GI Secretions

GI Secretions Overview

  • Dr. Shona McQuilken presents on gastrointestinal (GI) secretions.

Learning Objectives

  • After studying this topic, you should be able to:

    • Discuss secretions from major organs:

      • Salivary glands

      • Stomach

      • Pancreas

      • Liver and Gallbladder

      • Intestine

    • Provide an overview of control mechanisms for digestive system secretions.

Digestion Challenge

  • Question: How are nutrients transmitted to cells effectively?

Functions of GI Secretions

  • Main Functions:

    • Chemical Digestion: Breaks down food into absorbable units.

    • Lubrication: Facilitates smoother movement within the digestive tract.

    • Signalling: Communicates to facilitate digestion and absorption.

    • Protection: Guards against pathogens and mechanical damages.

    • Activation of Enzymes: Converts inactive forms to active forms needed for digestion.

    • Excretion of Waste: Eliminates undigested substances.

Gland Types

  • Exocrine Glands: Secrete substances onto epithelial surfaces via ducts.

  • Endocrine Glands: Secrete hormones directly into the bloodstream.

Blood and Chemical Secretions

  • Hormones are secreted into the blood through capillaries.

  • Chemicals are produced by glands and secreted onto surfaces.

Gastrointestinal Secretions by Organ

  • Salivary Glands:

    • Secrete a lubricating fluid with enzymes for carbohydrate breakdown.

  • Gastric Secretions:

    • Comprises hydrochloric acid, pepsinogen, and gastric lipase.

  • Pancreas:

    • Exocrine cells: Secrete digestive enzymes and buffers.

    • Endocrine cells: Secrete hormones such as insulin.

  • Liver/Gallbladder:

    • Secrete and modify bile.

  • Small Intestine:

    • Secretion of digestive enzymes.

Salivary Glands

  • Major Salivary Glands: Three pairs that secrete saliva via ducts.

  • Minor Salivary Glands: Approximately 600-1000 located in the mucosal lining of the oral cavity.

    • Each gland functions independently with its own duct.

Saliva Composition and Functions

  • Composition:

    • 99.4% water, 0.6% mucins, electrolytes (Na+, Cl-, HCO3-), antibodies, enzymes (amylase).

  • Functions:

    • Buffers to maintain pH ~7.

    • Keeps mucosa moist to prevent damage and aid speech.

    • Solvent for dissolving chemicals for taste detection.

Xerostomia

  • Xerostomia refers to dry mouth due to inadequate saliva production.

Gastric Secretions

  • Gastric Juice Composition:

    • Water, hydrochloric acid, pepsinogen, intrinsic factor, and mucus.

  • Engages both endocrine and exocrine processes.

Gastric Gland Cell Types

  • Mucous Neck Cells: Secrete mucus to provide a protective barrier.

  • Parietal Cells:

    • Secrete intrinsic factor (for Vitamin B12 absorption).

    • Produce hydrochloric acid, maintaining a pH of ~1-3.

  • Chief Cells: Secrete pepsinogen (activated to pepsin) and gastric lipase to digest lipids.

Pancreatic Secretions

  • Exocrine Secretions:

    • Produces 1.2-1.5L of pancreatic juice/day: water, bicarbonate (NaHCO3 to neutralize stomach acid), and digestive enzymes.

    • Enzymes Include:

      • Pancreatic amylase for starch digestion.

      • Pancreatic lipase for fat digestion.

      • Ribonuclease and deoxyribonuclease for RNA and DNA digestion.

Pancreatic Enzyme Activation

  • Proteases secreted in inactive forms (trypsinogen, chymotrypsinogen, procarboxypeptidase) are activated in the duodenum.

    • Key Activation Enzymes:

      • Enteropeptidase activates trypsinogen to trypsin.

      • Trypsin activates other proteases.

Endocrine Functions of Pancreas

  • Hormonal Secretions:

    • Alpha cells: Secrete glucagon in response to low blood glucose.

    • Beta cells: Secrete insulin in response to high blood glucose, inhibited by adrenaline.

    • Delta cells: Secrete somatostatin to inhibit gastrin release.

Bile Production and Direction

  • Produced by hepatocytes, proceeds through bile canaliculi to bile ducts.

Bile Composition and Functions

  • Composition:

    • 800-1000mL/day, pH 7.6-8.6, contains bile salts, bilirubin, cholesterol, neutral fats, phospholipids, electrolytes.

  • Functions:

    • Assists in lipid assimilation and neutralizes gastric acid.

Gallbladder

  • Stores and concentrates bile, releasing it through the cystic duct into the bile duct.

Intestinal Secretions

  • Various specialized cells (absorptive, goblet, enteroendocrine) contribute to digestive secretions in the small intestine.

Regulation of Secretion Phases

  • Cephalic Phase: Initiated by sensory stimuli, activates salivary and gastric secretion via CNS.

  • Gastric Phase: Distention of the stomach and chemoreceptors stimulate gastric juices.

  • Intestinal Phase: Entry of food into the small intestine inhibits gastric emptying and secretions; regulated via neural and hormonal mechanisms (CCK, secretin).

Further Reading References

  • Principles of Anatomy & Physiology by Tortora & Derrickson.

  • Netter's Essential Physiology 2nd Edition.