Four concentric layers from lumen outward:
Mucosal lining – innermost, epithelial surface in direct contact with chyme
Submucosa – connective tissue layer containing blood vessels, lymphatics, and nerves
Muscularis – three distinct smooth-muscle layers
Oblique (inner) layer
Circular (middle) layer
Longitudinal (outer) layer
Serosa (visceral peritoneum) – outermost protective covering
Autonomic Nervous System (ANS) modulates the Enteric Nervous System (ENS)
Parasympathetic (cholinergic) – stimulates motility, secretion, and sphincter relaxation ("rest-and-digest")
Sympathetic (adrenergic) – inhibits GI activity, constricts sphincters ("fight-or-flight")
Enteric Nervous System (ENS)
Can function independent of the CNS
Regulates motility & secretion along the entire tract through two plexuses:
Meissner (submucosal) plexus – controls secretion & blood flow
Auerbach (myenteric) plexus – controls muscle activity & peristalsis
Celiac artery – stomach & proximal duodenum
Superior mesenteric artery (SMA) – distal small intestine → mid-large intestine
Inferior mesenteric artery (IMA) – distal large intestine → anus
Parietal layer – lines abdominal cavity wall
Visceral layer – covers abdominal organs
Peritoneal cavity – potential (normally fluid-filled) space between the two; site for ascites/peritonitis
Mesentery & Omentum – double folds of peritoneum anchoring organs and carrying vessels, nerves, fat
Ultimate goal: supply nutrients to body cells through three overarching processes:
\text{Ingestion}
\text{Digestion}
\text{Absorption}
Additional process: Elimination of indigestible residue & waste
Appetite regulated by hypothalamic center + hormone ghrelin (secreted by stomach fundus)
Deglutition (swallowing) – three phases
Oral (voluntary) phase – bolus pushed by tongue to oropharynx
Pharyngeal (involuntary) phase – soft palate rises, epiglottis closes airway
Esophageal (involuntary) phase – peristaltic wave moves bolus to stomach
Chief cells – secrete pepsinogen (inactive) → converted to pepsin (active protease) in acid
Parietal cells – secrete hydrochloric acid (HCl), water, and intrinsic factor (for \text{B}_{12} absorption)
Physiology summary
Physical & chemical breakdown begins with saliva (amylase) in mouth
Proteins partially digested by pepsin in acidic stomach
Carbohydrates, fats, and remaining protein digestion completed in small intestine via pancreatic enzymes + bile
Extends from pylorus to ileocecal valve and consists of:
Duodenum
Jejunum
Ileum
Villi & microvilli massively increase mucosal surface area (→ "brush border")
Majority of nutrient absorption occurs here; end products cross epithelium into capillaries or lacteals
Segments: Cecum → Colon → Rectum → Anus
Main functions:
Absorption of water & electrolytes (≈ 100\text{–}200\,\text{mL} liquid lost in stool/day)
Bacterial synthesis of vitamin K & certain B vitamins
Formation, storage, & propulsion of fecal mass
Mucus secretion to lubricate passage
Defecation reflex can employ Valsalva maneuver (forced expiration against closed glottis → ↑ intra-abdominal pressure)
Largest internal organ; located in right upper quadrant beneath diaphragm
Divided into right & left lobes (plus caudate & quadrate subdivisions)
Functional unit = lobule containing:
Hepatocytes – metabolic, synthetic, detoxifying cells
Sinusoids – capillary channels mixing arterial & portal venous blood
Kupffer cells – macrophages that engulf bacteria & debris
Bile (water, bile salts, bilirubin, cholesterol) is essential for fat emulsification & absorption of fat-soluble vitamins (A, D, E, K)
Right & left hepatic ducts → common hepatic duct → (via cystic duct) gallbladder → common bile duct → duodenum (Ampulla of Vater)
Gallbladder concentrates & stores bile; contracts under cholecystokinin (CCK) stimulation after fatty meal
Derived from hemoglobin breakdown
Unconjugated (indirect) bilirubin = lipid-soluble; binds albumin for hepatic transport
Hepatocytes conjugate bilirubin (via glucuronidation) → water-soluble form excreted in bile
Gut bacteria convert bilirubin → urobilinogen → stercobilin (gives stool its brown color)
Anatomical parts: head, body, tail; empties via pancreatic duct (joins common bile duct at Ampulla of Vater)
Exocrine: acinar cells secrete digestive enzymes (lipase, amylase, proteases) + bicarbonate-rich fluid
Endocrine: islets of Langerhans (α, β, δ cells) release hormones (glucagon, insulin, somatostatin) into blood
Oral cavity: ↑ dental caries, periodontal disease; ↓ taste buds & smell; xerostomia (↓ saliva)
Esophagus: delayed emptying (↓ peristalsis, ↓ LES pressure)
Stomach: ↓ HCl secretion → altered iron & calcium absorption
Intestine: ↑ constipation (↓ motility, meds, ↓ fiber/fluids, inactivity)
Liver: ↓ size & perfusion → prolonged drug metabolism/clearance
Gallbladder: ↑ incidence of cholelithiasis
Nutritional risks: limited access, dentition issues, altered appetite/satiety
Health history (GI disorders, hepatitis, surgeries)
Medications (NSAIDs, anticoagulants, laxatives, antibiotics, opioids)
Surgical/other treatments (abdominal surgeries, transfusions)
Gordon’s functional health-patterns approach:
Health perception/management
Nutritional-metabolic
Elimination
Activity-exercise
Sleep-rest
Cognitive-perceptual
Self-perception–self-concept
Role–relationship
Sexuality–reproductive
Coping–stress tolerance
Value–belief
Inspection (lips, mucosa, gums, teeth, tongue, tonsils)
Palpation (lesions, masses, tenderness)
Preparation: good lighting, supine with knees slightly flexed, emptied bladder, warm hands/stethoscope
Order: Inspection → Auscultation → Percussion → Palpation (light then deep)
Inspection of perianal region, hemorrhoids, fistulas
Digital rectal exam (tone, masses, prostate in males)
Occult blood test (guaiac/FIT)
Radiologic
Barium enema (air-contrast) – outlines large intestine (Fig. 38-9)
Virtual colonoscopy (CT + 3-D reconstruction)
Endoscopy
Esophagogastroduodenoscopy (EGD)
Colonoscopy & sigmoidoscopy (direct visualization, polypectomy)
Endoscopic retrograde cholangiopancreatography (ERCP) – cannulation of biliary/pancreatic ducts (Fig. 38-10)
NPO 8h prior, consent, sedation, and possible antibiotics
After: VS, assess for perforation, infection, return of gag reflex
Capsule endoscopy – pill-sized camera traverses GI tract, data captured externally (Fig. 38-11)
Liver Biopsy
Open (surgical) vs. closed (needle, often ultrasound-guided)
Laboratory
Liver Function Tests (LFTs): ALT, AST, ALP, GGT, bilirubin (total/direct/indirect), albumin, PT/INR, ammonia, cholesterol