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Chapter 65: Secretory Functions of the Alimentary Tract

Guyton & Hall Textbook of Medical Physiology

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Overview

1. Digestive enzyme secretion - throughout the alimentary tract from mouth to ileum
2. Mucous secretion - from mouth to anus, for lubrication and mucosal protection

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I. Types of Alimentary Tract Glands

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II. Basic Mechanisms of Secretion

Secretion of Organic Substances

• Nutrients and raw materials are taken up from blood by acinar/secretory cells
• Rough ER synthesizes proteins
• Golgi apparatus processes and packages them into secretory vesicles (zymogen granules)
• Vesicles fuse with cell membrane and exocytose contents into the gland lumen
• This process requires energy (ATP) and is stimulated by neural/hormonal signals

Water and Electrolyte Secretion

• Active transport of Cl- and HCO3- into lumen creates osmotic gradient
• Water follows osmotically
• This provides the fluid vehicle to carry organic secretions

Stimuli for Secretion (3 categories)

1. Tactile stimulus - local contact/irritation of the mucosa
2. Chemical stimulus - presence of food breakdown products
3. Nervous/Hormonal stimulus - parasympathetic (ACh) stimulation greatly increases secretion; hormones (gastrin, CCK, secretin, GIP) modulate specific glands

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III. Salivary Secretion

Composition

• Parotid glands - serous (watery, enzyme-rich: ptyalin/amylase)
• Submandibular glands - mixed serous + mucous
• Sublingual glands - mainly mucous
• Daily output: 0.8 to 1.5 liters
• pH: 6.0-7.0

Functions of Saliva

1. Initiates starch digestion via ptyalin (salivary amylase) - cleaves alpha 1,4-glycosidic bonds
2. Lubrication for chewing and swallowing (mucin)
3. Antibacterial action - lysozyme, IgA, lactoferrin
4. Oral hygiene - washes bacteria, inhibits bacterial growth via thiocyanate + peroxidase system

Ionic Composition

• Secreted fluid is initially isotonic with plasma
• As it flows through ducts: Na+ and Cl- are reabsorbed, K+ and HCO3- are secreted
• Final saliva is hypotonic (markedly less Na+, more K+)
• At high flow rates (stimulated), saliva is more isotonic (less time for duct modification)

Neural Control

• Entirely autonomic - no voluntary control of secretory rate
• Parasympathetic (CN VII, IX): main stimulus - increases watery saliva profusely; ACh acts on muscarinic receptors
• Sympathetic: causes small amount of thick, enzyme-rich saliva
• Stimuli: taste, smell, tactile stimulation of mouth, conditioned reflexes (sight of food), nausea
• Sleep, dehydration, and fear decrease salivation

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IV. Esophageal Secretion

• Entirely mucous in nature
• Compound mucous glands in upper esophagus protect from irritation by food
• Simple mucous glands at the gastroesophageal junction protect gastric acid from refluxing upward
• Absence of mucous protection = esophageal ulceration

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V. Gastric Secretion

Glands of the Stomach

Secretory Cells of the Oxyntic Gland

• Highly active cells with massive mitochondria
• Secrete HCl at pH 0.8 (pure HCl = ~160 mEq/L)
• Mechanism:
  • H+ pumped into canaliculi by H+/K+-ATPase (proton pump)
  • Cl- actively transported alongside H+
  • CO2 + H2O → H2CO3 → H+ + HCO3- (carbonic anhydrase)
  • HCO3- exits basal side into blood ("alkaline tide")
• Stimulated by: ACh (vagal), gastrin, histamine (via H2 receptors)
• All three stimuli work synergistically; histamine has the greatest amplifying effect

• Converts pepsinogen → pepsin (active at pH < 5)
• Kills bacteria (antiseptic action)
• Provides optimal pH for gastric digestion
• Initiates protein denaturation

• Pepsinogen is inactive precursor
• Activated to pepsin by HCl (autocatalytic at pH 1.8-3.5)
• Pepsin is a protease - begins protein digestion (10-20% of total protein digestion)
• Inactive above pH 5; irreversibly denatured at pH 7+

• Secrete thin, soluble mucus (different from surface mucus cells which secrete thick, alkaline mucus)

• Essential for B12 absorption in terminal ileum
• Absence → pernicious anemia

Gastric Mucus Barrier

• Thick alkaline mucus layer (2-3 mm) protects gastric epithelium
• Maintains pH 6-7 at mucosal surface even when luminal pH is 2
• Prostaglandins stimulate mucus production (NSAIDs inhibit this - mechanism of peptic ulcers)

Phases of Gastric Secretion

Inhibition of Gastric Secretion

• Enterogastric reflex - when chyme enters duodenum: fat, acid, hyperosmolarity, or distension inhibit stomach
• GI hormones inhibiting acid: Secretin, GIP (gastric inhibitory peptide), CCK - collectively called enterogastrones
• Somatostatin from D cells inhibits both G cells and parietal cells

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VI. Pancreatic Secretion

Overview

• Pancreas secretes 1.2-1.5 liters/day into the duodenum
• Contains powerful enzymes for ALL three major food groups
• Also secretes large amounts of bicarbonate (pH 8.0) to neutralize acidic gastric chyme

Acinar Cells - Enzyme Secretion

• Enterokinase (brush border enzyme of duodenal epithelium) cleaves Trypsinogen → Trypsin
• Trypsin then autocatalytically activates more trypsinogen
• Trypsin also activates all other zymogens

• Pancreatic amylase - cleaves starch to maltose, dextrins
• Secreted in active form (not a zymogen - no risk of self-digestion)

• Pancreatic lipase - hydrolyzes triglycerides to monoglycerides + free fatty acids
• Cholesterol esterase - hydrolyzes cholesterol esters
• Phospholipase A2 - hydrolyzes phospholipids
• Also secreted in active form; lipase requires colipase to work in the presence of bile salts

• Ribonuclease (RNase) and Deoxyribonuclease (DNase)

Ductal Cells - Bicarbonate Secretion

• Large volumes of NaHCO3-rich fluid
• Mechanism: CO2 diffuses into ductal cells → carbonic anhydrase → H+ + HCO3-; H+ exchanged for Na+ (blood side); HCO3- secreted into duct lumen with Na+
• Purpose: raise duodenal pH from ~2 to ~6-7, creating optimal pH for pancreatic enzymes

Regulation of Pancreatic Secretion

• Secretin + CCK are synergistic - together produce far more secretion than either alone

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VII. Bile Secretion and Liver Function

Bile Composition

• Water, bile salts, bilirubin, cholesterol, fatty acids, lecithin, electrolytes
• pH ~7.8-8.0 (alkaline)
• Volume: 0.6-1 liter/day
• Not an enzyme - bile does not chemically digest fat; it emulsifies it

Bile Salts - Structure and Function

• Synthesized in liver from cholesterol
• Primary bile acids: cholic acid, chenodeoxycholic acid
• Conjugated to glycine or taurine → glycocholate, taurocholate (increases water solubility and ionization)
• In colon, bacterial action converts to secondary bile acids: deoxycholate, lithocholate

1. Emulsification of fat - bile salts have hydrophilic + hydrophobic ends (amphipathic); surround fat droplets, breaking them into tiny particles (increasing surface area for lipase)
2. Micellar solubilization - form micelles with fat products (fatty acids, monoglycerides) enabling them to reach the intestinal brush border for absorption
3. Without bile: 40% of fat is not absorbed (steatorrhea)

Enterohepatic Circulation

• Bile salts are reabsorbed in the terminal ileum (active transport - very efficient)
• Return via portal blood to liver → re-secreted into bile
• Full cycle: 2-3 times per meal, 6-15 times per day
• Only 5% of bile salts lost in feces per cycle; liver replenishes what is lost
• Interruption (e.g., ileal disease/resection) → fat malabsorption + bile salt diarrhea

Gallbladder

• Concentrates bile 5-20x by absorbing Na+, Cl-, water
• Stores bile between meals
• CCK (from duodenum in response to fat/protein) is the primary stimulus for gallbladder contraction
• Vagal stimulation also causes mild contraction
• Sympathetic inhibits contraction

Regulation of Bile Secretion (Choleresis)

• Bile salts themselves (returning via enterohepatic circulation) are the strongest choleretic stimulus
• Secretin increases watery bicarbonate component from bile ductules
• Vagal stimulation increases bile secretion

Bilirubin

• Breakdown product of hemoglobin from RBC destruction
• Old RBCs destroyed in spleen/liver → heme → biliverdin → bilirubin
• Unconjugated (indirect) bilirubin - water insoluble; bound to albumin; can cross BBB
• Conjugated in liver by UDP-glucuronosyltransferase to conjugated (direct) bilirubin - water soluble
• Excreted in bile → intestine → urobilinogen (some absorbed, excreted in urine + feces as stercobilin = brown color)

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VIII. Small Intestinal Secretion

Crypts of Lieberkühn

• Secrete 1-2 liters/day of isotonic extracellular fluid (nearly identical to plasma)
• This serves as a vehicle to carry nutrients from the intestinal chyme to the villi for absorption
• Goblet cells interspersed throughout - secrete protective mucus
• Secretion driven by: local neural reflexes (myenteric), VIP, serotonin; stimulated by tactile irritation

Brunner's Glands (Duodenum only)

• Located in submucosa of first few centimeters of duodenum
• Secrete alkaline mucus (pH 8.0-9.0)
• Purpose: protect duodenal wall from highly acidic gastric juice
• Inhibited by: sympathetic stimulation (ulcer risk), stress
• Strongly stimulated by: vagal stimulation, secretin, gastric irritants

Digestive Enzymes of Small Intestine

• Enterocytes of the brush border bear peptidases, disaccharidases (maltase, sucrase, lactase), intestinal lipase
• These are not secreted into the lumen but remain bound to the brush border membrane
• Digestion occurs at the membrane surface as food contacts the villi

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IX. Large Intestinal Secretion

• Almost entirely mucous - no digestive enzymes
• Goblet cells are the predominant secretory cells
• Mucus protects the colonic wall from mechanical trauma and bacterial enzymes
• Stimuli: pelvic nerve parasympathetics; extreme stimulation → "mucous colitis" with massive mucus secretion

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Key High-Yield Points for Exams

Must-Know Facts

Zymogen Summary (Critical!)

• Trypsinogen → Trypsin (by enterokinase)
• Chymotrypsinogen → Chymotrypsin (by trypsin)
• Procarboxypeptidase → Carboxypeptidase (by trypsin)
• Proelastase → Elastase (by trypsin)

Hormones Regulating GI Secretion

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Seminar Discussion Points

1. Why do peptic ulcers occur in the duodenum more than stomach? - Gastric mucus barrier is highly protective; Brunner's glands only cover the first few cm; beyond that the duodenum is vulnerable to acid
2. Why does ileal resection cause fat malabsorption? - Terminal ileum is the sole site of bile salt reabsorption; loss disrupts enterohepatic circulation; depleted bile salt pool cannot emulsify fat normally
3. Mechanism of acute pancreatitis - Trypsinogen activated inside acinar cells (e.g., by alcohol, gallstones obstructing duct) → trypsin activates all other zymogens → pancreatic autodigestion
4. Why do H2 blockers and PPIs treat acid reflux and ulcers differently? - H2 blockers block histamine's synergistic effect on parietal cells; PPIs directly block H+/K+-ATPase (the final common pathway)
5. Pernicious anemia mechanism - Autoimmune destruction of parietal cells → loss of IF → B12 cannot be absorbed in terminal ileum → megaloblastic anemia + neurological damage

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Source: Guyton and Hall Textbook of Medical Physiology (ISBN 9780443111013), Chapter 65 - Secretory Functions of the Alimentary Tract, pp. 797-815