Digestive System – Chapter 26 Review

Introduction to the Digestive System

• General purpose
  • Reduce ingested food to absorbable monomers, absorb them into blood or lymph, eliminate indigestible residue.
  • Maintain fluid, electrolyte, acid–base homeostasis; harbor immunological & endocrine cells.
• Core functions (occur at specific anatomical sites)
  • Ingestion – oral cavity.
  • Propulsion – peristalsis (esophagus 6.6 cm/min, stomach, SI, LI), deglutition.
  • Mechanical digestion – mastication, churning, segmentation.
  • Chemical digestion – enzymatic hydrolysis beginning in mouth (salivary amylase) and ending in small intestine.
  • Absorption – primarily SI; limited absorption in stomach (alcohol, aspirin) and LI (water, electrolytes, vitamins K/B).
  • Defecation – terminal rectum/anus.

Organs of the GI Tract

• Continuous muscular tube ≈ 9\,\text{m} from mouth to anus.
  • Mouth → Pharynx (oro-, laryngo-) → Esophagus → Stomach → Small Intestine (duodenum, jejunum, ileum) → Large Intestine (cecum, ascending, transverse, descending, sigmoid, rectum) → Anal canal.
• Accessory organs
  • Teeth, tongue, salivary glands (parotid, submandibular, sublingual, numerous minor), liver, gallbladder, pancreas, appendix, greater & lesser omenta, mesenteries.

Gastrointestinal Wall (4 Tunics)

• Mucosa (innermost)
  • Epithelium: non-keratinized stratified squamous (mouth → esophagus, anal canal); simple columnar with goblet cells (stomach → rectum).
  • Lamina propria: loose areolar CT, MALT.
  • Muscularis mucosae: thin smooth muscle creating local folds.
• Submucosa
  • Dense irregular CT, blood & lymph vessels, glands, \texttt{Meissner}\,(=\,submucosal) plexus.
• Muscularis externa
  • Inner circular & outer longitudinal smooth muscle; responsible for peristalsis, segmentation, churning.
  • \texttt{Auerbach}\,(=\,myenteric) plexus sandwiched between layers.
• Serosa / Adventitia
  • Visceral peritoneum of intraperitoneal organs; fibrous adventitia where retroperitoneal.

Nervous Regulation

• Enteric Nervous System (ENS)
  • ~100\,\text{million} neurons; autonomous but modulated by ANS.
  • Coordinates motility, secretion, local blood flow via short (intrinsic) reflex arcs.
• Autonomic Nervous System (ANS)
  • Parasympathetic (vagus, pelvic splanchnics) – ↑ motility & secretion.
  • Sympathetic (thoracolumbar) – ↓ motility, constricts sphincters, vasoconstricts.

Serous Membranes & Mesenteries (Lab Emphasis)

• Peritoneum: parietal vs visceral.
• Mesenteries: double-layered peritoneal folds suspending GI organs, housing vessels/nerves (e.g., mesentery proper, transverse & sigmoid mesocolon, greater & lesser omentum, falciform ligament).
• Intraperitoneal organs: stomach, jejunum, ileum, transverse & sigmoid colon, appendix, spleen, liver.
• Retroperitoneal organs: duodenum (parts 2–4), pancreas (except tail), ascending & descending colon, rectum, kidneys, adrenal glands.

Upper GI Tract Overview

Oral Cavity

• Functions: ingestion, sensory analysis, mechanical processing, limited digestion of carbs & lipids, speech, immune defense (lingual tonsil).
• Boundaries: lips (labia), cheeks (buccinator m.), palate, tongue, floor.
• Hard palate: maxillae & palatine bones → rigid surface for tongue.
• Soft palate & uvula: muscular; close off nasopharynx during swallowing.
• Epithelium: non-keratinized stratified squamous for abrasion resistance.

Tongue

• Intrinsic & extrinsic skeletal muscles (CN XII) → manipulate food, form bolus, initiate swallowing, speech, house taste buds (CN VII, IX).
• Lingual lipase secreted by serous glands beneath vallate papillae – activated in stomach (pH optimum ≈ 4!–!5).

Salivary Glands

• Functions of saliva
  • Lubrication, taste dissolution, cleansing, buffering (bicarbonate HCO_3^-), chemical digestion (amylase, lipase), IgA, lysozyme, growth factors (EGF), etc.
• Composition ≈ 97\% water, pH 6.7!–!7.0.
  • Electrolytes Na^+, K^+, Cl^-, HCO_3^-; mucin; enzymes; metabolic wastes (urea, uric acid).
• Major (extrinsic)
  • Parotid – purely serous, \uparrow amylase.
  • Submandibular – mixed; serous > mucous.
  • Sublingual – mixed; mucous > serous.
• Minor (intrinsic) scattered throughout mucosa – secrete lingual lipase, mucous.
• Regulation
  • Parasympathetic (CN VII, IX) → copious, watery saliva.
  • Sympathetic (T1–T3) → viscous, mucin-rich saliva; extreme activation → xerostomia.
  • Stimuli: chemoreceptors (acidic foods) & mechanoreceptors, higher centers (smell, sight, thought), nausea (↑), fear/sleep (↓).

Mastication

• Voluntary + reflexive jaw movements (CN V trigeminal motor); increases surface area for enzymes.

Teeth (Brief)

• Dentition pattern Adult: 2\,1\,2\,3 \times 2 = 32 (incisors, canines, premolars, molars).
• Functions: cutting, tearing, grinding.
• Dental caries: demineralization by bacterial acids (Streptococcus mutans) fed by sucrose → enamel erosion → dentin/pulp infection.

Deglutition (Swallowing)

• Phases (≈ 1\,\text{s} total)
  1. Voluntary (buccal) – tongue pushes bolus → oropharynx.
  2. Pharyngeal – reflex (medulla + pons), soft palate & uvula elevate, epiglottis closes laryngeal inlet, upper esophageal sphincter (UES) relaxes.
  3. Esophageal – peristaltic waves (2–4 cm/s) propel bolus; LES relaxes.
• Bolus exclusion from nasal cavity by uvular elevation; from trachea by epiglottic closure & vocal fold adduction.

Pharynx & Esophagus

• Oropharynx & laryngopharynx: skeletal muscle walls, propel bolus to esophagus.
• Esophagus: ≈ 25\,\text{cm}, upper 1⁄3 skeletal, middle mixed, distal 2⁄3 smooth muscle; adventitia; stratified squamous epithelium.
• Functions: transport via peristalsis; mucus lubrication.
• Heartburn / GERD
  • Reflux of acidic chyme → burning substernal pain.
  • Chronic GERD → metaplasia (Barrett esophagus: non-ciliated simple columnar w/ goblet cells replacing stratified squamous) → adenocarcinoma risk.

Stomach

• Regions: cardia, fundus, body, pylorus (antrum + canal); lesser/greater curvatures; rugae (mucosal folds) allow expansion to \approx 4\,\text{L}.
• Chyme: semifluid mixture of food + gastric juice.
• Gastric pits → glands housing 5 cell types
  • Surface mucous & mucous neck cells – alkaline mucus (HCO₃⁻-rich) protecting epithelium.
  • Parietal (oxyntic) cells – HCl\,(pH \sim 0.8 \text{ in canaliculi}) → activates pepsinogen, denatures proteins, kills microbes, breaks plant walls; intrinsic factor (IF) essential for vitamin B_{12} absorption in ileum.
  • Chief cells – pepsinogen (→ pepsin @ pH < 3), gastric lipase.
  • G cells (enteroendocrine) – gastrin hormone → ↑ HCl secretion & motility.
  • D cells – somatostatin (paracrine inhibitor).
• Only indispensable stomach function = secretion of intrinsic factor; lack → pernicious anemia.
• Mucosal barrier: tight junctions, thick bicarbonate mucus, rapid epithelial turnover (3–6 days).

Motility

• Gastric mixing: peristaltic waves (3/min) generated by interstitial cells of Cajal, grinding in antrum.
• Gastric emptying: squirts \sim 3\,\text{mL} chyme per wave through pyloric sphincter; rate depends on volume, fluidity, nutrient content (fat slows).

Regulation Reflexes (watch video)

1. Cephalic phase
   • Sight/smell/thought → vagus → ↑ gastric juice (≈ 30%).
2. Gastric phase
   • Distention & peptides ↑ gastrin, ENS, ACh → maximal secretion/motility (≈ 60%).
3. Intestinal phase
   • Initial chyme entry briefly ↑ gastrin (intestinal gastrin) then dominant enterogastrones (CCK, secretin, GIP), enterogastric reflex → ↓ gastric activity.

Vomiting (Emesis)

• Coordinated by medullary emetic center/Chemoreceptor Trigger Zone (CTZ).
• Stimuli: distention, irritants, pathogens, motion sickness (vestibular), sight/smell, pregnancy hormones, drugs (opiates, emetics).
• Sequence: deep breath → soft palate rises, glottis closes, LES relaxes, diaphragm & abdominal muscles contract, stomach squeezed → expulsion.

Small Intestine

• Regions
  • Duodenum (25 cm): receives acidic chyme, bile, pancreatic juice; Brunner0s glands secrete alkaline mucus.
  • Jejunum (≈ 2.5 m): primary site of chemical digestion & nutrient absorption.
  • Ileum (≈ 3.6 m): absorbs bile salts, vitamin B_{12}, electrolytes; Peyer patches.
• Surface amplifiers
  • Plicae circulares (1-cm folds) → slow chyme.
  • Villi (1 mm) – columnar epithelium; core of lamina propria w/ capillaries + lacteal.
  • Microvilli (brush border; \sim 200\,\text{million}/mm^2) – digestive enzymes.
• Lacteal: blind lymphatic capillary in villus → absorbs chylomicrons.
• Intestinal glands (crypts of Lieberkühn)
  • Enterocytes – enzymes, absorption.
  • Goblet cells – mucus.
  • Paneth cells – defensins/lysozyme.
  • Enteroendocrine – CCK, secretin, GIP, motilin.
• Handling acidic chyme
  • Bicarbonate HCO_3^- from pancreas + duodenal glands neutralize ⇢ pH ≈ 7.4.
• Enzymes
  • Brush-border: maltase, sucrase, lactase, peptidases, enteropeptidase (activates trypsin).
  • Pancreatic: amylase, trypsin, chymotrypsin, carboxypeptidase, elastase, lipase, colipase, phospholipase, nucleases.
• Motility
  • Segmentation (12 → 8 contractions/min from duodenum → ileum): mixing, contact with mucosa.
  • Migrating Motor Complex (MMC) every 90 min in fasting; clears debris.

Accessory Organs

• Liver
  • Produces bile (900 mL/day), processes nutrients, detoxifies, stores glycogen/vitamins, synthesizes plasma proteins (albumin, clotting factors), Kupffer cells.
• Bile composition
  • Water, bile salts (derivatives of cholesterol; amphipathic), bilirubin, cholesterol, phospholipids, electrolytes.
  • Function: emulsify fats → micelles ↑ surface area for lipase; route for cholesterol & bilirubin excretion.
• Gallbladder
  • Stores & concentrates bile (up to 10×; removes water/ions), releases via cystic → common bile duct when CCK stimulates.
  • Gallstones: cholesterol or pigment precipitates; risk ↑ with “female, forty, fertile, fat”; block ducts → biliary colic, jaundice, pancreatitis.
• Pancreas
  • Exocrine acini → pancreatic juice (1.5 L/day): water, HCO_3^- (duct cells), enzymes (acinar cells).
  • Juice pH ≈ 8 neutralizes acid, provides optimal pH for enzymes.
  • Enzymes secreted as zymogens (trypsinogen, chymotrypsinogen) except amylase, lipase.

Hormonal Regulation

• CCK (I cells)
  • Trigger: fatty/ protein-rich chyme.
  • Actions: contracts gallbladder, relaxes hepatopancreatic sphincter, ↑ acinar enzyme secretion, ↓ gastric emptying.
• Secretin (S cells)
  • Trigger: acidic chyme (pH < 4.5).
  • Actions: ↑ pancreatic HCO_3^- rich juice, ↑ bile production, ↓ gastric secretion.

Large Intestine

• Length 1.5\,\text{m}; frames small intestine.
• Functions: reclaim water (≈ 400 mL/day) & electrolytes, absorb vitamins K, B{7}, B{5} from flora, produce mucus, store & expel feces.
• No circular folds/villi; abundant goblet cells; teniae coli create haustra.
• Bacterial flora
  • Ferment indigestible carbs → short-chain fatty acids (propionate, butyrate) for colonocytes.
  • Synthesize vitamins K and B_{7}.
  • Outcompete pathogens, modulate immunity.
• Motility
  • Haustral contractions (slow segmenting, every 30 min).
  • Mass movements: powerful peristaltic waves 3–4 × daily, usually post-meal (gastrocolic reflex).
• Appendix
  • Lymphoid tissue reservoir; appendicitis when lumen blocked (fecalith) → inflammation, potential rupture -> peritonitis.

Anal Canal & Defecation Reflex

• Internal anal sphincter: smooth muscle, involuntary.
• External anal sphincter: skeletal muscle (levator ani & pudendal innervation), voluntary.
• Reflex
  1. Feces stretch rectal wall → afferents to sacral spinal cord.
  2. Parasympathetic efferents contract rectum/sigmoid, relax internal sphincter.
  3. Conscious cortex decides; relaxation of external sphincter + Valsalva → defecation.
  4. If delayed, reflex wanes, rectum relaxes until next mass movement.
• Constipation: slow transit, excessive water reabsorption; causes – low fiber, dehydration, inactivity, opioids.
• Diarrhea: reduced absorption/increased secretion, rapid transit; causes – infection (cholera: cAMP \uparrow Cl⁻ secret.), intolerance, inflammation.

Nutrients & Their Digestion

• Importance: polymers ↓ to absorbable monomers → maintain osmotic balance, prevent immune reaction.

Carbohydrates

• Digestion
  • Mouth: salivary amylase cleaves starch → maltose/oligosaccharides (inactive @ pH < 4).
  • Lumen SI: pancreatic amylase continues.
  • Brush border: lactase, sucrase, maltase → monosaccharides (glucose, galactose, fructose).
• Absorption
  • Glucose/galactose via \text{SGLT1} (Na⁺ cotransport); fructose via \text{GLUT5} facilitated diffusion; all exit via \text{GLUT2}.
  • Enter portal blood → liver glycogenesis/glycolysis.
• Lactose intolerance: lactase deficiency → undigested lactose → ↑ osmotic load, bacterial fermentation → gas, diarrhea.
• Fiber
  • Insoluble cellulose adds bulk ⇢ stimulates motility; soluble pectins lower cholesterol by binding bile salts.

Proteins

• Digestion
  • Stomach: pepsin (optimal pH ≈ 2) → polypeptides.
  • SI lumen: pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) → oligopeptides.
  • Brush border: dipeptidases, aminopeptidases → amino acids.
• Absorption: Na⁺- or H⁺-dependent cotransporters; short peptides via \text{PepT1}.

Lipids

• Emulsification: bile salts + agitation ↓ droplet size ↑ SA.
• Digestion: pancreatic lipase (with colipase) → fatty acids + 2-mono-glycerides.
• Micelles ferry lipids to enterocyte membrane → diffusion.
• Inside cell: re-esterified → triglycerides, packaged with phospholipids, cholesterol, apoproteins → chylomicrons.
• Chylomicrons exocytosed → lacteals → thoracic duct → blood.

Nucleic Acids

• Pancreatic ribonuclease/deoxyribonuclease → nucleotides.
• Brush border nucleosidases, phosphatases → nitrogenous bases, pentose sugars, phosphate → absorbed via active transport.

Vitamins

• Fat-soluble A,D,E,K follow lipids in chylomicrons.
• Water-soluble (C, B-complex) via diffusion or transporters; B_{12} requires IF + ileal receptor.

Electrolytes & Water

• Na^+ cotransported with sugars/AA; K^+ absorbed via solvent drag, secreted if lumen hypotonic; Ca^{2+} & Fe^{2+} regulated by vitamin D & hepcidin.
• Water: \sim 9\,\text{L}/day enter lumen (food + drink ≈ 2 L; secretions ≈ 7 L); \approx 90\% absorbed in SI, most of remainder in LI; net stool water ≈ 100 mL.

Clinical Correlations & Ethical/Practical Notes

• GERD management: lifestyle (weight loss, avoid late meals), PPIs; long-term acid suppression ↔ micronutrient malabsorption (Mg²⁺, Ca²⁺, B₁₂).
• NSAID ulcers: inhibit COX-1 → ↓ prostaglandins → weak mucosal barrier.
• Celiac disease: auto-immune attack on transglutaminase → villus atrophy; gluten-free diet restores.
• Pancreatitis: gallstones or alcohol; auto-digestion → high serum amylase/lipase.
• Ethical issue: bariatric surgery alters anatomy/absorption; requires nutritional counseling & lifelong supplements.
• Public health: fiber intake & colorectal cancer risk; probiotic therapy to restore microbiota after antibiotics.