RC

Gastrointestinal System – Comprehensive Study Notes

Layers of the Gastrointestinal (GI) Tract

  • 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

Innervation of the GI System

  • 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

Arterial Blood Supply

  • Celiac artery – stomach & proximal duodenum

  • Superior mesenteric artery (SMA) – distal small intestine → mid-large intestine

  • Inferior mesenteric artery (IMA) – distal large intestine → anus

Peritoneum & Potential Spaces

  • 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

Core Functions of the GI System

  • Ultimate goal: supply nutrients to body cells through three overarching processes:

    • \text{Ingestion}

    • \text{Digestion}

    • \text{Absorption}

  • Additional process: Elimination of indigestible residue & waste

Ingestion & Propulsion of Food

  • 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

Gastric Secretion & Initial Digestion

  • 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

Small-Intestinal Digestion & Absorption

  • 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

Large-Intestinal Elimination

  • 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)

Accessory Organs

Liver

  • 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

Biliary Tract

  • 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

Bilirubin Metabolism

  • 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)

Pancreas

  • 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

Gerontologic Considerations

  • 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

Nursing Assessment of the GI System

Subjective Data

  • 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

Objective Physical Examination

Mouth
  • Inspection (lips, mucosa, gums, teeth, tongue, tonsils)

  • Palpation (lesions, masses, tenderness)

Abdomen
  • Preparation: good lighting, supine with knees slightly flexed, emptied bladder, warm hands/stethoscope

  • Order: Inspection → Auscultation → Percussion → Palpation (light then deep)

Rectum & Anus
  • Inspection of perianal region, hemorrhoids, fistulas

  • Digital rectal exam (tone, masses, prostate in males)

  • Occult blood test (guaiac/FIT)

Diagnostic Studies

  • 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