of large intestine Cycles of contraction • Churn & fragment the bolus • Mix contents with intestinal secretions Doesn’t follow a set pattern • Does NOT push materials in any one direction • Control of Digestive Functions o Local Factors Prostaglandins, histamine, & other chemicals released into interstitial fluid May affect adjacent cells within small segment of digestive tract • Ex) Histamine release at lamina propria causes increase in stomach acid release Coordinate response to changing conditions • Variations in local pH • Chemical or physical stimuli o Intestine wall stretching causes local contraction Affect only a portion of the GI tract o Neural MechanismsBSC2086 A&P II Final Exam Study Guide – Lesson 16-18 5 Motor neurons located in myenteric plexus • Short Reflexes: Responsible for local reflexes o Operate entirely outside of CNS control o Control smooth muscle contraction (peristalsis) & glandular secretion (secretory functions) • Long Reflexes: CNS control of digestive & glandular activities o Involves CNS interneurons and motor neurons o May involve parasympathetic fibers that synapse onto myenteric plexus o Controls large-scale peristaltic waves o Hormonal Mechanisms At least 18 peptide hormones produced by enteroendocrine cells in digestive tract • Affect: o Most aspects of digestive function o Activities of other systems • Reach target organs after distribution in bloodstream The Oral Cavity • Functions of the Oral Cavity: o Sensory analysis of material before swallowing o Mechanical processing through actions of teeth, tongue, and palatal surfaces o Lubrication – mixing with mucus & salivary gland secretions o Limited digestion of
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