Lower Digestive Tract, Accessory Organs & Disorders

Small Intestine

• Tube measuring ≈ 2.5\,\text{cm} in diameter and 6\,\text{m} (≈ 600\,\text{cm}) in length
• Extends from the pyloric sphincter to the ileocecal valve
• Divisions
  • Duodenum
  • Uppermost C-shaped segment; length ≈ 25\,\text{cm}
  • Receives chyme plus bile and pancreatic juice; major site of continued chemical digestion
  • Jejunum
  • Mid-section; length ≈ 2.5\,\text{m}
  • Thick wall, numerous plicae → maximal nutrient absorption
  • Ileum
  • Distal section; length ≈ 3.5\,\text{m}
  • Ends at ileocecal valve; absorbs vitamin B_{12} and bile salts; houses Peyer’s patches (immune function)

Microscopic Structure of the Small Intestine

• Plicae circulares (circular folds) covered by finger-like villi → ↑ surface area
• Villi Anatomy
  • Core contains arteriole, venule, lacteal, and smooth muscle
  • Apical brush border carries ≈ 1700 microvilli per enterocyte → membrane-bound enzymes (e.g., disaccharidases, peptidases)
• Crypts of Lieberkühn
  • Invaginations between villi; secrete intestinal juice rich in enzymes and water (≈ 1-2\,\text{L}\,\text{day}^{-1})
  • House intestinal stem cells → continuous renewal (turnover ≈ every 3–5 days)
  • Contain Paneth cells (defensins) → antimicrobial defense
• Cell lineages originating from crypt stem cells
  • Enterocytes (nutrient absorption)
  • Goblet cells (mucus → lubrication)
  • Enteroendocrine cells (hormone release: CCK, secretin, GIP)
  • Tuft cells (chemosensory/immune)
  • Paneth cells (innate immunity)

Large Intestine

• Dimensions: Diameter ≈ 6\,\text{cm}; length 1.5–1.8\,\text{m}
• Primary functions: Water & electrolyte reclamation, vitamin K & B production (microbiota), feces formation & storage
• Divisions
  • Cecum (with ileocecal valve)
  • Colon
  • Ascending (right vertical)
  • Transverse (horizontal—superior to small intestine)
  • Descending (left vertical)
  • Sigmoid (S-shaped; leads to rectum)
  • Rectum & Anal Canal
  • Last 7–8 in; terminal ≈ 2.5\,\text{cm} = anal canal → anus
  • Internal (involuntary) & external (voluntary) sphincters regulate defecation
• Unique anatomical features
  • Taeniae coli (three longitudinal muscle bands) → formation of haustra (sacculations)
  • Epiploic appendages (fat tags)
  • Mucosa contains abundant goblet cells; crypts present but no villi

Vermiform Appendix

• Slender tube 8–10\,\text{cm} long protruding from the cecum
• Lymphoid tissue reservoir; hypothesized “safe house” for beneficial gut flora

Peritoneum & Mesenteries

• Peritoneum: Largest serous membrane; visceral layer coats organs, parietal lines cavity
• Intraperitoneal vs Retroperitoneal
  • Retroperitoneal examples: pancreas (except tail), duodenum (parts 2–4), ascending & descending colon, rectum, kidneys, bladder
• Special folds & reflections
  • Mesentery proper: suspends jejunum & ileum
  • Transverse mesocolon: supports transverse colon
  • Greater omentum: “fatty apron” draping from stomach to transverse colon; limits infection spread (“abdominal policeman”)
  • Lesser omentum: links stomach & liver
  • Peritoneal pouches (e.g., rectouterine) → potential sites for fluid accumulation (ascites)

Liver

• Largest gland; weight ≈ 1.5\,\text{kg} (3–4 lb); protected by rib cage under diaphragm
• Lobes
  • Right (≈ \frac{5}{6} total mass; subdivided into right proper, caudate, quadrate)
  • Left (≈ \frac{1}{6})
• Supporting ligaments: falciform, coronary, round (ligamentum teres—fetal umbilical vein remnant)

Lobular Architecture

• Hexagonal lobules (≈ 1\,\text{mm} diameter) made of radiating plates of hepatocytes
• Portal triad at each corner: branch of hepatic artery, portal vein, and interlobular bile duct
• Blood flow
  • Portal vein (nutrient-rich) + hepatic artery (oxygen-rich) → sinusoids → central vein → hepatic vein → IVC
  • Countercurrent bile flow from hepatocytes → canaliculi → bile duct → right & left hepatic ducts → common hepatic duct

Key Functions

• Detoxification (e.g., alcohol, drugs via cytochrome P450 systems)
• Bile production ≈ 1\,\text{pint} = 473\,\text{mL}\,\text{day}^{-1} → emulsifies lipids
• Metabolism
  • Carbohydrates: glycogenesis, glycogenolysis, gluconeogenesis
  • Lipids: cholesterol & lipoprotein synthesis
  • Proteins: deamination (→ urea), plasma protein (albumin, clotting factors) synthesis
• Storage: glycogen, vitamins A,D,B_{12}, and iron
• Immune role: Kupffer cells (fixed macrophages) remove microbes & debris

Gallbladder

• Pear-shaped sac (length 7–10\,\text{cm}; width ≈3\,\text{cm}) on liver undersurface
• Layers: serosa → muscular → mucosa (folded)
• Functions
  • Stores & concentrates bile 5- to 10-fold
  • Contracts (via cholecystokinin) → bile through cystic duct + common bile duct → duodenum (ampulla of Vater)
• Clinical note: Gallstones (cholesterol or pigment) may obstruct ducts → biliary colic, pancreatitis

Pancreas

• Retroperitoneal gland 12–15 cm long; weight ≈ 60\,\text{g}
• Exocrine component
  • Acinar cells → pancreatic enzymes (trypsinogen, chymotrypsinogen, lipase, amylase)
  • Ducts merge into main pancreatic duct (duct of Wirsung) that joins common bile duct at hepatopancreatic ampulla
• Endocrine component (Islets of Langerhans)
  • Alpha cells → glucagon (↑ blood glucose)
  • Beta cells → insulin (↓ blood glucose)

Major Digestive Disorders (Lower GI, Liver, Pancreas)

• Intestinal
  • Diarrhea / constipation (motility & absorption imbalance)
  • Malabsorption syndromes (e.g., celiac disease, lactase deficiency)
  • Diverticulosis (& inflammation = diverticulitis)
  • Colitis (ulcerative) & Crohn disease (transmural inflammation)
  • Irritable bowel syndrome (functional), colorectal cancer (adenocarcinoma)
  • Hemorrhoids, proctitis, anal fissures (painful tears)
• Hepatic / Pancreatic
  • Hepatitis (viral A–E, toxins)
  • Cirrhosis (fibrosis → portal hypertension, ascites)
  • Pancreatitis (enzyme autodigestion; acute vs chronic)
  • Pancreatic carcinoma (often silent until obstructive jaundice; poor prognosis)

Clinical & Practical Relevance / Connections

• Portal circulation vital in nutrient processing & drug metabolism; portal hypertension causes esophageal varices (GI bleeding risk)
• Peritoneal reflections guide surgical incisions and predict fluid/pus spread
• Small-intestine surface area concepts underpin malabsorption understanding & bariatric surgery physiology
• Ethical/Philosophical: organ donation (liver transplantation, living donors), allocation fairness, screening for transplant candidacy
• Public health: colorectal cancer screening via colonoscopy; vaccination for hepatitis A & B; lifestyle modification for NAFLD/diabetes

Core Numerical Facts & Quick-Recall Values

• Small intestine length 6\,\text{m}; villus microvilli density ≈ 1700\,/\text{cell}
• Large intestine length 1.5–1.8\,\text{m}, diameter 6\,\text{cm}
• Bile output ≈ 473\,\text{mL}\,\text{day}^{-1}; gallbladder concentrates bile 5–10\times
• Liver mass 1.5\,\text{kg}; pancreas length 12–15\,\text{cm}

These bullet-point notes integrate gross anatomy, histology, physiology, and clinical correlations for the lower digestive tract and associated accessory organs, providing a comprehensive foundation for exam preparation.