Digestive System

Oral Cavity & Salivary Glands - Mouth Matters

• Teeth: Types: Incisors, Canines, Premolars, Molars. Sets: Deciduous (20), Permanent (32).
• Tongue: Muscular; taste (papillae: circumvallate, fungiform, foliate), speech, swallowing.
• Palate: Roof of mouth. Hard (anterior, bony), Soft (posterior, muscular, uvula).

Salivary Glands:

Pharynx & Esophagus - Gullet Gateway

• Pharynx:
  • Nasopharynx: Skull base to soft palate; Respiratory epithelium.
  • Oropharynx: Soft palate to epiglottis; Strat. Sq. Non-keratinized.
  • Laryngopharynx: Epiglottis to C6; Strat. Sq. Non-keratinized.
• Esophagus: ~25cm muscular tube.
  • Constrictions (📌 C6, T4/5, T10): Cricopharyngeal, Aortic/Bronchial, Diaphragmatic.
  • Histology: Mucosa (Strat. Sq. Non-keratinized), Submucosa, Muscularis Externa (striated → mixed → smooth), Adventitia.
  • Sphincters: UES (cricopharyngeus), LES (physiological).
  • Blood: Inf. thyroid, aortic, L. gastric aa. Nerves: Vagus, sympathetic chain.
  • Porto-systemic anastomosis: Lower esophagus (L. gastric vein ↔ azygos vein).

⭐ Killian's dehiscence: Posterolateral muscular defect between thyropharyngeus & cricopharyngeus (parts of inferior constrictor); common site of Zenker's diverticulum.

Stomach & Duodenum - Acidic Adventures

• Stomach: J-shaped; parts: cardia, fundus, body, pylorus. Curvatures: lesser, greater.
  • Histology: Gastric glands with key cells:

    Cell	Secretion
    Parietal	HCl, Intrinsic Factor
    Chief	Pepsinogen
    Mucous Neck	Mucus
    G-cells	Gastrin

  • Arterial: Celiac trunk. Venous: Portal vein. Lymph: Celiac nodes.
  • Nerve: Vagus (Parasympathetic), Celiac plexus (Sympathetic).
• Duodenum: C-shaped; 4 parts (D1-D4).
  • Features: D1-Brunner's glands (alkaline mucus); D2-Major Duodenal Papilla (Ampulla of Vater); Ligament of Treitz (suspends Duodenojejunal flexure).

⭐ Arteries: Lesser curve supplied by Left & Right Gastric Arteries; Greater curve by Left & Right Gastro-omental Arteries.

Small & Large Intestines - Absorption Arena

• Small Intestine (SI): Primary nutrient absorption.

  • Jejunum vs. Ileum:

    Feature	Jejunum	Ileum
    Wall	Thicker, more vascular	Thinner, less vascular
    Plicae Circulares	Large, numerous, closely set	Smaller, fewer, sparse distally
    Arterial Arcades	Fewer (1-2 tiers), long vasa recta	More (3-4+ tiers), short vasa recta
    Peyer's Patches	Few/Absent	Numerous, large

• Large Intestine (LI): Water absorption, waste compaction.

  • Features: Taeniae coli (3 longitudinal bands), Haustra (sacculations), Epiploic appendages (fat tags).
  • Parts: Cecum, Appendix, Colon (Ascending, Transverse, Descending, Sigmoid), Rectum, Anal Canal.
  • Blood Supply: SMA (to splenic flexure), IMA (distal).
  • Watershed areas: Splenic flexure (Griffiths' point), Rectosigmoid junction (Sudeck's point).
  • Pectinate line (Anal Canal): Divides endoderm (above) from ectoderm (below) origin; changes innervation, blood supply, lymphatics.

⭐ McBurney's point: Located one-third of the distance from the anterior superior iliac spine to the umbilicus, commonly corresponds to the base of the appendix (site of maximum tenderness in acute appendicitis).

Liver, Gallbladder & Pancreas - Accessory All-Stars

• Liver: 4 lobes (R, L, Caudate, Quadrate); 8 Couinaud segments. Portal triad (Portal Vein, Hepatic Artery, Bile Duct). Histo: Hepatocytes, sinusoids (Kupffer cells), Space of Disse. Functions: Bile prod., metabolism. , biliary apparatus, and pancreatic ducts)
• Gallbladder: Stores, concentrates bile. Calot's Triangle (boundaries: Cystic Duct, Common Hepatic Duct, Inf. Liver edge; content: Cystic Artery). 📌 Mnemonic: "3 C's" (Cystic Duct, Common Hepatic Duct, Cystic Artery).
• Pancreas: Head, neck, body, tail. Exocrine: Acini (enzymes: amylase, lipase, proteases). Endocrine (Islets):

  Cell	Hormone
  Alpha	Glucagon
  Beta	Insulin
  Delta	Somatostatin
  Ducts: Wirsung (main), Santorini (accessory).

⭐ Portal vein: formed by Splenic Vein & Superior Mesenteric Vein union.

High-Yield Points - ⚡ Biggest Takeaways

• Meckel's diverticulum: Rule of 2s (2% population, 2 feet from ICJ, 2 inches long).
• Gut arterial supply: Foregut by celiac trunk, Midgut by SMA, Hindgut by IMA.
• Portal triad: Proper hepatic artery, portal vein, common bile duct in hepatoduodenal ligament.
• Calot's triangle: Borders: cystic duct, common hepatic duct, liver edge; contains cystic artery.
• GI wall layers: Mucosa, Submucosa (Meissner's plexus), Muscularis externa (Auerbach's plexus), Serosa.
• Achalasia cardia: LES non-relaxation due to Auerbach's plexus loss in esophagus.