Gastrointestinal Secretions and Function

Intestinal Hormones and the Stomach

  • Intestinal Gastrin:

    • Briefly stimulates the stomach.
    • Initiates gastric secretion before being inhibited by other hormones.

  • Cholecystokinin (CCK):

    • Released by the duodenum when chyme enters.
    • Inhibits gastric secretion and motility.
    • Stimulates gallbladder contractions to release bile into the intestine.

  • Interogastric Reflex:

    • Involved in inhibiting gastric activity.

  • Sympathetic Nerve Fibers:

    • Suppress gastric activity.

  • Vagal Parasympathetic Stimulation:

    • Initially stimulates the stomach but is eventually inhibited.

The Liver

  • Function:

    • Cleans blood and produces bile.
    • Contains fenestrated sinusoidal capillaries which allow blood component filtration.

  • Liver Regeneration:

    • Capable of regenerating approximately every seven years.

  • Bile Composition:

    • Bile produced continuously by liver cells and stored in the gallbladder.

  • Cystic Duct:

    • Important for bile transport.
    • Blockage could lead to severe health issues, including jaundice and cholestasis.

Pancreatic Function

  • Pancreatic Duct:

    • Important for transporting pancreatic juices, including digestive enzymes.
    • Works with the major duodenal papillae to control enzyme release into the duodenum.

  • Pancreatic Juice Composition:

    • Contains sodium bicarbonate and digestive enzymes that act on carbohydrates, proteins, and fats.
    • Sodium Bicarbonate:
    • Buffers gastric acid arriving in the duodenum.

  • Digestive Enzymes:

    • Examples include trypsin (from trypsinogen), chymotrypsinogen, and procarboxypeptidase for protein digestion and amylase for carbohydrate digestion.

  • Zymogens:

    • Inactive forms of enzymes (e.g., trypsinogen), activated only in the intestine to prevent damage to the pancreas.

Hormonal Regulation in Digestion

  • CCK Role:

    • Stimulates gallbladder contraction and relaxation of the sphincter to allow bile flow.

  • Secretin:

    • Stimulated by acidity of chyme in the duodenum; encourages pancreatic and liver secretion of bicarbonate.

Nutrient Absorption and Digestion

  • Carbohydrates:

    • Digestion starts in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase.

  • Absorption:

    • Glucose, fructose, and galactose are absorbed via specific transport mechanisms.

  • Protein:

    • Broken down into amino acids and dipeptides, primarily through pancreatic enzymes.

  • Fats:

    • Digestion facilitated by bile salts which emulsify fats into smaller droplets for lipase action.

  • Lacteals:

    • Specialized lymphatic vessels in the intestinal villi that absorb fatty acids and glycerol into the lymphatic circulation instead of the blood directly.

GI Tract Structure and Function

  • Small Intestine:

    • Key site for digestion and absorption, featuring circular folds and villi to increase surface area.

  • Peristalsis:

    • Muscle contractions that move food through the digestive tract. Essential for effective digestion and absorption.

  • Mucosa Layer:

    • Contains microvilli and brush border enzymes that facilitate nutrient absorption.

Nutritional Considerations

  • Thermic Effect of Food:

    • Estimated 10% of caloric intake used on digestion and processing of food.

  • Meal Composition Impact:

    • Mixed meals with a combination of sugars may improve nutrient absorption and energy levels.

  • Hormones Affecting Appetite:

    • Ghrelin: Increases hunger.
    • Leptin: Signals satiety.
    • PYY and CCK: Contribute to feeling full, with increased insulin release post-meal suppressing hunger signals.