Chapter 26

Chapter 26: Digestive System Anatomy & Physiology

26.1 Organization of the Digestive System

1. Categories of Digestive Organs
A. Gastrointestinal Tract (Digestive Tract, Alimentary Canal)
  1. Forms a continuous tube lined with a mucous membrane
  2. Components:
    • Oral Cavity and Pharynx
    • Esophagus and Stomach
    • Small Intestine
    • Large Intestine
    • Anus
  3. Function: Within the lumen, food is broken down into smaller components to be absorbed.
B. Accessory Digestive Organs
  1. Assist in the breakdown of food
  2. Produce secretions that empty into the GI tract
  3. Components:
    • Salivary Glands
    • Liver
    • Pancreas
    • Teeth and Tongue
    • Gallbladder (stores liver secretions)
  4. Participation: Chewing and swallowing.

26.1b General Functions of the Digestive System

2. Six Main Functions

  1. Ingestion

    • Definition: Introduction of solid and liquid nutrients into the oral cavity.
    • Significance: First step in digesting and absorbing nutrients.

  2. Motility

    • Definition: Voluntary and involuntary muscular contractions.
    • Purpose: Mixing and moving materials through the GI tract.

  3. Secretion

    • Definition: Production and release of fluid products that facilitate digestion (e.g., digestive enzymes, acid, bile).

  4. Digestion

    • Breakdown into smaller structures:
      • Mechanical Digestion: Physically breaking down material (chewing and mixing).
      • Chemical Digestion: Enzyme-driven process to break chemical bonds, converting large complex molecules into smaller molecules.

  5. Absorption

    • Definition: Transport of digested molecules, electrolytes, vitamins, and water from the GI tract into blood or lymph.

  6. Elimination

    • Definition: Expulsion of indigestible components that are not absorbed.

26.1c Gastrointestinal Tract Wall

3. Components of GI Tract Wall
  • Composed of 4 concentric layers called tunics:
    1. Mucosa: Inner lining, consisting of:
    • Epithelium (simple columnar epithelium in most areas for secretion and absorption; nonkeratinized, stratified squamous epithelium in abrasion-resistant areas like the esophagus).
    • Lamina Propria: Areolar tissue with small blood vessels and nerves supporting absorption.
    • Muscularis Mucosae: Smooth muscle that aids secretion and increases material contact with the mucosa.
    1. Submucosa: Areolar and dense irregular connective tissue containing blood vessels and nerves, including the submucosal nerve plexus.
    • Contains mucosa-associated lymphoid tissue (MALT) for immune functions (e.g., Peyer patches in ileum).
    1. Muscularis:
    • Composed of smooth muscle:
      • Inner circular layer: Constricts the lumen.
      • Outer longitudinal layer: Shortens the tube.
    • Myenteric Nerve Plexus: Controls contractions.
    1. Adventitia or Serosa:
    • Adventitia: Areolar connective tissue outside the peritoneal cavity.
    • Serosa: Same as adventitia but has visceral peritoneum covering (only on intraperitoneal organs).
4. Functionality of Layers
  • Mucosa: Important for absorption and secretion; muscularis mucosae enhances contact of chyme with epithelial surface for absorption.
  • Submucosa: Provides elasticity and contains the nerve plexus that innervates glands and muscles.
  • Muscularis: Coordinates peristalsis and segmentation for mixing and propulsion of contents.
  • Adventitia/Serosa: Provides structural support and anchoring in the abdomen.
5. Accessory Digestive Organs: Salivary Glands
A. Salivary Glands
  1. Intrinsic Salivary Glands: Located within the oral cavity; continuously release saliva regardless of eating.
    • Salivary components include lingual lipase (begins fat digestion).
  2. Extrinsic Salivary Glands: Located externally and release saliva in response to food presence:
    • Parotid, Submandibular, Sublingual Glands.
6. Functions of Salivary Glands
  1. Produce 1.0-1.5 liters of saliva daily, mainly during meals.
  2. Composed of 99.5% water and solutes (amylase, mucin, lysozyme).
  3. Functions include:
    • Moistening food for bolus formation.
    • Initiation of starch and fat digestion.
    • Cleaning oral cavity of bacteria and debris.
7. Teeth
  1. Types: Incisors, Canines, Premolars, Molars.
  2. Anatomy of a Tooth:
    • Composed of enamel, dentin, and pulp cavity (contains nerves and blood vessels).
  3. Clinical Note: Dental caries result from bacterial acids damaging tooth structure.
8. Deciduous and Permanent Teeth
  1. Deciduous Teeth: 20 teeth, erupting from 6-30 months.
  2. Permanent Teeth: 32 teeth, replacing deciduous teeth.
  3. Classification of teeth based on function.
9. Pharynx and Esophagus
  1. Pharynx: Funnel-shaped muscular passageway for air and food.
    • Formed by three muscle pairs; lined with non-keratinized stratified squamous epithelium for protection.
  2. Esophagus: A collapsed tube starting at cricoid cartilage; passes through the diaphragm at esophageal hiatus.
  3. Histology: Lined with stratified squamous epithelium, elastic submucosa for distention, and muscularis combining both skeletal muscle (upper third) and smooth muscle (lower third).
10. Swallowing Process
  1. Phases: Voluntary (food manipulation in mouth), Pharyngeal (involuntary reflex), Esophageal (wave-like muscle contractions).
  2. Swallowing pathway prevents food from entering the trachea.

26.2d Stomach Anatomy and Function

1. Gross Anatomy
  1. Location: Superior left abdominal quadrant, beneath the diaphragm.
  2. Shape: J-shaped with curvatures; regions include Cardia, Fundus, Body, and Pylorus.
  3. Function: Mechanical and chemical digestion; forms chyme, absorbs nutrients minimally, and serves as a holding bag before emptying into the small intestine.
2. Histology of Stomach
  1. Lined by simple columnar epithelium with gastric pits/glands for secretion.
  2. Contains specialized cells (e.g., parietal cells for HCl production, chief cells for pepsinogen).
3. Gastric Secretions
  1. Secreted by five types of cells; functions include protection from abrasion and facilitating digestion.
  2. Importance of HCl in digestion and the role of intrinsic factor in vitamin B12 absorption.
4. Motility in Stomach
  1. Functions to mix and propel chyme, regulated by pacemaker cells and hormonal signals.
  2. Distinction between gastric mixing and emptying into the duodenum is crucial for digestion.
5. Clinical Notes on Gastrointestinal Disorders
  • Conditions such as reflux esophagitis and gastric ulcers highlight the importance of gastric secretions and their role in digestion.

26.3 Overview of the Lower GI Tract and Associated Accessory Organs

1. Lower GI Tract Composition
  1. Consists of the small intestine (duodenum, jejunum, ileum) and large intestine, where further digestion, absorption, and waste elimination occurs.
  2. Accessory organs (liver, gallbladder, pancreas) contribute enzymes and bile critical for digestion.
2. Small Intestine Functionality
  1. Segments: Duodenum primarily responsible for chemical digestion; jejunum and ileum focus on nutrient absorption.
  2. Histological Features: Villi and microvilli increase surface area for absorption.
3. Large Intestine Functionality
  1. Absorbs water and electrolytes; compacts feces before elimination.
  2. Composed of cecum (with appendix), colon (ascending, transverse, descending, sigmoid), and rectum.
4. Clinical Views on Digestive Health
  • Pathologies such as inflammatory bowel disease, colorectal cancer, and their implications are crucial for understanding digestive function.

26.4 Digestion and Absorption of Nutrients

1. Carbohydrate Digestion
  1. Salivary Amylase initiates digestion in the oral cavity; pancreatic amylase continues in the small intestine.
  2. Brush Border Enzymes finalize the digestion of carbohydrates.
2. Protein Digestion
  1. Begins in the stomach with pepsin and continues in the small intestine with activated pancreatic enzymes.
  2. Regulation of enzyme activity is vital to prevent autodigestion.
3. Lipid Digestion
  1. Enzymes: Lingual lipase and gastric lipase digest triglycerides to varying extents, with pancreatic lipase completing digestion in the small intestine.
  2. Role of Bile: Required for fat emulsification and effective absorption.
4. Nucleic Acid Digestion
  1. Nucleases break down DNA and RNA, facilitating absorption of nucleotides across the small intestine.
5. Absorption Mechanisms
  1. Water and nutrients absorbed primarily in the small intestine through specific mechanisms including osmosis and active transport.
6. Clinical Considerations
  • Conditions like celiac disease and other digestive disorders highlight the necessity for proper nutrient absorption in maintaining health.