Principles of Human Physiology - Gastrointestinal System

Principles of Human Physiology: Gastrointestinal System

Chapter Outline

  • 20.5 Gastrointestinal Secretion and Its Regulation

Learning Outcomes

  • Describe the functions of saliva, stomach acid, pancreatic juice, and bile.
  • Explain how the secretion of each of these substances is regulated.

Gastrointestinal Secretion and Its Regulation (20.5)

  • Key components of gastrointestinal secretion include:
    • Saliva secretion from chief cells.
    • Acid and pepsinogen secretion within the stomach.
    • Secretion of pancreatic juice and bile.
    • Rates of fluid movement throughout the digestive tract.

Acid and Pepsinogen Secretion in the Stomach

  • Acid Secretion:

    • Influenced by the parasympathetic nervous system.
    • Gastrin Hormone: Increases acid secretion.
    • Histamine (paracrine): Enhances acid secretion.

  • Pepsinogen Secretion:

    • Regulated synchronously with acid secretion.
    • Influenced by:
    • Parasympathetic nervous system
    • Gastrin
    • Histamine

Mechanisms of Gastric Secretion Regulation

  • Cephalic Phase Control:

    • Initial stimuli include:
    • Sight, smell, taste of food
    • Chewing and swallowing
    • Triggering response leads to:
    • Increased parasympathetic activity
    • Increased gastrin secretion from G cells
    • Resulting increase in plasma gastrin levels
    • Activation of parietal and chief cells, leading to elevated acid and pepsinogen secretion.

  • Gastric Phase Control:

    • Activated when food enters the stomach.
    • Stimuli for gastric secretion include:
    • Chemoreceptors: Detect proteins, peptides, and amino acids in stomach.
    • Mechanoreceptors: Respond to stomach distension.
    • Resultant actions involve:
    • Short and long reflex pathways to release gastrin, acid, and pepsinogen.

Secretion of Pancreatic Juice and Bile

  • Pancreatic Juice:

    • Produced by acinar cells:
    • Small volume of primary secretion containing water, electrolytes, and digestive enzymes.
    • Secreted by duct cells:
    • Larger volume comprising bicarbonate-rich fluid, which is crucial to counteract acidity from the stomach.

  • Mechanism of Bicarbonate Secretion:

    • Triggered by:
    • Increased acid presence in duodenum.
    • Chemoreceptors: Detect acid levels and initiate response through short and long reflexes leading to:
    • Increased secretion of bicarbonate-rich fluid from pancreatic duct cells via secretin secretion.

Regulation of Bile Entry into the Duodenum

  • Triggered by:
    • Increased acidity present in the duodenum.
    • Presence of protein digestion products and fats.
  • Increased levels of:
    • Secretin
    • Cholecystokinin (CCK)
  • Physiological responses involve:
    • Upregulation of bile secretion into the duodenum to aid digestion.