A&P II: Module 8: The Digestive System (Part 1)

Basic Anatomy and Functions of the Digestive System

• The digestive system is responsible for breaking down food, releasing nutrients, and absorbing those nutrients into the body.
• Divided into two main categories:
  • Accessory Digestive Organs:
  • Teeth, tongue, salivary glands, gallbladder, liver, pancreas.
  • Aid in the breakdown of food:
    • Mechanical Digestion: Teeth and tongue
    • Chemical Digestion: Salivary glands (secrete enzymes), gallbladder, liver, and pancreas (release bile and enzymes).
  • Alimentary Canal Organs:
  • One-way tube from mouth to anus includes:
  • Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.
  • Main function is to nourish the body.

Histology of Alimentary Canal

• Comprised of four layers:
  1. Mucosa:
  • Mucous membrane with three components:
    • Epithelium: Direct contact with digested food; includes goblet cells (secrete mucus) and enteroendocrine cells (secrete hormones).
    • Lamina Propria: Connective tissue layer with blood and lymphatic vessels; houses lymphocytes.
    • Muscularis Mucosa: Smooth muscle layer that folds the mucosa.
  1. Submucosa:
  • Dense connective tissue layer with blood/lymphatic vessels and submucosal glands; releases digestive secretions.
  1. Muscularis:
  • Consists of two layers of smooth muscle; promotes mechanical digestion and moves food along the canal.
  1. Serosa:
  • Visceral peritoneum layer in the abdominal cavity; loose connective tissue.

Neural Control of the Digestive System

• Intrinsic Innervation (Enteric Nervous System):
  • Runs from esophagus to anus and contains two plexuses:
  • Myenteric Plexus: Motility (muscle contractions).
  • Submucosal Plexus: Regulates digestive secretions.
• Extrinsic Innervation:
  • Autonomic nervous system (sympathetic/parasympathetic):
  • Sympathetic activation decreases GI motility and secretions.
  • Parasympathetic activation increases GI motility and secretions.

Blood Supply of the Digestive Tract

• Blood supplies transport absorbed nutrients and provide oxygen to digestive organs.
• Arteries: Hepatic, splenic, left gastric, inferior and superior mesenteric arteries.
• Hepatic portal circulation:
  • Drains nutrient-rich blood from digestive organs to liver for processing.

Membranes of the Digestive Tract

• The peritoneum holds digestive organs in place, composed of:
  • Parietal Peritoneum: Lines abdominal cavity.
  • Visceral Peritoneum: Envelopes organs, contains large folds.
  • Major Peritoneal Folds:
  • Greater omentum, falciform ligament, lesser omentum, mesentery, mesocolon.

Digestive Processes

• The digestive system employs mechanical and chemical activities involving:
  1. Ingestion: Food enters the alimentary canal through the mouth.
  2. Propulsion: Movement of food (swallowing and peristalsis).
  3. Mechanical Digestion: Physical breakdown without altering food's chemical nature (chewing, stomach churning, segmentation).
  4. Chemical Digestion: Enzymatic breakdown into chemical building blocks; starts in mouth and finishes in small intestine.
  5. Absorption: Nutrients are absorbed into the bloodstream, primarily in the small intestine.
  6. Defecation: Undigested materials are expelled from the body as feces.

Flow of Food from Mouth to Anus

• Pathway: Mouth → oral cavity → tongue → pharynx → esophagus → stomach → small intestine (duodenum → jejunum → ileum) → large intestine (cecum → colon → rectum → anus).
• In the mouth, saliva moistens food and initiates carbohydrate digestion (salivary amylase).
• Peristalsis moves bolus from the pharynx to the esophagus, with esophageal mucus aiding lubrication.
• The stomach serves as a mixing chamber, where food becomes chyme through mechanical and chemical digestion (e.g., HCl activates pepsin).

Key Takeaways

1. The digestive system breaks down food and absorbs nutrients.
2. Organs are classified as accessory or alimentary canal organs.
3. Accessory organs aid digestion, while alimentary canal organs nourish the body.
4. The alimentary canal has its own nervous system and receives input from autonomic nerves.
5. Digestive organs work together through mechanical and chemical means.
6. Digestion includes ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.