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