EP

In-Depth Notes on the Digestive System

Overview of the Digestive System

  • Organs: Mouth, Esophagus, Stomach, Small Intestine, Large Intestine.
  • General Functions:
    • Breakdown of food
    • Nutrient absorption
    • Elimination of waste

Digestive System Components

Accessory Digestive Organs/Structures

  • Salivary Glands: Secrete saliva containing enzymes for carbohydrate breakdown.
  • Liver: Produces bile for fat emulsification.
  • Gallbladder: Stores bile and releases it into the small intestine.
  • Pancreas: Produces pancreatic juice with enzymes and bicarbonate.

Alimentary Canal

  • Mouth: Mechanical breakdown and initial carbohydrate digestion.
  • Pharynx: Connects mouth with esophagus.
  • Esophagus: Transports food via peristalsis to the stomach.
  • Stomach: Acid and enzyme secretion; begins protein digestion.
  • Small Intestine: Final enzymatic breakdown and nutrient absorption.
  • Large Intestine: Absorbs water and forms feces.
  • Rectum: Regulates feces elimination.
  • Anus: Facilitates fecal expulsion.

Structure of the GI Tract Layers

  • Layers vary based on function:
    1. Mucosa
    2. Submucosa
    3. Muscularis Externa
    4. Serosa (Visceral Peritoneum)
    • Note: Structure can differ in various GI regions (e.g., adventitia)

Digestive Processes

  1. Ingestion: Intake of food.
  2. Motility: Involves swallowing and peristalsis, which is a wave-like contraction that moves food through the GI tract.
  3. Secretion: Release of substances such as enzymes, acids, bile, and mucus.
    • Total digestive secretions approximately 9,000 mL.
  4. Digestion:
    • Mechanical Digestion: Involving chewing and segmentation (contractions push food forward then backward).
    • Chemical Digestion: Involves hydrolysis of food using digestive enzymes.
  5. Absorption: Passage of nutrients, vitamins, and minerals into blood or lymph.
  6. Elimination: expulsion of indigestible waste as feces.

Mouth and Salivary Glands

  • Mechanics: Mechanical breakdown occurs through chewing; chemical digestion via saliva.
  • Salivary Glands:
    • Types: Parotid, Sublingual, Submandibular.
    • Composition of saliva: 97% water, electrolytes, enzymes, mucin, and antimicrobial agents.

Pharynx

  • Composed of 3 regions:
    • Nasopharynx
    • Oropharynx
    • Laryngopharynx

Esophagus

  • Connects pharynx to stomach (about 10 inches long).
  • Muscularis externa comprised of different muscle types:
    • Superior 1/3: Skeletal muscle
    • Middle: Mixed skeletal and smooth muscle
    • Inferior 1/3: Smooth muscle.
  • Gastroesophageal Sphincter: Prevents acid reflux; dysfunction results in heartburn.

Stomach Functions

  • Storage: Holds food.
  • Mixing & Digestion: Breaks down food into chyme; absorbs some substances.
  • Muscularis Externa Layers:
    1. Longitudinal layer
    2. Circular layer
    3. Oblique layer.

Stomach Mucosa

  • Cell Types:
    • Surface Mucous Cells: Secrete alkaline fluid (mucin).
    • Mucous Neck Cells: Secrete acidic fluid (mucin).
    • Parietal Cells: Secrete hydrochloric acid and intrinsic factor.
    • Chief Cells: Produce pepsinogen and gastric lipase.
  • Gastric Pits and Glands: Lead to secretion of digestive juices.

Hormonal Signaling in Digestion

  • Signaling Types:
    • Autocrine: Cell affects itself.
    • Paracrine: Cell affects nearby cells.
    • Endocrine: Hormones travel through blood to distant target cells (e.g., gastrin from G-cells).