Anatomy and Physiology of the Gastrointestinal System Notes
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NOTES NOTES ANATOMY & PHYSIOLOGY ANATOMY osms.it/gastrointestinal-anatomy-physiology ▪ 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 osms.it/gastrointestinal-anatomy-physiology 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 osms.it/gastrointestinal-anatomy-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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