Digestive System Comprehensive Notes

Digestive System – General Overview

• System functions
  • Mechanical & chemical breakdown of food
  • Absorption of nutrients into the body
  • Elimination of undigested waste (defecation)
• Five overlapping stages of digestion
  • Ingestion
  • Propulsion (peristalsis & mass movements) of material through the GI tract
  • Digestion
  • Mechanical (mastication, churning, segmentation)
  • Chemical (enzymatic hydrolysis)
  • Absorption across the mucosa into blood & lymph
  • Elimination of indigestible residue
• Structural divisions
  • Gastro-intestinal (GI) tract / alimentary canal
  • Convoluted cylindrical tube; lumen is technically "outside" the body (donut-hole analogy)
  • Accessory organs
  • Connected to the tube by ducts; secrete substances that facilitate digestion (saliva, bile, pancreatic juice, etc.)

Embryological Development of the Abdominal Cavity

• Primordial gut tube (≈ 4 weeks gestation)
  • Endoderm-lined tube anchored to posterior body wall by a double layer of peritoneum – the mesentery
  • Divided into three regions by blood-supply
  • Foregut
    • Gives rise to esophagus, stomach, proximal duodenum, liver, pancreas, spleen
    • Supplied by the celiac trunk
  • Midgut
    • Forms jejunum, ileum, ascending & proximal 2/3 of transverse colon
    • Supplied by the superior mesenteric artery (SMA)
  • Hindgut
    • Forms distal 1/3 transverse colon, descending & sigmoid colon, rectum
    • Supplied by the inferior mesenteric artery (IMA)
• Liver bud
  • Outgrowth of duodenum that projects ventrally within ventral mesentery; connected to anterior wall by the falciform ligament
• Stomach formation
  • Dilatation between esophagus & duodenum within the same double layer of mesentery
  • Ventral stomach peritoneum + liver = lesser omentum
• Mid-gut herniation & rotation (≈ 6 weeks)
  • Midgut temporarily herniates through umbilicus into yolk sac; stays attached by mesentery
  • Elongates → intestinal loops; retracts back, rotates 270^{\circ} counter-clockwise → adult configuration
  • Transverse colon retained by transverse mesocolon; stomach-to-transverse colon peritoneum folds into the greater omentum (quadruple layer)
• Retroperitoneal relationships
  • Primary retroperitoneal: kidneys, adrenal glands, aorta, IVC, pancreas (except tail), etc.
  • Secondarily retroperitoneal: ascending & descending colon pressed against & fused with posterior wall

Peritoneum & Mesenteries

• Serous membrane analogous to pleura & pericardium
  • Parietal peritoneum – lines abdominopelvic walls; continuous with transversalis fascia, diaphragm, pelvic muscles
  • Visceral peritoneum – invests many abdominal organs
• Positional terminology
  • Intraperitoneal organs – completely enveloped by visceral peritoneum (e.g., stomach, jejunum, ileum, transverse & sigmoid colon, liver, spleen)
  • Retroperitoneal organs – only anterior surface covered (e.g., pancreas, duodenum (parts 2–4), ascending/descending colon)
• Major mesenteric reflections (double layers of visceral peritoneum)
  • Mesentery (of small intestine) – jejunum & ileum; conveys SMA, SMV, vagus nn.
  • Transverse mesocolon – transverse colon neurovascular plane
  • Coronary ligament – diaphragm → superior liver; leaves the bare area devoid of peritoneum
• Secondary (visceral-to-visceral) folds
  • Lesser omentum
  • Hepatogastric ligament (liver ↔ stomach)
  • Hepatoduodenal ligament (liver ↔ duodenum); contains portal triad (portal vein, proper hepatic artery, common bile duct)
  • Greater omentum – stomach → transverse colon; mobile “apron” containing fat, immune cells; walls-off infection
  • Gastrosplenic ligament – stomach → spleen
  • Splenorenal (lienorenal) ligament – spleen → posterior wall at left kidney
• Peritoneal cavity
  • Potential space between parietal & visceral layers; contains a thin film of serous fluid for friction-free movement

General Histological Plan of the GI Tract

• Four concentric tunics (lumen → exterior)
  1. Mucosa
  • Epithelium (protective, secretory, or absorptive depending on site)
  • Lamina propria (areolar CT, capillaries, lymphatics)
  • Muscularis mucosae (thin smooth muscle enhances local folds)
  1. Submucosa
  • Dense irregular/areolar CT; large blood & lymph vessels, glands, and Meissner’s (submucosal) plexus; MALT prominent in ileum & appendix
  1. Muscularis externa
  • Inner circular & outer longitudinal smooth muscle layers → peristalsis & segmentation
  • Myenteric (Auerbach) plexus sandwiched between layers
  1. Adventitia / Serosa
  • Areolar CT; if intraperitoneal, covered by visceral peritoneum (serosa)

Esophagus

• Muscular tube ≈ 25\,\text{cm} from pharynx to stomach; only distal \sim 3\,\text{cm} within abdomen
• Passes diaphragm at esophageal hiatus (T10) where the lower esophageal sphincter (LES) guards reflux (pyrosis)
• Histology
  • Non-keratinized stratified squamous epithelium (protection from abrasion)
  • Muscularis externa transitions:
  • Upper 1/3 – skeletal muscle (rapid voluntary deglutition)
  • Middle 1/3 – mixed skeletal/smooth
  • Lower 1/3 – smooth muscle (autonomic, slower ≈ 5–8\,\text{s} transit)
  • Z-line = abrupt epithelium change to gastric mucosa at cardiac orifice
• Function
  • Peristaltic waves propel bolus; LES relaxes on arrival

Stomach

• J-shaped intraperitoneal reservoir
  • Lesser curvature (concave, attachment of lesser omentum)
  • Greater curvature (convex, attachment of greater omentum)
• Four anatomical regions
  1. Cardia – immediately distal to LES; cardiac notch above
  2. Fundus – dome superior to cardia; gas bubble when fasted, expands post-prandially
  3. Body – largest; intense mixing, prominent gastric rugae
  4. Pylorus
  • Pyloric antrum (wider, inferiorly “sags”)
  • Pyloric canal with thickened circular muscle → pyloric sphincter (usually closed; squirts chyme into duodenum)
• Internal features
  • Gastric folds (rugae) especially along greater curvature & pylorus; allow expansion and aid in mixing
  • Mucosa arranged in columnar folds; surface mucous cells, deeper gastric pits housing parietal (HCl, intrinsic factor), chief (pepsinogen), mucous neck, and enteroendocrine cells
• Muscularis externa uniquely has 3 layers
  • Outer longitudinal, middle circular, inner oblique → vigorous churning

Small Intestine

Duodenum (≈ 25\,\text{cm}; C-shaped)

• Begins at pylorus → ends at duodenojejunal flexure
• Mostly retroperitoneal (except proximal \sim2\,\text{cm} – duodenal cap)
• Four parts
  1. Superior (intraperitoneal cap); attachment of hepatoduodenal ligament
  2. Descending – retroperitoneal alongside pancreatic head; receives common bile duct + main pancreatic duct at hepatopancreatic ampulla → major duodenal papilla
  3. Horizontal – crosses midline inferior to pancreas
  4. Ascending – left of aorta; becomes intraperitoneal at flexure, suspended by ligament of Treitz (skeletal muscle from right crus of diaphragm)

Jejunum & Ileum (≈ 6\,\text{m} together)

• Jejunum – primarily LUQ of infracolic compartment
• Ileum – RLQ, ends at ileocecal valve
• Gradual transition; distinguishing trends
  • Wall: jejunum thicker, heavier; ileum thinner, lighter
  • Circular folds (plicae circulares): tall & numerous in jejunum; diminish distally, absent in terminal ileum
  • Mesenteric fat: sparse in proximal jejunum; abundant in ileum (“fatty mesentery”)
  • Arterial pattern
  • Jejunum: fewer large arterial arcades, long vasa recta
  • Ileum: many short arcades, short vasa recta

Large Intestine

• Wider, thinner-walled tube; functions in water/electrolyte absorption & fecal storage
• Hallmarks
  • Teniae coli – 3 ribbon-like longitudinal muscle bands with elastic recoil
  • Haustra – sacculations formed by tonic teniae contraction
  • Semilunar folds – mucosal invaginations between haustra
  • Omental (epiploic) appendices – fat-filled peritoneal pouches hanging from teniae
• Regional anatomy
  1. Cecum – blind pouch RLQ; receives ileal contents via ileocecal valve (frenula close when cecum distends)
  • Vermiform appendix – narrow diverticulum from posteromedial cecum; variable position
  1. Ascending colon – retroperitoneal; cecum → right colic (hepatic) flexure
  2. Transverse colon – intraperitoneal; hepatic → left colic (splenic) flexure; suspended by transverse mesocolon
  3. Descending colon – splenic flexure → sigmoid; secondarily retroperitoneal
  4. Sigmoid colon – intraperitoneal S-shaped loop, terminates at rectosigmoid junction (S3) where teniae coli end

Accessory Digestive Organs

Salivary Glands

• Parotid (largest, over masseter & ramus; Stensen’s duct), Submandibular, Sublingual
• Secrete saliva containing
  • Lysozyme (antibacterial)
  • Salivary amylase (initiates starch digestion)

Liver

• Mass \approx 3{\text{–}}5\,\text{lb}; occupies RUQ, shielded by rib cage
• Anchors
  • Falciform ligament (ventral mesentery) dividing anatomical right/left lobes; inferior free edge becomes ligamentum teres (obliterated umbilical vein)
  • Coronary ligaments → bare area of liver in direct contact with diaphragm
• Functional anatomy
  • Right & left lobes (blood supply & bile drainage follow)
  • Caudate (post-sup) & Quadrate (ant-inf) lobes on visceral surface belong functionally to left lobe
• Porta hepatis (hilum) within hepatoduodenal ligament – contains
  1. Proper hepatic artery (oxygenated blood, \approx 25\% total flow)
  2. Hepatic portal vein (nutrient-rich, deoxygenated blood, \approx 75\% flow)
  3. Common hepatic/bile duct system (bile outflow)
• Microscopic architecture
  • Lobules: hexagonal “wheels” of hepatocyte cords radiating to central vein; portal triads at corners (branch of portal vein, hepatic artery, bile duct)
  • Blood from artery & portal vein mixes in hepatic sinusoids → central vein → hepatic veins → IVC
  • Bile flows counter-current in canaliculi toward bile duct branches → right/left hepatic ducts
• Major functions
  • Metabolic regulation (glucose homeostasis via insulin/glucagon sensitivity, glycogen storage & mobilization)
  • First-pass processing of absorbed nutrients (except chylomicron fats)
  • Synthesis of plasma proteins, detoxification, vitamin storage
  • Continuous production of bile salts for fat emulsification; stored in gall bladder between meals

Gall Bladder

• Muscular sac on visceral liver; fundus, body, neck → cystic duct
• Cystic duct + common hepatic duct = common bile duct → merges with pancreatic duct at ampulla
• Stores & concentrates bile while fasting; CCK-mediated contraction & sphincter relaxation release bile during digestion

Pancreas

• Retroperitoneal, oblique leaf behind stomach; parts: tail (contacts spleen), body, head wedged in duodenal C with uncinate process hooking posterior to SMA/SMV
• Main pancreatic duct courses centrally; joins common bile duct at hepatopancreatic ampulla (alternatively accessory duct drains uncinate)
• Histology
  • Exocrine acini → digestive enzymes (trypsinogen, chymotrypsinogen, lipase, amylase) & alkaline fluid
  • Endocrine Islets of Langerhans
  • (\alpha)-cells: glucagon (raises plasma glucose)
  • (\beta)-cells: insulin (lowers plasma glucose)
• Functions
  • Major digestive enzyme source
  • Endocrine regulation of carbohydrate metabolism (liver, skeletal muscle)

Spleen (Immunological, not digestive but embryologically foregut)

• Large, variably sized lymphatic organ in LUQ above splenic flexure
• Held by gastrosplenic & splenorenal ligaments
• Functions: filters aged RBCs, recycles iron/heme, immune surveillance; non-essential (can be removed)