Gastrointestinal Secretions

Physiology of the Gastrointestinal Tract

Introduction

  • Department of Biomedical Sciences, College of Veterinary Medicine, Tuskegee University.

  • Instructor: Ayman I. Sayegh.

Gastrointestinal Secretions

Sources of Gut Secretions

  • Epithelial Cells

    • Salivary Glands

    • Stomach

    • Pancreas and Liver

Epithelial Secretions

  • Components:

    • Water & Electrolytes

    • Mucus

    • Special Molecules

Water and Electrolytes

  • Functions:

    • Secreted throughout the gut.

    • Liquefies and dissolves food.

    • Serves as a reaction medium.

    • Releases H$^+$ and Bicarbonate.

Mucus

  • Functions:

    • Lubricates gastrointestinal content.

    • Protects the epithelial surfaces.

Specialized Molecules

  • Functions of specialized molecules:

    • Aid in digestion and absorption of nutrients.

Salivary Secretions

Types of Salivary Secretions

  • Major Salivary Glands:

    • Parotid: Serous secretions.

    • Submandibular: Mixed secretions (serous & mucous).

    • Sublingual: Mixed secretions (serous & mucous).

Characteristics of Salivary Glands

  • High permeability.

  • High vascularity and blood flow, approximately 20 times more than muscles.

Cellular Composition of Salivary Glands

  • Components:

    • Serous cells

    • Myoepithelial cells

    • Salivary duct (secretory)

    • Intercalated duct

    • Mucous cells

    • Demilune of serous cells

    • Basement membrane

Functions of Saliva

  • Key Functions:

    • Digestion: Initiates carbohydrate digestion.

    • Lubrication: Facilitates swallowing.

    • Protection: Protects oral mucosa and prevents decay.

Digestion by Saliva

  • Key Enzymes and Functions:

    • α-Amylase (Ptyalin): Breaks down α-1,4 glycosidic bonds.

    • Products include maltose, maltotriose, and dextrins.

    • Active at neutral pH; pancreatic amylase active at acidic pH.

    • Lingual lipase: Begins lipid digestion, active at acidic pH.

Lubrication and Protection

  • Functions:

    • Lubrication: Provided by mucus.

    • Protection:

    • Diluting effect of saliva.

    • Bactericidal properties due to lysozymes and lactoferrins.

Compositions of Saliva

  • Organic Composition:

    • Digestive enzymes, mucus/glycoproteins,

    • Lysozymes, lactoferrin, kallikrein (converts plasma protein to bradykinin, a vasodilator).

  • Inorganic Composition:

    • Ions (Cl$^-$, Na$^+$, K$^+$, HCO$_3^-$) with a notable increase during meals.

Gastric Secretions

Composition of Gastric Secretions

  • Major Components:

    • Hydrochloric acid (HCl)

    • Pepsinogen

    • Mucus

    • Intrinsic Factor

Functions of Gastric Secretions

  • HCl:

    • Converts pepsinogen to pepsin, bacteriostatic properties.

  • Pepsin:

    • Involved in protein digestion.

  • Mucus:

    • Provides protection, lubrication, and forms a barrier.

  • Intrinsic Factor:

    • Essential for Vitamin B12 absorption.

Sources of Gastric Secretions

  • Fundus: (Oxyntic Glands, 80%)

    • Acid-producing parietal cells, pepsinogen from chief or peptic cells, intrinsic factor from chief cells.

  • Antrum: (Pyloric Glands, 20%)

    • Mucus cells and gastrin-producing cells.

Physiological Divisions of the Stomach

  • Esophagus

  • Fundus

  • Cardiac Region

  • Lower Esophageal Sphincter

  • Oxyntic Zone

  • Pylorus and Duodenum.

Rates of Gastric Acid Secretion

  • Basal Secretion: Interdigestive state, no gastrointestinal stimulation.

  • Peak Secretion: Actual digestive state with food intake.

Sources of Gastric Acid Secretion - Two Component Theory

  • Non-parietal (Independent of ingestion): Basal levels of NaCl.

  • Parietal (Digestive): Peak levels of HCl.

Characteristics of Gastric Acid Secretion

  • Primarily meal-dependent, some hormonal/neuronal influences.

  • Acid secretion does not linearly match gastrin levels.

  • Potentiation effects noted during meal consumption.

Control of Gastric Acid Secretion

  • Phases:

    • Cephalic Phase (30%): Triggered by sight or thought of food, vagally mediated, increased by hypoglycemia and insulin.

    • Gastric Phase (60%): Influenced by stomach distension, alcohol, calcium, and caffeine; unaffected by vagotomy.

    • Intestinal Phase (10%): Involves duodenal amino acids, independent of gastrin.

Gastric Diseases

  • Common Issues:

    • Ulceration: Differentiate between gastric vs. duodenal ulcers.

    • Elevated pepsinogen I/II levels.

    • Gastrinoma presence.

    • Best exemplified in small animals via H. pylori and NSAID use.

Pepsin Enzymatic Activity

  • Activation: Secreted as pepsinogen, cleaved by pH; functions in digesting proteins by targeting interior peptide bonds.

  • Stimulated by: Vagus nerve and secretin.

Pepsinogen Groups

  • Group I (PG I): Secreted from oxyntic glands in the stomach.

  • Group II (PG II): Secreted from pyloric glands in the duodenum.

Gastric Mucus Types

  • Water-Soluble Mucus:

    • Secreted by mucous neck cells; provides lubrication and is stimulated vagally.

  • Water-Insoluble Mucus:

    • Secreted by surface mucous cells; serves as an alkaline physical barrier, stimulated mechanically or chemically.

Parietal Gland Structure

  • Components:

    • Gastric pit (foveolus); surface mucus cells; mucus neck cells; parietal cells; chief cells.

Intrinsic Factor Role

  • Produced by parietal cells; fundamental for binding and absorption of Vitamin B12; deficiency can lead to pernicious anemia.

Pancreatic Secretions

Pancreatic Anatomy

  • Components:

    • Duodenum

    • Pancreas

    • Islands of Langerhans

    • Acinar cells

    • Ductal cells.

Pancreatic Secretions Overview

  • Types:

    • Endocrine: Hormonal secretions (4% of total secretion).

    • Exocrine: Ions and enzymatic secretions (90% of total secretion).

Endocrine and Exocrine Secretions

  • Islands of Langerhans: Secrete insulin and glucagon.

  • Ductal Cells: Secrete ions (NaCl and bicarbonates).

  • Acinar Cells: Release digestive enzymes (lipase, amylase, trypsinogen).

Endocrine Secretions and Blood Sugar Control

  • Low Blood Sugar:

    • Glucagon release: elevates blood sugar by stimulating glycogen breakdown in the liver.

  • High Blood Sugar:

    • Insulin release: lowers blood sugar by promoting glucose uptake and glycogen formation.

Ionic Secretions of Ductal Cells

  • Low Rate Products: NaCl.

  • High Rate Products: Bicarbonates; consistent with the two component theory.

Exocrine Secretions and Digestion

  • Active Enzymes: Lipase, Amylase.

  • Inactive Enzyme: Trypsinogen, activated by enterokinase.

Control of Pancreatic Secretions

  • Mechanism:

    • Secretions are stored as inactive to prevent autodigestion by trypsin.

Phases of Pancreatic Secretions

  • Cephalic Phase: 20%; initiated by the thought of food, vagally mediated with low volume and high concentration of enzymes.

  • Gastric Phase: 5-10%; maintains secretion via vagal mechanisms.

  • Intestinal Phase: 70%; hormonal control through cholecystokinin (CCK).

Clinical Considerations for Pancreatic Health

  • Pancreatitis: Acute or chronic states may show elevated amylase within 3-6 hours and lipase over 1-14 days.

  • Best Tests: Measure secretin and CCK; 80% damage results in steatorrhea (fat in stools).

  • Exocrine Pancreatic Insufficiency: Indicates low serum trypsinogen-like immunoreactivity.

Gallbladder and Bile Secretions

Gallbladder Anatomy

  • Structure: Sac-like, closely related to the liver and pancreas.

  • Control Mechanisms: Vagal and hormonal regulation.

Functions of Bile

  • Digestive Roles:

    • Fat digestion, emulsification, and absorption.

    • Formation of micelles, critical for fatty acid diffusion and absorption.

    • Elimination of various products (cholesterol, heavy metals, bile pigments, drugs).

Micelle Formation

  • Process Involvement:

    • Formation of emulsion droplets involving triglycerides, bile salts, and lipase leads to the diffusion of monoglycerides and fatty acids into cells.

Composition of Bile

  • Organic Components:

    • Bile acids (50%), phospholipids (30-40%), cholesterol (4%), bile pigments (2%).

  • Inorganic Components:

    • Water, bicarbonate, and ions.

Characteristics of Bile Secretions

  • Daily synthesis approximately 0.5 gm; in dogs, concentration <25 umol/L.

  • Synthesized from cholesterol, with 90-95% bound to plasma proteins.

Bile Acids Production and Regulation

  • Produced continuously by hepatocytes, with secretion peaked 30 minutes after a meal, regulated by hormonal and vagal stimuli.

  • Reabsorption:

    • 50% passively reabsorbed in the upper intestine (unconjugated).

    • 45% actively reabsorbed in the lower intestine (conjugated).

Phospholipids in Bile

  • Functionality:

    • Amphipathic properties facilitating micelle formation and lipid absorption.

Cholesterol in Bile

  • Health Implications:

    • Excess cholesterol may contribute to gallstone formation.

Bile Pigment (Bilirubin)

  • Source: Breakdown of red blood cells.

  • Clinical Significance: Elevated bilirubin may lead to jaundice; may serve as a nidus for gallstones.

Types of Gallstones

  • Two Main Types:

    • Cholesterol Gallstones: 50-75% of cases.

    • Pigment Gallstones: Formed from unconjugated bilirubin with calcium.

Gallstones and Clinical Outcomes

  • Associated Conditions:

    • Bile duct obstruction leading to jaundice, abdominal pain, and vomiting.

    • Diagnostic tools include GGT, X-ray, ultrasound, liver enzymes (ALT, AST, ALP).

Treatment for Gallstones

  • Medical Management Options:

    • Extracorporeal shock wave lithotripsy (ESWL).

    • Dietary modifications, statins, Vitamin B3 (niacin).

    • Cholecystectomy: surgical removal of the gallbladder, no direct effect on digestion.

Inorganic Components of Gallbladder Secretions

  • Constituents:

    • Water, bicarbonate, and various ions.

    • Concentration of bile achieved through reabsorption processes.