Anatomy and Physiology of the Gastrointestinal System Notes


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Gastrointestinal system anatomy and physiology

NOTES NOTES ANATOMY & PHYSIOLOGY ANATOMY ▪ Alimentary/GI tract: continuous muscular tube from mouth to anus ▪ Many digestive organs reside in abdominal, pelvic cavity; covered by mesentery PERITONEUM ▪ Thin connective tissue composed of mesothelium, connective tissue supporting layer, simple squamous epithelium ▪ Lines abdominal, pelvic cavities; binds organs together, holds them in place ▪ Contains blood vessels, lymphatics, nerves innervating abdominal organs ▫ Parietal peritoneum: lines abdominal, pelvic cavities ▫ Visceral peritoneum: covers organ surfaces ▫ Peritoneal cavity: potential space between parietal, visceral layers ▪ Intraperitoneal organs: digestive organs; keep mesentery during embryological development, remain in peritoneal cavity (e.g. stomach) ▪ Retroperitoneal organs: lose mesentery during embryological development, lay posterior to peritoneum (e.g. kidneys, pancreas, duodenum) ▪ Mesentery: double layer of parietal peritoneum on dorsal peritoneal cavity, provides routes for vessels, lymphatics, nerves to digestive organs Omentum ▪ Visceral peritoneum layer covering stomach, intestines; contains adipose tissue, many lymph nodes ▫ Expands during weight gain; “fat skin” ▪ Lesser omentum: double layer arises from lesser curvature of stomach, extends to liver ▪ Greater omentum: four layers (double sheet folds back upon itself); arises from greater curvature of stomach, covers intestines GI tract layers ▪ Four basic tissue layers from esophagus to anus ▪ Serosa/adventitia ▫ Outermost layer of intraperitoneal organs; also visceral peritoneum ▫ Primarily composed of simple squamous epithelial cells, connective tissue ▫ Secretes slippery fluid, prevents friction between viscera, digestive organs ▫ Esophagus has adventitia instead of serosa ▫ Retroperitoneal organs have serosa, adventitia ▪ Muscularis propria ▫ Outer longitudinal, inner circular smooth muscle for involuntary contractions; regions of thickened circular layer forms sphincters ▫ Skeletal muscle in esophagus for voluntary swallowing ▫ Contains myenteric plexus (between longitudinal, circular layers of smooth muscle) ▫ Myenteric plexus responsible for peristalsis, mixing ▪ Submucosa ▫ Connective tissue that binds muscularis, provides elasticity, distensibility ▫ Contains Meissner’s plexus ▫ Richly vascularized, innervated OSMOSIS.ORG 293
▪ Mucosa ▫ Innermost layer composed of epithelial membrane lining entire GI tract ▫ Functions: exocrine glands secrete water, mucus, digestive enzymes, hormones; absorb digested nutrients; provides protective surface ▫ Muscularis mucosae: smooth muscle layer responsible for mucosa movement; contains folds to increase surface area ▫ Lamina propria: loose areolar connective tissue; contains blood, lymphatic vessels; contains MALT (lymphoid tissue that protects against pathogens) ▫ Epithelium: mouth, esophagus, anus composed of stratified squamous cells; rest of GI tract simple columnar with mucus secreting cells BLOOD CIRCULATION ▪ Splanchnic circulation ▪ Celiac trunk: supplies stomach, liver, spleen ▪ Superior mesenteric artery: supplies small intestine ▪ Inferior mesenteric artery: supplies large intestine INNERVATION ▪ Supplied by autonomic nervous system (ANS) ▪ Sympathetic component: thoracic splanchnic nerves → celiac plexus ▪ Parasympathetic component: vagus nerve ▪ Enteric division provides local control of GI activity; “the brain in the gut”; can function independently of ANS Figure 36.1 Cross section from small intestine showing the four basic tissue layers that line gastrointestinal tract: (from the outermost) serosa/adventitia, muscularis propria, submucosa, and mucosa. 294 OSMOSIS.ORG
Chapter 36 Gastrointestinal System: Anatomy & Physiology STRUCTURES ORAL (BUCCAL) CAVITY Function ▪ Ingestion, mechanical, chemical digestion, propulsion ▪ Saliva contains antibacterial properties that cleanses, protects oral cavity, teeth from infection ▪ Propulsion: swallowing (performed by tongue) propels food into pharynx, starts propulsion through GI tract ▪ Mechanical digestion: via mastication by teeth, tongue ▪ Chemical digestion: salivary amylase starts carbohydrate chemical breakdown Secretions ▪ Chemical digestion: salivary amylase starts carbohydrate chemical breakdown; mucin, water provide lubrication ▪ Lysozyme: kills some microbes ▪ Lingual lipase: digests some lipids ESOPHAGUS ▪ Muscular tube extending from laryngopharynx to stomach ▪ Esophageal hiatus: diaphragm opening where esophagus, vagus nerve pass through to abdominal cavity ▪ Cardiac orifice: junction of esophagus, stomach Function ▪ Propulsion/peristalsis ▪ Epiglottis closes larynx, routes food into esophagus ▪ Lower end of esophagus contains mucous cells to protect esophagus from stomach acid reflux Sphincters ▪ Upper esophageal sphincter: skeletal muscle; regulates movement from pharynx to esophagus ▪ Cardiac sphincter: AKA lower esophageal sphincter; smooth muscle at cardiac orifice that prevents acidic contents of stomach from moving upward into esophagus Histology ▪ Mucosa ▫ Nonkeratinized stratified squamous epithelium (simple columnar epithelium near cardiac orifice) ▪ Mucosa, submucosa form longitudinal folds when empty ▪ Submucosa ▫ Mucus secreting glands ▪ Muscularis externa ▫ Superior ⅓: skeletal muscle ▫ Middle ⅓: skeletal, smooth muscle ▫ Inferior ⅓: smooth muscle ▪ Adventitia instead of serosa Secretions ▪ Mucus: lubrication, protection from gastric acid STOMACH ▪ Located in upper left abdominal cavity quadrant ▪ Contains rugae (mucosa, submucosa) when stomach empty → expands to accommodate food Function ▪ Churning, digestion, storage ▪ Beginning of chemical digestion turning food into chyme to be delivered into small intestine Regions ▪ Cardia: most superior area surrounding cardiac orifice where food from esophagus enters stomach ▫ Defined by Z-line of gastroesophageal junction ▫ Z-line: epithelium changes from stratified squamous → simple columnar OSMOSIS.ORG 295
▪ Fundus: area lying inferior to diaphragm, upper curvature ▫ Food storage ▪ Body: central, largest area of the stomach ▪ Pylorus: connects to duodenum via pyloric sphincter ▫ Controls gastric emptying, prevents backflow from duodenum into stomach Histology ▪ Muscularis contains regular GI tract layers with three-layered muscularis propria unique to stomach allowing for vigorous contractions, churning ▫ Inner oblique layer ▫ Middle circular layer (contains myenteric plexus) ▫ Outer longitudinal layer Glands ▪ Lined with simple columnar epithelium; forms gastric pits (tube-like opening for gastric glands) ▪ Cardia, pylorus glands mainly secrete mucus ▪ Fundus, body glands secrete majority of digestive stomach secretions ▪ Pyloric antrum glands mainly secrete mucus, hormones (mainly gastrin) Figure 36.2 Stomach anatomy. 296 OSMOSIS.ORG Secretions ▪ Mucous cells: neck, basal regions of glands; produce mucus that protects stomach lining, lubricates food ▪ Parietal cells: gland apical region amongst chief cells; produce HCl, intrinsic factor ▪ Chief cells: gastric gland base; produce pepsinogen (protein digestion) ▪ Enteroendocrine cells (ECL cells): located deep in glands; secretes histamine, somatostatin, serotonin, ghrelin ▪ G-cells: gastrin ▪ D-cells: somatostatin SMALL INTESTINE Function ▪ Primary organ of digestion, nutrient absorption; segmentation (localized mixing area), peristalsis ▪ Absorption: food breakdown products absorbed ▪ Contains circular folds, villi, microvilli to maximize absorption surface area ▫ Circular folds are permanent, composed of mucosa, submucosa
Chapter 36 Gastrointestinal System: Anatomy & Physiology Innervation ▪ Relayed through celiac, superior mesenteric plexus ▪ Sympathetic: thoracic splanchnic ▪ Parasympathetic: vagus Blood supply ▪ Arterial: superior mesenteric artery ▪ Veins from small intestine → hepatic portal vein → liver Histology ▪ Epithelium of villus: simple columnar absorptive cells ▫ Main function is absorbing nutrients ▪ Mucus secreting goblet cells in epithelium ▪ Mucosa contains pits called intestinal crypts ▫ Crypt cells: secrete intestinal juice containing mucus ▫ Enteroendocrine cells: within crypts, intraepithelial lymphocytes (T cells) ▫ Paneth cells: located deep in crypts, release defensins, lysozyme to protect against pathogens Sections ▪ Duodenum ▫ Mostly retroperitoneal ▫ Curves around head of pancreas, receives bile from liver via bile duct, pancreatic secretions from pancreas via main pancreatic duct ▫ Ampulla of vater: bulb-like point where bile duct, main pancreatic duct unite, deliver secretions into duodenum ▫ Major duodenal papilla: ampulla opening into duodenum releasing bile/ pancreatic secretions ▫ Hepatopancreatic sphincter: controls bile entry, pancreatic secretions ▫ Duodenal glands (Brunner’s) in duodenal submucosa secrete alkaline mucus to neutralize acidic chyme ▪ Jejunum ▫ Intraperitoneal ▫ Suspended from posterior abdominal wall by mesentery ▪ Ileum ▫ Intraperitoneal ▫ Joins large intestine at ileocecal valve ▫ Suspended from posterior abdominal wall by mesentery ▫ Peyer’s patches: lymphatic tissue sections composed predominantly of proliferating B lymphocytes, mostly located in ileal lamina propria as protection against pathogenic bacteria; B lymphocytes release IgA Secretions ▪ Brush border enzymes on microvilli complete food digestion (e.g. mucus, water, peptidases, disaccharidases) ▪ Pancreas, liver contribute to most small intestine digestion LARGE INTESTINE ▪ Retroperitoneal except for transverse, sigmoid parts ▫ Intraperitoneal transverse, sigmoid sections anchored to posterior abdominal wall by mesocolon (mesentery) ▫ Connects ileum via ileocecal valve, sphincter Function ▪ Digestion, absorption, propulsion, defecation ▪ Digestion: enteric bacteria digests remaining food ▫ Bacteria also produce vitamin K, other B vitamins ▪ Absorption: absorbs mainly water, electrolytes, vitamins to concentrate, form feces ▪ Propulsion: propels feces towards rectum ▪ Defecation: stores, eliminates feces from body Unique features ▪ Tenia coli: three longitudinal ribbons of smooth muscle on ascending, transverse, descending, sigmoid colons that contract to produce haustra ▪ Haustra: small pouches/segments of large intestine created by tenia coli ▪ Epiploic appendages: small pouches of peritoneum filled with fat OSMOSIS.ORG 297
Regions ▪ Cecum → ascending colon → right colic/ hepatic flexure → transverse colon → left colic/splenic flexure → descending colon → sigmoid colon → rectum → anal canal → anus ▫ Cecum: pouch that lies below ileocecal valve at large,small intestine junction; beginning of large intestine ▫ Appendix: pouch of lymphoid tissue (part of MALT) located in cecum, harbors bacteria to recolonize gut when needed ▪ Anal canal has two sphincters ▫ Internal anal sphincter: involuntary, composed of smooth muscle ▫ External anal sphincter: voluntary, composed of skeletal muscle Flora ▪ Large intestine contains largest bacterial ecosystem in body ▪ Function of bacteria ▫ Synthesize vitamins (vitamin K, some B vitamins) ▫ Ferment indigestible carbohydrates (e.g. cellulose) ▫ Metabolism/digestion of certain molecules (e.g. hyaluronic acid, mucin) ▫ Live symbiotically with host ▫ Present pathogens to nearby lymphoid tissue (MALT) Secretions ▪ Mucus Histology ▪ Muscularis mucosae consists of inner circular, outer longitudinal layers ▪ Large intestine mucosa: simple columnar epithelium ▪ Anal canal: stratified squamous epithelium ▪ Does not contain folds, villi, microvilli as in small intestine ▪ Many crypts with goblet cells Pectinate line ▪ Divides upper ⅔ from lower ⅓ of anal canal where many distinctions made ▪ Embryological origin ▫ Above: endoderm ▫ Below: ectoderm ▪ Epithelium ▫ Above: columnar epithelium ▫ Below: stratified squamous epithelium ▪ Innervation ▫ Above: inferior hypogastric plexus ▫ Below: inferior rectal nerves ▪ Lymph drainage ▫ Above: internal iliac ▫ Below: superficial inguinal lymph nodes ▪ Vascularization ▫ Above: superior rectal artery, superior rectal vein (drains into inferior mesenteric vein → hepatic portal system) ▫ Below: middle, inferior rectal arteries; middle, inferior rectal veins 298 OSMOSIS.ORG Figure 36.3 Large intestine anatomy.
Chapter 36 Gastrointestinal System: Anatomy & Physiology ACCESSORY ORGANS ▪ Gallbladder, liver, pancreas ▪ Liver ▫ Hepatocytes produce bile which emulsifies lipid globules, aids in absorption ▫ Stores glucose in form of glycogen ▪ Gallbladder ▫ Bile storage; releases bile into small intestine in response to hormonal stimulus ▪ Pancreas ▫ Exocrine function: acini secrete various digestive enzymes; “pancreatic juice;” e.g. secretin, cholecystokinin (CCK) ▫ Endocrine function: islets produce glucagon, insulin to maintain normal glucose levels; somatostatin, pancreatic polypeptide production Figure 36.4 Overview of gastrointestinal tract, accessory organs structures. PHYSIOLOGY PROCESSING OF FOOD 1. Ingestion 2. Mechanical digestion ▫ Carried out by teeth; increases surface area to facilitate enzymatic digestion 3. Propulsion ▫ Movement, mixing of food through GI tract, starts with swallowing 4. Secretion ▫ Exocrine glands secrete various digestive juices into digestive tract lumen 5. Digestion ▫ Complex food broken down via enzymes 6. Absorption ▫ Digested nutrients absorbed by GI mucosal cells into blood/lymph 7. Elimination ▫ Indigestible substances eliminated via anus in form of feces GI MUSCLE PROPERTIES ▪ Smooth muscle of GI tract acts as syncytium ▫ Muscle fibers connected by gap junctions allowing electrical signals to initiate muscle contractions from one muscle fiber to next rapidly along length of bundle ▪ Normal resting membrane potential of GI smooth muscles: -50mV to -60mV ▪ Two types of electrical waves contributing to membrane potential Slow waves ▪ Generated, propagated by interstitial cells of Cajal (pacemaker cells) ▪ Slow-wave threshold: potential that must be reached by slow wave to propagate smooth muscle ▪ Does not cause smooth muscle contraction ▪ Slow-wave threshold reached → L-type calcium channels activated → calcium influx → motility initiation OSMOSIS.ORG 299
▪ Occur at 12 cycles/minute in duodenum, decreases towards colon ▪ Regulated by innervation, hormones ▫ Excitatory stimulants (e.g. acetylcholine, substance P), inhibitory stimulants (e.g. VIP, nitric oxide) Spikes ▪ True action potentials occurring automatically when GI smooth muscle potential becomes more positive than -40mV ▪ Digestive activity controls ▫ Involves regulation by autonomous smooth muscle, intrinsic nerve plexuses, external nerves (ANS), GI hormones ENTERIC NERVOUS SYSTEM ▪ Intrinsic nervous system of the GI system ▪ Division of ANS ▪ Provides major nerve supply to GI tract controlling GI function, motility ▫ Parasympathetic system activates digestion ▫ Sympathetic system inhibits digestion ▫ Also capable of self-regulation, autonomous function Receptors and plexus ▪ Chemoreceptors respond to chemicals from food in gut lumen ▪ Stretch receptors respond to food distending GI tract wall ▪ Two plexus consist of motor neurons, interneurons, sensory neurons ▫ Submucosal (Meissner’s) nerve plexus: innervates secretory cells → controls digestive secretions ▫ Myenteric nerve plexus: innervates smooth muscle layers of muscularis → controls GI motility ▪ Segmentation, peristalsis mostly automatic mediated by pacemaker cells, reflex arcs Reflex mediation ▪ Short reflexes: intrinsic control (enteric nervous system) ▪ Long reflexes: extrinsic control outside of GI tract (e.g. CNS, autonomic nerves) 300 OSMOSIS.ORG GASTROINTESTINAL MOTILITY Gastric motility ▪ Peristaltic contractions originate in upper fundus, move to pyloric sphincter ▪ Moves gastric chyme forward → gastric emptying into duodenum Small intestinal motility ▪ Mix chyme, digestive enzymes, pancreatic secretions, bile → digestion ▪ Expose nutrients to mucosa → maximize absorption ▪ Advance chyme along small intestine via segmentation actions → ileocecal valve → ileocecal sphincter → large intestine Large intestinal motility ▪ Unabsorbed small intestine material → large intestine ▫ Contents now feces (destined for excretion) ▪ Segmental contractions (cecum, proximal colon) associated with haustra (sac-like segments characteristic of large intestine) mixes contents ▪ Mass movements ▫ Function: move contents long distances (e.g. transverse → sigmoid) ▫ Occur 1–3 times daily ▫ Water absorption: fecal contents → increasingly solid (hard to mobilize) ▫ Final mass movements propel contents to rectum → stored until defecation ▪ Gastrocolic reflex ▫ Stomach distension → ↑ colonic motility → ↑ mass movements ▫ Afferent limb (from stomach) → parasympathetic nervous system mediates → efferent limb → CCK, gastrin production → ↑ colonic motility
Chapter 36 Gastrointestinal System: Anatomy & Physiology ▪ Defecation ▫ Rectum 25% full → defecation urge ▫ Rectum fills with feces → rectal wall distends → stretch receptors send afferent signals to spinal cord → to brain (awareness of need to defecate) + afferent signals to myenteric plexus → peristaltic waves → move feces forward → internal anal sphincter relaxes → external anal sphincter remains tonically contracted (striated skeletal muscle under voluntary control) → when appropriate, external anal sphincter relaxed voluntarily → rectal smooth muscle contracts → ↑ pressure → Valsalva maneuver (expire against closed glottis) → ↑ intra-abdominal pressure → ↑ defecation pressure → feces forced out through anal canal OSMOSIS.ORG 301

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This Osmosis High-Yield Note provides an overview of Anatomy and Physiology of the Gastrointestinal System essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Anatomy and Physiology of the Gastrointestinal System by visiting the associated Learn Page.