Gastrointestinal Physiology – 2025-2026

DIGESTIVE SECRETIONS

  • Digestive secretions are produced by various glands including the salivary glands, gastric glands, exocrine pancreas, and liver. They can be classified into three types based on the mechanism of release: neurocrine, endocrine, or paracrine.

  • Mucous glands secrete mucus which serves functions of lubrication and protection within the alimentary tract.

SALIVARY SECRETION

Composition of Saliva

  1. Water: Main component of saliva.

  2. Electrolytes: Includes sodium (Na+), potassium (K+), calcium (Ca²+), bicarbonate (HCO₃⁻), chloride (Cl⁻), and phosphate (P).

  3. Organic Constituents:

    • Mucoproteins: ABO blood group substances and mucins.

    • Enzymes:

      • α-amylase (breaks down starch).

      • Lingual lipase (breaks down fats).

      • Ribonuclease (breaks down RNA).

      • Antibacterial agents (e.g., muramidase).

    • Other: R-protein and epidermal growth factor.

Production of Saliva

  • Saliva formation occurs in a two-step process:

    1. Acinar Cells: Na+ and Cl⁻ enter cells from the blood via the Na⁺/K⁺-ATPase pump. Cl⁻ diffuses into the duct lumen while Na⁺ is pumped out. HCO₃⁻ and K⁺ enter passively.

    2. Ductal Cells: The initial saliva is modified here; Na+ and Cl⁻ are reabsorbed, which alters the ionic composition of saliva to be hypotonic compared to plasma.

Influence of Flow Rate on Ionic Composition of Saliva

  • Salivary electrolyte composition changes with flow rate:

    • At higher flow rates, Na+ and Cl⁻ levels rise in saliva as efficiency of reabsorption decreases.

    • HCO₃⁻ secretion increases initially with flow but stabilizes while K+ levels remain unchanged.

Autonomic Nervous System Influence

  • Salivary secretion is controlled by the autonomic nervous system:

    • Sympathetic Stimulation: Provides a small increase in saliva output.

    • Parasympathetic Stimulation: Leads to a large increase in saliva output, affecting both electrolyte and flow rates.

    • Neurotransmitters involved include acetylcholine, norepinephrine, and vasoactive intestinal peptide (VIP).

Clinical Conditions Associated with Salivary Secretion

  1. Mumps: Infectious parotitis affecting parotid glands.

  2. Sjögren's Syndrome: Autoimmune condition leading to decreased salivary secretions.

  3. Xerostomia: Condition defined as dry mouth; prevalent in elderly populations contributing to difficulties in chewing, swallowing, and increased dental caries.

PANCREATIC SECRETION

Anatomy and Function of the Pancreas

  • The pancreas has both exocrine and endocrine functions. The exocrine pancreas secretes pancreatic juice into the duodenum, involving:

    • HCO₃⁻-Containing Aqueous Component.

    • Digestive Enzymes: Critical for digestion of nutrients.

    • The pancreas has a lobular structure, with acinar and duct cells contributing to secretion.

Bicarbonate Secretion Mechanism

  • The bicarbonate secretion process is complex, primarily involving carbonic anhydrase which converts CO₂ into HCO₃⁻ and H+. Mechanisms include:

    • H+ is exchanged for Na+ across the membrane.

    • Exchanging Cl⁻ for HCO₃⁻ in the ducts.

Electrolyte Composition of Pancreatic Juice

  • Pancreatic juice is isotonic to plasma with varying chloride and bicarbonate levels affecting the ionic balance.

  • Contains cations K+ and Na+, and trace elements (Ca²+, Mg²+, Zn²+).

Regulation of Pancreatic Secretion

  • Hormonal regulation includes:

    1. Secretin: Stimulates bicarbonate secretion in response to acid in the duodenum.

    2. Cholecystokinin (CCK): Triggers enzyme release from acinar cells in response to amino acids and fatty acids.

  • Neural regulation occurs via vagal reflexes and sympathetic pathways.

  • Potentiation exists where multiple stimulants enhance secretory response.

Pathophysiology Related to Pancreatic Function

  1. Pancreatitis: Inflammatory condition potentially caused by alcohol or gallstones.

  2. Steatorrhea: Loss of pancreatic lipase leading to fat malabsorption.

  3. Cystic Fibrosis: Genetic condition affecting chloride conductance and pancreatic secretions.

BILIARY SYSTEM

Structure of the Liver and Gallbladder

  • The liver is structured into lobules that facilitate blood flow and substance processing. Hepatocytes synthesize bile, which is essential for lipid digestion/absorption.

  • Bile travels through canaliculi to larger ducts and ultimately to the duodenum via the common bile duct.

Bile Formation and Composition

  • Formation: Liver cells secrete bile acids from cholesterol.

  • Composition: Bile consists of electrolytes, bile acids, pigments (bilirubin), phospholipids, and cholesterol.

Enterohepatic Circulation

  • About 95% of bile salts are reabsorbed in the ileum and return to the liver, maintaining a necessary pool for digestion.

Gallbladder Motility and Function

  • The gallbladder stores and concentrates bile, releasing it into the duodenum during digestion via CCK stimulation.

Clinical Conditions Related to Gallbladder and Liver Function

  1. Jaundice: Condition resulting from excess bilirubin, with causes like hemolytic disease or liver dysfunction.

  2. Cholecystitis: Gallbladder inflammation often related to gallstone obstruction.

  3. Cholestasis: Condition leading to suppressed bile secretion, causing accumulation of bile components.

GASTRIC SECRETION

Structure of Gastric Mucosa

  • Divided into cardiac, oxyntic, and pyloric regions with specialized functions.

  • Secretions Include:

    • HCl (from parietal cells)

    • Pepsinogen (from chief cells)

    • Mucus and bicarbonate for protection.

Acid Production by Oxyntic Cells

  • Oxyntic cells utilize a Na+/K+-ATPase mechanism to produce hydrogen ions actively secreted into the gastric lumen.

Stimulation of Gastric Secretion

  • Cephalic, Gastric, and Intestinal Phases: Each phase triggers gastric acid secretion through neural, hormonal, and mechanical means.

Mucosal Defense Mechanisms

  • Mucus and bicarbonate secretion creates a protective gastric barrier against acid. Prostaglandins enhance this protective role.

Clinical Conditions Related to Gastric Function

  1. Gastric Mucosal Atrophy: Loss of acid-producing cells affecting vitamin B12 absorption.

  2. Ulcer Formation: Strongly associated with H. pylori infection.

  3. Gastroesophageal Reflux Disease (GERD): Resulting from an incompetent lower esophageal sphincter.

DIGESTION AND ABSORPTION

Carbohydrate Digestion and Absorption

  • Begins in the mouth with salivary amylase, with major digestion occurring in the intestine via pancreatic amylase. Final absorption occurs as monosaccharides.

Protein Digestion and Absorption

  • Initiated by pepsin in the stomach, with further digestion via pancreas-derived proteases in the small intestine.

Lipid Digestion and Absorption

  • Emulsification by bile salts is essential for lipid digestion, with absorption into enterocytes occurring through both passive and facilitated transport mechanisms.

Mineral Absorption

  • Essential minerals are absorbed across intestinal segments via active and passive transport mechanisms, highly regulated by physiological cues.

Abnormalities in Digestion and Absorption

  • Conditions include lactase deficiency leading to intolerance, sprue affecting gluten metabolism, and fat malabsorption resulting from enzyme deficiencies.