Ch. 40: Digestion and Ingestion

Overview of the Digestive System

  • Primary Function: The digestive system's primary goal is to provide essential nutrients for the internal environment of the body, making these the nutrients available to every cell.

  • Digestive Tract: It acts as an extension of the external environment; substances only enter the body upon absorption into the internal milieu.

Mechanisms of Digestive Function

  • Ingestion: The process of taking food into the body.

  • Digestion: The breakdown of food into simpler, absorbable nutrients.

  • Motility: The movement of food through the digestive tract is essential for physically reducing large food items and assisting in moving food along.

  • Secretion: Involves digestive enzymes that facilitate chemical digestion.

  • Absorption: The phase where nutrients pass into the internal environment.

  • Elimination: The excretion of unabsorbed material.

  • Regulation: Coordination of various digestive activities.

Enteric Nervous System

  • Read and discussion: Refer to Box 40.1 for insights on the Enteric Nervous System.

Types of Digestion

1. Mechanical Digestion

  • Purpose: Transforms large food particles into smaller ones, assisting chemical digestion.

  • Process:

    • Churning: Mixes food with digestive juices, ensuring contact with intestinal mucosa for absorption.

    • Mastication: The Chewing process to break down food.

    • Deglutition (Swallowing):

    • Oral Stage: Voluntary control to form a bolus.

    • Pharyngeal Stage: Involuntary movement of the bolus into the esophagus.

    • Esophageal Stage: Involuntary movement into the stomach.

2. Types of Motility

  • Peristalsis: A Wavelike ripple of muscle contraction, creating a stepwise movement of food down the tract.

  • Segmentation: Mixing movements create a forward and backward motion to enhance food particle interaction with digestive juices.

Gastric Motility

  • Types: Peristalsis and segmentation are key in the GI tract for moving and mixing food.

  • Chyme: Resulting semi-liquid material from the stomach that moves into the duodenum, typically ejected every 20 seconds; gastric emptying takes 2 to 6 hours.

  • Hormonal Regulation:

    • Gastric Inhibitory Peptide (GIP): Released from the duodenum, slows peristalsis, thus controlling the rate of chyme entering the small intestine.

Intestinal Motility

  • Function: Peristalsis moves the food along the intestinal tract; segmentation mixes contents with digestive juices from accessory organs.

  • Cholecystokinin (CCK): Hormone stimulating increased peristalsis and digestive activity as chyme approaches the jejunum.

Chemical Digestion

  • Definition: Involves biochemical breakdown, chiefly through hydrolysis.

  • Digestive Enzymes: Reaction-specific organic catalysts that operate in the lumen of digestive structures.

    • Properties of Enzymes:

    • Specific action (lock and key model).

    • Function optimally at a designated pH.

    • Notably, many are inactivated proenzymes initially synthesized and require continual replacements.

Carbohydrate Digestion

  • Components of Saccharides:

    • Polysaccharides: Multiple saccharides (C₆H₁₀O₅).

    • Disaccharides: Two saccharide units (sucrose, lactose, maltose).

    • Monosaccharides: Basic units like glucose, fructose, and galactose.

  • Hydrolysis Pathway:

    • Amylases (from saliva and pancreas) convert polysaccharides to disaccharides.

    • Enzymatic Breakdown: Disaccharides are broken down into monosaccharides by sucrase, lactase, and maltase in the intestinal mucosa.

Protein Digestion

  • Enzymatic Action: Proteases catalyze protein hydrolysis into peptides, which are further broken down into amino acids.

    • Types of Proteases: Pepsin (gastric juice), trypsin & chymotrypsin (pancreatic juice).

    • Peptidases: Located in the intestinal brush border for peptide breakdown.

Fat Digestion

  • Emulsification Definition: Process of dispersing fats into small droplets.

    • Agents: Lecithin and bile salts are crucial for emulsification in the small intestine.

  • Role of Lipases: Digest emulsified fats into fatty acids, monoglycerides, and glycerol.

Residues of Digestion

  • Compounds Rejected by Digestion:

    • Cellulose: Dietary fiber.

    • Undigested Connective Tissue: Commonly from meats.

    • Unprocessed Fats: Resulting in fecal matter alongside bacteria, pigments, water, and mucus.

Secretion in Digestion

  • Saliva Composition: Consists primarily of water, mucus, and enzymes (e.g., amylase, lipase).

  • Gastric Juice Components:

    • Pepsin: Initiates protein digestion.

    • Hydrochloric Acid (HCl): Lowers pH, enabling pepsin function; secreted by parietal cells.

    • Intrinsic Factor: Facilitates vitamin B12 absorption.

    • Mucus: Protects and lubricates chyme.

  • Pancreatic Juice Components:

    • Proteases: Digest proteins and polypeptides (e.g., trypsin, chymotrypsin).

    • Lipases: Enzymes for fat digestion.

    • Nucleases: Work on nucleic acids (DNA, RNA).

    • Amylase: Acts on starches.

    • Bicarbonate: Buffers pH in the small intestine.

Bile Functionality

  • Source and Storage: Produced and secreted by the liver, stored in the gallbladder.

  • Components: Lecithin and bile salts emulsify fats by encasing them in shells to form tiny spheres, called micelles. Sodium bicarbonate raises pH for enzyme optimization.

  • Waste Products: Bilirubin and other detoxification products are expelled via feces.

Intestinal Juice Production

  • Composition: Secreted by intestinal exocrine cells, consisting of mucus and water for mixing chyme.

  • Regulation: Mechanisms are not fully understood but likely involve hormonal stimuli from the intestinal mucosa.

Absorption Process

  • Overview: Passage of nutrients through the intestinal mucosa into capillaries predominantly occurs in the small intestine.

  • Nutrient Pathways:

    • Proteins: Absorbed as amino acids into the bloodstream.

    • Carbohydrates: Converted to simple sugars before entering capillaries.

    • Fats: Absorbed as glycerol, monoglycerides, and fatty acids via lymphatic lacteals.

  • Absorptive Surface Increase: Intestinal villi and microvilli enhance absorptive capacity significantly.

Elimination Process

  • Defecation Mechanism: Involves expelling fecal residues; the rectum remains typically empty, and peristaltic movements transfer fecal matter, stimulating the desire to defecate.

  • Constipation: Characterized by the delayed movement of contents through the lower colon and rectum.

  • Diarrhea: Arises from increased intestinal motility, resulting in reduced water absorption, leading to watery stools.

Suggested Reading

  • Review text pages 915-938 for detailed diagrams, case studies, and a comprehensive overview of digestive processes.