Alimentary System – Comprehensive Review

Terminology

• Ampulla = Dilated end of a duct

• Concave = Curves inward

• Convex = Curves outward

• Gastro- = Relating to stomach

• Hepato- = Relating to liver

• Vestibule = Chamber / channel that opens into another

Clinical Importance for Medical Professionals

• Mastery of abdominal procedures (surgery, endoscopy, radiology)

• Improves assessment, differential diagnosis & clinical communication

• Foundation for research into digestion, nutrition, GI pathology

Overview of Content (Organs & Topics)

• Oral Cavity – Lip → Oropharynx

• Salivary Glands – Parotid, Sub-mandibular, Sublingual

• Palate – Hard vs. Soft (with muscles)

• Tongue – Gross anatomy, papillae, muscles, neurovascular supply

• Esophagus

• Stomach

• Small Intestine – Duodenum, Jejunum, Ileum

• Large Intestine – Caecum → Anal canal

• Accessory Glands – Liver, Gall bladder, Pancreas

General Principles of the Alimentary Tract

• Continuous epithelium-lined muscular tube, mouth → \text{anal canal}

• Glands embedded in wall secrete enzymes, mucus, hormones

• Major extramural glands: 3 pairs of salivary glands, liver, gall-bladder, pancreas

Functional Process of Digestion

1. Mechanical breakdown by teeth

2. Propulsion (swallowing & peristalsis)

3. Chemical digestion via enzymes ⇒ macromolecules → absorbable units

4. Absorption through epithelium → blood / lymph

5. Indigestible residue expelled as faeces

ORAL CAVITY

• Bounded by 2 lips anteriorly

• Sub-regions
  • Oral cavity proper (inside dental arches)
  • Vestibule (teeth ↔ cheeks)

• Dentition per quadrant: 2 incisors, 1 canine, 2 premolars, 3 molars

Roof & Floor

• Roof: Hard palate (maxilla + palatine bone) & Soft palate (no bone, ends in uvula)

• Floor: Muscular (mylohyoid oral diaphragm) + tongue

Palate Musculature (Soft Palate)

• Tensor veli palatini

• Levator veli palatini

• Palatoglossus

• Palatopharyngeus

• Musculus uvulae

SALIVARY GLANDS

• Parotid – Largest, serous, opens via Stensen’s duct, target of mumps virus

• Sub-mandibular – Mixed, opens via Wharton’s duct, common site of calculi

• Sublingual – Mucous, many small ducts on sublingual fold

TONGUE

• Apex (free) & Root (fixed); dorsal surface = dorsum

• Landmarks: sulcus terminalis, foramen caecum, oral part (ant \tfrac{2}{3}), pharyngeal part (post \tfrac{1}{3}), median/lateral glosso-epiglottic folds, valleculae

• Papillae
  • Filiform (most numerous; tactile)
  • Fungiform (taste buds)
  • Circumvallate (≈ 8!–!12 in V-shape ant. to sulcus terminalis)
  • Foliate (posterolateral)

• Taste zones: Sweet (tip), Salty (lateral anterior), Sour (lateral posterior), Bitter (posterior)

Muscles

• Extrinsic: Styloglossus (up/back), Palatoglossus (shuts oropharynx), Genioglossus (protrudes), Hyoglossus (depresses)

• Intrinsic: Longitudinal, Transverse, Vertical – alter shape

• Motor nerve: Hypoglossal (CN XII) except Palatoglossus (pharyngeal plexus via cranial Accessory CN XI + Vagus)

Vascular & Neural Supply

• Artery: Lingual (ECA branch)

• Veins: Lingual → Internal jugular

• Sensation: Ant \tfrac{2}{3} – Lingual n.; Post \tfrac{1}{3} – Glossopharyngeal; Root/epiglottic – Vagus

• Taste: Ant \tfrac{2}{3} – Chorda tympani; remainder as above

ESOPHAGUS

• Length \approx 25\,\text{cm}, from C6 (cricoid) → T11 (cardia of stomach)

• Layers: Mucosa, Submucosa, Muscularis (upper 1/3 striated, middle mixed, lower 1/3 smooth)

• Constrictions (from upper incisors)
  • 15\,\text{cm} – Cricopharyngeal
  • 23\,\text{cm} – Aortic arch / Left bronchus
  • 40\,\text{cm} – Diaphragmatic (LES)

• Blood supply & drainage differ in neck, thorax, abdomen (inferior thyroid, aortic branches, left gastric)

• Nerve: Vagal trunks + sympathetic chains (forms esophageal plexus)

STOMACH

• Muscular J-shaped organ; capacity: newborn 30\,\text{mL}, adult 1000!–!1500\,\text{mL}

• Parts: Cardia, Fundus, Body, Pylorus (antrum + canal)

• Orifices: Cardiac (T11, no anatomical sphincter) & Pyloric (L1, true sphincter)

• Curvatures: Greater vs. Lesser (angular/incisura notch)

• Layers of muscle: Outer longitudinal, Middle circular, Inner oblique

• Functions: Food reservoir, enzymatic digestion, limited absorption (water, \text{glucose}, alcohol), gastrin secretion; emptying time 3!–!4 h

• Blood
  • Arteries: Left & Right Gastric, Left & Right Gastro-epiploic, Short Gastrics
  • Veins: Parallel arteries → Portal system
  • Lymph: Along arteries → Celiac nodes

• Nerves: Parasympathetic (Anterior & Posterior vagal trunks – N. of Latarjet & Grassi), Sympathetic (T6–T9)

• "Rule of 2’s" mnemonic: 2 ends, 2 surfaces, 2 curvatures, 2 openings, 2 sphincters, 2 vagal trunks, 2 sets of gastric + gastro-epiploic arteries, volume ≈ 2 pints

• Glands: Cardiac (mucus), Fundic/Body (chief, oxyntic, mucus neck), Pyloric (mucus + gastrin)

SMALL INTESTINE

• Extends Pylorus → Ileo-cecal junction; length ≈ 6!–!8\,\text{m}

• Duodenum (\approx 25\,\text{cm}: 1st 5\,\text{cm}, 2nd 7.5\,\text{cm}, 3rd 12.5\,\text{cm}, 4th 2.5\,\text{cm})
  • Fixed retroperitoneal except initial cap; related closely to pancreas, bile / pancreatic ducts empty at Ampulla of Vater (Sphincter of Oddi)
  • Blood: Superior (gastroduodenal) & Inferior (SMA) pancreatico-duodenal vessels

• Jejunum (proximal \tfrac{2}{5}) vs. Ileum (distal \tfrac{3}{5})
  • Jejunum – wide lumen, thick wall, tall plicae, long vasa recta, less mesenteric fat, no Peyer’s patches
  • Ileum – narrow, thin, sparse folds, many arterial arcades, more fat, Peyer’s patches

• Ileal (Meckel’s) diverticulum = persistent vitelline duct remnant (antimesenteric border, rule of 2: 2\% pop., 2 feet from ileocecal valve, 2 inches long)

LARGE INTESTINE

• Ileo-cecal junction → Anal canal; functions: water absorption, vitamin synthesis (flora), fecal storage

• Characteristic features: Taeniae coli (3 bands), Haustra (sacculations), Appendices epiploicae (fat pouches)

Caecum & Appendix

• Caecum: blind sac 6\,\text{cm} long × 7.5\,\text{cm} wide; RLQ; ileocecal valve guards opening

• Appendix: average 9\,\text{cm}; base at McBurney’s point; variable tip (retrocecal most common)

• Blood: Ileocolic → Anterior/Posterior cecal branches; Appendicular artery (end-artery)

• Referred pain at umbilicus (T10)

Colon Segments

• Ascending (15\,\text{cm}), Retroperitoneal → Hepatic flexure

• Transverse (\approx 50\,\text{cm}), intraperitoneal with transverse mesocolon; splenic flexure higher, attached to diaphragm via phrenico-colic ligament

• Descending (25\,\text{cm}) Retroperitoneal → Iliac fossa

• Sigmoid (40\,\text{cm}) intraperitoneal, sigmoid mesocolon (inverted V root over left common iliac bifurcation)

• Blood: SMA branches (ileocolic, right colic, middle colic) for midgut colon; IMA branches (left colic, sigmoid) for hindgut; marginal artery of Drummond provides collateral

Rectum

• Starts at S3, ends at anorectal junction; length 12\,\text{cm}

• No taeniae, haustra, epiploic fat; three lateral curves & A-P curves (sacral, perineal)

• Peritoneum: upper 1/3 anterior & lateral; middle 1/3 anterior only; lower 1/3 none

• Folds: 3 transverse (Houston’s valves)

• Blood: Superior (IMA), Middle (internal iliac), Inferior (internal pudendal) rectal arteries; venous plexus forms portal-systemic anastomosis (clinical hemorrhoids)

Anal Canal (Length 3.8\,\text{cm})

• Upper 15\,\text{mm} (columnar mucosa, columns/valves of Morgagni, endoderm)

• Pectinate line – embryology, blood, lymph & nerve watershed

• Intermediate 15\,\text{mm} – non-keratinised stratified squamous

• Lower 8\,\text{mm} – true skin (anoderm)

• Sphincters: Internal (involuntary) vs. External (voluntary, pudendal nerve)

ASSOCIATED GLANDS

Pancreas

• Retroperitoneal; head–body–tail; dual exocrine (digestive enzymes via main pancreatic duct) & endocrine (islets) functions

• Joins common bile duct → Ampulla of Vater in 2nd part of duodenum (Sphincter of Oddi)

Liver

• Largest gland; four lobes; suspended by falciform ligament; porta hepatis contains hepatic artery, portal vein, bile ducts

• Microscopic unit: Hepatic lobule with central vein, sinusoids, cords of hepatocytes, Kupffer & stellate cells; bile canaliculi drain to bile ductules

• Produces bile (salts, bilirubin, cholesterol, phospholipids, electrolytes)

Gall Bladder

• Pear-shaped sac in gall-bladder fossa; concentrates bile \times 10 via water absorption

• Releases bile via cystic → common bile duct on cholecystokinin stimulus (fatty chyme)

• Gallstones may obstruct ducts / ampulla

SELF-PRACTICE: Jejunum vs. Ileum (Key Points)

• Diameter: Jejunum wide, Ileum narrow

• Wall: Jejunum thick, Ileum thin

• Circular folds: Numerous/tall vs. sparse/low/absent distally

• Arteries: Few arcades, long vasa recta vs. many arcades, short vasa

• Mesenteric fat: Less near intestine in jejunum, abundant in ileum

• Peyer’s patches: Absent vs. Present (aggregated lymphoid nodules)

ETHICAL & CLINICAL CORRELATIONS

• Endoscopy landmarks: Esophageal constrictions, gastric canal (Magenstrasse)

• Surgical significance: McBurney’s point (appendicitis), Taeniae converge to appendix (intra-op identification), Pre-pyloric vein of Mayo (locates pylorus)

• Portal–systemic anastomosis at lower esophagus & rectum – implications for varices & hemorrhoids in portal hypertension

• Gallstones & biliary colic – blockage of cystic/common bile duct or Ampulla leads to jaundice, pancreatitis

• Meckel’s diverticulum – mimic appendicitis; rule of 2

SUMMARY

The alimentary system is a continuous, region-specialised tube supported by accessory glands and neurovascular networks. Understanding its layered structure, vascular territories, innervation, embryological divisions, and surface anatomy is critical for diagnostics, surgical intervention, and managing pathologies ranging from reflux and peptic ulcer to inflammatory bowel disease and hepatobiliary disorders.