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.