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Digestive System Β· Anatomy
Digestive System
Anatomy Study Notes
15 Concepts
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expand β†— Overview & Two Organ Groups Key Terms
GI tractalimentary canalaccessory digestive organsmuscle tonus
GI Tract (Alimentary Canal)
  • Continuous tube: mouth β†’ anus through thoracic + abdominal cavities
  • Organs: Mouth, most of pharynx, esophagus, stomach, small intestine, large intestine
  • Living length: 16.5–23 ft (muscle tonus keeps it shorter) Β· Cadaver: 23–29.5 ft
Analogy: Long assembly line for food processing
Accessory Digestive Organs
  • Structures: Teeth, tongue, salivary glands, liver, gallbladder, pancreas
  • Direct contact: Teeth + tongue touch food Β· Secretory: others produce/store secretions via ducts
Mnemonic: Gastrointestinal Accessory
expand β†— Six Basic Processes of the GI Tract Key Terms
ingestionsecretionmixing & propulsionmechanical digestionchemical digestionabsorptiondefecationhydrolysis
Mnemonic: In Some Mixed Dishes All Digest
1Ingestion β€” taking in foods and liquids (eating)
2Secretion β€” water, acid, buffers, enzymes into intestinal lumen
3Mixing & Propulsion β€” smooth muscle contractions mix and move food
4Digestion β€” Mechanical (cutting, grinding, churning) + Chemical (hydrolysis by enzymes = catabolism)
5Absorption β€” crossing epithelial barrier from lumen β†’ blood/lymph
6Defecation β€” elimination of feces (wastes, indigestible material, bacteria, sloughed cells)
expand β†— Layers of the GI Tract Wall Key Terms
mucosasubmucosamuscularisserosa / adventitialamina propriamuscularis mucosaemyenteric plexusNKSSET
Mnemonic (deep β†’ superficial): Most Surgeons Make Serious
1. Mucosa (Innermost)
  • Epithelium: NKSSET (mouth, pharynx, esophagus, anal canal) β€” abrasion protection Β· Simple columnar (stomach, intestines) β€” secretion + absorption
  • Lamina propria: Areolar CT with blood/lymphatic vessels + MALT tissue (immunoprotection)
  • Muscularis mucosae: Smooth muscle β€” folds mucosa to ↑ surface area
Mucosa mnemonic: Every Lawyer Makes money (Epithelium, Lamina propria, Muscularis mucosae)
2. Submucosa
  • Areolar CT Β· Binds mucosa to muscularis Β· Blood/lymphatic vessels Β· Submucosal plexus (Meissner's) controls secretion
3. Muscularis
  • Skeletal: Mouth, pharynx, superior/middle esophagus, external anal sphincter
  • Smooth (rest): Inner circular + outer longitudinal Β· Myenteric plexus (Auerbach's) between layers β€” controls motility
4. Serosa / Adventitia (Outermost)
  • Serosa: Visceral peritoneum β€” areolar CT + simple squamous epithelium
  • Adventitia: Areolar CT only (esophagus β€” no mesothelium)
expand β†— Neural Innervation & Enteric Nervous System Key Terms
enteric nervous systemmyenteric plexus (Auerbach's)submucosal plexus (Meissner's)vagus nerve (CN X)chemoreceptorsmechanoreceptors
Enteric Nervous System ("Brain of the Gut")
  • ~100 million neurons from esophagus to anus Β· Contains motor, interneurons, and sensory neurons
  • Myenteric plexus (Auerbach's): Between longitudinal + circular muscle layers β†’ controls GI motility (frequency/strength)
  • Submucosal plexus (Meissner's): In submucosa β†’ controls secretion
Autonomic Nervous System
  • Parasympathetic: Vagus nerve (CN X) β€” most of GI tract Β· Pelvic splanchnic nerves β€” last Β½ large intestine Β· Effect: ↑ secretion and motility
  • Sympathetic: Thoracic + upper lumbar cord Β· Effect: ↓ secretion and motility
Sensory Receptors
  • Chemoreceptors: Detect chemicals in food
  • Mechanoreceptors: Respond to stretch when food distends walls
expand β†— Peritoneum & Peritoneal Folds Key Terms
peritoneummesotheliumparietal peritoneumvisceral peritoneumgreater omentumlesser omentummesenteryfalciform ligamentretroperitoneal
Structure
  • Largest serous membrane in body Β· Simple squamous epithelium (mesothelium) + areolar CT
  • Parietal: Lines abdominal cavity wall Β· Visceral: Covers organs (their serosa) Β· Peritoneal fluid lubricates
Five Folds: Great Folds Link Mesenteries Magically
Five Major Peritoneal Folds
  • Greater Omentum ("Fat Apron"): Largest fold; drapes over transverse colon + small intestine; 4 layers; adipose + lymph nodes; expands with weight gain
  • Falciform Ligament: Attaches liver to anterior abdominal wall + diaphragm; contains ligamentum teres (remnant fetal umbilical vein); only organ attached anteriorly
  • Lesser Omentum: Anterior fold, stomach/duodenum β†’ liver; carries hepatic portal vein, common hepatic artery, common bile duct
  • Mesentery: Fan-shaped; binds jejunum + ileum to posterior wall; largest peritoneal fold; fat + vessels + lymph nodes
  • Mesocolon: Binds transverse + sigmoid colon to posterior wall; carries blood/lymphatic vessels
Retroperitoneal Organs
Mnemonic: Kidneys And Duodenum Posterior (KADP)
  • Kidneys Β· Ascending + descending colons Β· Duodenum Β· Pancreas
expand β†— Mouth Anatomy Key Terms
faucesoral vestibuleoral cavity properhard palatesoft palateuvulalabial frenulumpalatine tonsilslingual tonsilsbuccinator
Boundaries
Mnemonic: Cheeks Hard Tongue Lips Fauces
  • Lateral: Cheeks Β· Superior: Hard + soft palate Β· Inferior: Tongue Β· Anterior: Lips Β· Posterior: Fauces
Key Structures
  • Cheeks: External skin / internal NKSSET mucosa / buccinator muscle + CT between layers
  • Lips (Labia): External skin / internal NKSSET mucosa / orbicularis oris between Β· Labial frenulum attaches lip to gum
  • Oral Vestibule: Space between cheeks/lips and gums/teeth
  • Hard Palate (ant.): Maxillae + palatine bones Β· Bony partition between oral + nasal cavities
  • Soft Palate (post.): Arch-shaped muscular partition Β· Separates oropharynx from nasopharynx
  • Uvula: Finger-like muscle from soft palate Β· Elevates during swallowing to close nasopharynx
  • Palatine Tonsils: Between palatoglossal (ant.) and palatopharyngeal (post.) arches
  • Lingual Tonsils: Base of tongue
expand β†— Salivary Glands Key Terms
parotid glandsubmandibular glandsublingual glandsalivary amylaseIgAlysozymemucusserous
Major Salivary Glands (3)
GlandLocationDuctSecretion
ParotidInferior/anterior to ears, between skin + masseterPierces masseter β†’ opposite 2nd maxillary molarWatery serous + salivary amylase
SubmandibularFloor of mouth, medial + partly inferior to mandibleOpens lateral to lingual frenulumSerous + amylase + some mucus
SublingualBeneath tongue, superior to submandibularLesser sublingual ducts β†’ floor of mouthMostly mucus Β· minimal amylase
Saliva Composition
  • Water: 99.5% Β· Solutes: 0.5%
  • Ions: Na⁺, K⁺, Cl⁻, HCO₃⁻, phosphate Β· Urea, uric acid, mucus, IgA, lysozyme, salivary amylase
Small Salivary Glands
Mnemonic: Labial Buccal Palatal Lingual (in lips, cheeks, palate, tongue)
Clinical Note
Lysozyme and IgA in saliva provide first-line antimicrobial defense. Salivary amylase begins starch digestion in the mouth before stomach acid inactivates it.
expand β†— Tongue Anatomy Key Terms
median septumextrinsic musclesintrinsic muscleslingual frenulumpapillaelingual lipasehyoglossusgenioglossusstyloglossus
Structure
  • Skeletal muscle organ covered with mucous membrane Β· Median septum divides into symmetrical halves
  • Attachments: Hyoid bone, styloid process, mandible
  • Lingual Frenulum: Mucous membrane fold on undersurface; limits posterior tongue movement
Extrinsic Muscles β€” Move Tongue
Mnemonic: How Good Speech (Hyoglossus Β· Genioglossus Β· Styloglossus)
  • Move tongue side-to-side and in/out; form floor of mouth
Intrinsic Muscles β€” Change Shape
Mnemonic: Longitudinal Longitudinal Transverse Vertical
  • Alter tongue shape + size for speech and swallowing
Papillae
  • Lamina propria projections covered with stratified squamous epithelium
  • Upper + lateral tongue surfaces Β· Functions: Taste buds (taste receptors) + touch/friction receptors
Lingual Glands
  • Secrete mucus + watery serous fluid Β· Enzyme: Lingual lipase β€” breaks down triglycerides
expand β†— Pharynx & Esophagus Key Terms
nasopharynxoropharynxlaryngopharynxesophageal hiatusUESLESadventitia
Pharynx β€” Three Regions
Mnemonic: Nasty Old Larynx
  • Nasopharynx: Respiratory only Β· Posterior to nasal cavity, superior to soft palate
  • Oropharynx: Digestive + respiratory Β· Posterior to oral cavity
  • Laryngopharynx: Digestive + respiratory Β· Posterior to larynx
Food route: Mouth β†’ oropharynx β†’ laryngopharynx β†’ esophagus β†’ stomach
Esophagus β€” Structure
  • Collapsible tube Β· 10 inches long Β· Posterior to trachea
  • Esophageal hiatus at T10 β€” pierces diaphragm
Esophagus β€” Histology (Deep β†’ Superficial)
  • Mucosa: NKSSET epithelium (wear + tear protection) Β· Mucous glands near stomach
  • Submucosa: Areolar CT + blood vessels + mucous glands
  • Muscularis: Superior β…“ skeletal Β· Middle β…“ mixed Β· Inferior β…“ smooth
  • Adventitia: Areolar CT only (no mesothelium β€” anchors to mediastinum)
Two Sphincters
  • UES (Upper Esophageal): Skeletal muscle
  • LES (Lower Esophageal): Smooth muscle near heart
Clinical β€” GERD
LES weakness allows acidic stomach contents to reflux into esophagus. NKSSET lacks the mucous protection of gastric epithelium β†’ heartburn, esophagitis.
expand β†— Stomach Anatomy Key Terms
cardiafundusbodypyloric antrumpyloric canalpylorusrugaepyloric sphincterlesser curvaturegreater curvature
Functions
Mnemonic: Mixing Digestion Semisolid Absorption
  • Mixing chamber + holding reservoir Β· Digests starch, triglycerides, proteins
  • Converts semisolid bolus β†’ liquid (chyme) Β· Absorbs certain substances
Four Regions
Mnemonic: Cats Frequently Bother People
CardiaSurrounds esophageal opening
FundusRounded, superior portion left of cardia
BodyLarge central portion inferior to fundus
PyloricThree sub-regions: Antrum β†’ Canal β†’ Pylorus (β†’ duodenum)
Surface Features
  • Rugae: Mucosal folds β€” allow stretch to 2–3 liters Β· Pyloric sphincter: Smooth muscle β†’ duodenum
  • Lesser curvature: Concave superior border Β· Greater curvature: Convex inferior border
expand β†— Stomach Histology Key Terms
surface mucous cellsmucous neck cellschief cellsparietal cellsgastric pitspepsinogenintrinsic factorHClG cellsgastrin
1. Mucosa
  • Surface epithelium: Simple columnar Β· Surface mucous cells secrete mucus (protect stomach wall)
  • Gastric glands β€” three secretory cell types:
  • Mucous neck cells: Secrete mucus
  • Chief cells: Secrete pepsinogen + gastric lipase
  • Parietal cells: Secrete intrinsic factor (B12 absorption) + HCl
  • Gastric pits: Narrow channels collecting gland secretions β†’ stomach lumen
2. Submucosa
  • Areolar CT Β· G cells in pyloric antrum β€” neuroendocrine cells secreting gastrin
3. Muscularis (Unique β€” 3 Layers)
  • Outer longitudinal Β· Middle circular Β· Inner oblique (body only) β€” enables churning motion
4. Serosa
  • Lesser curvature β†’ extends as lesser omentum Β· Greater curvature β†’ extends as greater omentum
Clinical β€” Pernicious Anemia
Destruction of parietal cells (autoimmune or H. pylori) β†’ ↓ intrinsic factor β†’ impaired B12 absorption β†’ megaloblastic anemia and neurological deficits.
expand β†— Small Intestine Key Terms
duodenumjejunumileumileocecal sphinctercircular folds (plicae circulares)villimicrovilli / brush borderBrunner's glandsPeyer's patcheslactealcrypts of LieberkΓΌhn
Three Regions
Mnemonic: Dogs Jump Into
  • Duodenum: 12 inches Β· C-shaped Β· Retroperitoneal Β· Pyloric sphincter β†’ jejunum
  • Jejunum: 3 ft Β· "Empty" at death Β· Extends to ileum
  • Ileum: 6 ft Β· Terminal section Β· Joins large intestine at ileocecal sphincter
Histology β€” Cell Types (Crypts of LieberkΓΌhn)
  • Absorptive cells: Microvilli; absorb nutrients
  • Goblet cells: Secrete mucus
  • Paneth cells: Secrete lysozyme; phagocytosis; regulate gut microbes
  • Enteroendocrine: S cells (secretin) Β· CCK cells (cholecystokinin) Β· K cells (GIP)
  • Brunner's glands (submucosa): Alkaline mucus β€” neutralizes gastric acid in duodenum
  • Peyer's patches: Aggregated lymphatic follicles in ileum lamina propria
Surface Area Adaptations
Mnemonic: Circular Villi Microvilli
  • Circular folds (plicae circulares): Permanent ridges in mucosa + submucosa Β· Proximal duodenum β†’ middle ileum Β· Cause chyme to spiral
  • Villi: Finger-like projections (20–40/mmΒ²) Β· Core has arteriole, venule, capillaries, lacteal (lymph)
  • Microvilli (brush border): Apical membrane projections Β· 200 million/mmΒ² Β· Secrete brush border enzymes
expand β†— Large Intestine Key Terms
cecumascending/transverse/descending/sigmoid colonrectumanal canalhaustrateniae colivermiform appendixinternal / external anal sphincter
Functions
Mnemonic: Haustral Bacteria Absorption Formation Defecation
  • Haustral churning, peristalsis, mass peristalsis Β· Bacteria β†’ amino acids + B vitamins + vitamin K
  • Absorption of water, ions, vitamins Β· Formation + defecation of feces
Structure: 5 ft long Β· 2.5 in diameter
Regions: Cecum Colon Rectum Anal
  • Cecum: Pouch inferior to ileocecal sphincter Β· Vermiform appendix attached (with mesoappendix)
  • Ascending colon: Retroperitoneal Β· Right colic (hepatic) flexure
  • Transverse colon: NOT retroperitoneal Β· Below spleen
  • Descending colon: Retroperitoneal Β· Left colic (splenic) flexure
  • Sigmoid colon: NOT retroperitoneal Β· S-shaped β†’ rectum
  • Rectum: 6 in Β· Anterior to sacrum + coccyx Β· Temporary fecal storage
  • Anal canal: Last 2–3 cm Β· Anal columns (mucosal folds) Β· Internal sphincter (involuntary smooth) Β· External sphincter (voluntary skeletal)
Special Structures
  • Teniae coli: 3 thickened longitudinal muscle bands Β· Run the length of colon
  • Haustra: Pouches from tonic teniae contractions Β· Give colon "puckered" appearance
Clinical Note
Large intestine has NO circular folds or villi (unlike small intestine). Fewer microvilli on absorptive cells. Much less absorption β€” specialized for water reclamation and fecal compaction only.
expand β†— Pancreas Key Terms
acinipancreatic islets (of Langerhans)duct of Wirsungaccessory duct (Santorini)hepatopancreatic ampulla (ampulla of Vater)sphincter of Oddimajor duodenal papilla
Location & Components
  • Retroperitoneal Β· Posterior to greater curvature of stomach
  • Head: Near duodenal curve Β· Body: Superior + left of head Β· Tail: Tapered portion
Two Ducts
  • Pancreatic duct (Wirsung) β€” larger: Joins common bile duct β†’ forms hepatopancreatic ampulla (ampulla of Vater) β†’ enters duodenum at major duodenal papilla Β· Sphincter of Oddi regulates flow
  • Accessory duct (Santorini): Empties duodenum above ampulla
Histology β€” Two Functional Portions
  • Acini (99% β€” Exocrine): Secrete pancreatic juice (1,200–1,500 mL/day) β€” water, salts, NaHCO₃, digestive enzymes
  • Pancreatic islets (1% β€” Endocrine): Secrete glucagon (Ξ± cells), insulin (Ξ² cells), somatostatin, pancreatic polypeptide into blood
Clinical β€” Pancreatitis
Premature activation of pancreatic enzymes within the pancreas β†’ autodigestion. Common causes: gallstones blocking ampulla of Vater, or alcohol. Elevated serum amylase and lipase are diagnostic markers.
expand β†— Liver, Bile Pathway & Gallbladder
Liver β€” Location & Gross Anatomy
  • Below diaphragm Β· Right hypochondriac + epigastric regions
  • Primary lobes: Right + left, separated by falciform ligament
  • Additional lobes: Quadrate (inferior) + caudate (posterior) β€” both part of left lobe
  • Falciform ligament: Suspends liver Β· Contains ligamentum teres (remnant fetal umbilical vein)
  • Coronary ligaments: Narrow extensions of parietal peritoneum β†’ suspend from diaphragm
Liver Histology
  • Hepatocytes: 80% of liver Β· Specialized epithelial cells (5–12 sides) Β· Metabolic, secretory, endocrine roles
  • Hepatic laminae: 3D plates one cell thick, bordered by hepatic sinusoids
  • Hepatic sinusoids: Highly permeable capillaries Β· Receive oxygenated blood (hepatic artery) + nutrient-rich blood (hepatic portal vein)
  • Bile canaliculi: Grooves between hepatocytes Β· Secrete bile
  • Stellate reticuloendothelial cells (Kupffer cells): Macrophages β€” destroy worn RBCs, bacteria, foreign matter
  • Portal triad: Bile duct + hepatic artery branch + portal vein branch
Bile Pathway
Bile canaliculi β†’ bile ductules β†’ bile ducts β†’ R + L hepatic ducts β†’ common hepatic duct (exits liver) β†’ joins cystic duct β†’ common bile duct β†’ duodenum
Liver Blood Supply & Flow
  • Hepatic artery: Oxygenated blood
  • Hepatic portal vein: Deoxygenated, nutrient/drug/toxin-rich blood from GI + spleen
  • Flow: Hepatic sinusoids β†’ central vein β†’ hepatic veins β†’ IVC β†’ right atrium
Liver Functions (10)
  • Bile secretion Β· Carbohydrate metabolism (glycogen storage/release) Β· Lipid metabolism (emulsification) Β· Protein metabolism (deamination) Β· Drug/hormone processing Β· Bilirubin excretion Β· Bile salt synthesis Β· Storage (glycogen, vitamins A, B12, D, E, K) Β· Phagocytosis (Kupffer cells) Β· Vitamin D activation
Gallbladder
  • Pear-shaped sac Β· Depression on posterior liver surface
  • Fundus: Projects below liver's inferior border
  • Body: Central portion Β· stores and concentrates bile
  • Releases bile via cystic duct β†’ common bile duct β†’ duodenum
Hormonal Regulation
  • CCK (cholecystokinin): CCK cells in SI Β· Stimulus: amino acids + fatty acids in chyme β†’ stimulates bile release + pancreatic enzymes
  • Secretin: S cells in SI Β· Stimulus: acidic chyme β†’ stimulates NaHCO₃ secretion
  • Others: Motilin, substance P, bombesin (↑ motility) Β· VIP (ion/water secretion) Β· Somatostatin (inhibits gastrin)