GI Tract Organ Structure and Functions

Introduction to the GI Tract Organs and Structure

  • In part two of chapter 24, the focus shifts to the gastrointestinal (GI) tract organs and their general structure.

Elementary Canal Organs

  • The organs that compose the elementary canal itself include:

    • Mouth

    • Pharynx (throat)

    • Esophagus

    • Stomach

    • Small intestine

    • Large intestine

    • Rectum

    • Anal canal

    • Anus

  • These constitute the parts of the elementary canal proper.

Accessory Digestive Organs

  • Accessory digestive organs are connected to the GI tract via ducts, but they are not part of the elementary canal. They include:

    • Teeth

    • Tongue

    • Salivary glands (with ducts that secrete saliva)

    • Liver (with hepatic duct)

    • Gallbladder (with cystic duct)

    • Pancreas

  • Detailed insights will be provided on the ducts associated with each organ.

Structure of the GI Tract

  • Understanding the layers of the gastrointestinal tract is crucial. The wall of the GI tract, which extends from the lower esophagus to the anal canal, consists of four basic layers, generally organized from the inside out:

    1. Mucosa

    2. Submucosa

    3. Muscularis

    4. Serosa

Mucosa

  • Definition: The innermost layer, moist and mucus-secreting.

  • Function: Secretion and absorption.

  • **Structure:

    • Composed of three sub-layers:**

    • Epithelium:

      • The most common type of epithelium lining the GI tract is simple columnar epithelium, which serves the functions of protection, secretion, and absorption.

    • Lamina Propria:

      • A layer of connective tissue containing blood and lymphatic vessels, as well as Mucosa Associated Lymphatic Tissue (MALT), which is part of the immune system.

    • Muscularis Mucosa:

      • A thin layer of smooth muscle that tenses mucosa, creating grooves and ridges to enhance surface area and contact with food.

Submucosa

  • Definition: The layer beneath the mucosa.

  • Formation: Composed of areolar connective tissue binding to the muscularis mucosa.

  • Characteristics: Contains numerous blood and lymphatic vessels and houses the submucosal plexus, an intrinsic network of nerve cells responsible for controlling secretions.

Muscularis Externa

  • Definition: The third layer of muscle; distinguished from muscularis mucosa.

  • Composition:

    • Formed of voluntary skeletal muscle in certain locations:

    • Mouth

    • Pharynx

    • Upper two-thirds of the esophagus

    • Anal sphincter

    • Composed of involuntary smooth muscle elsewhere.

  • Arrangement: Contains inner circular fibers (contracts to constrict) and outer longitudinal fibers (contracts to shorten), aiding in movement and mixing of food.

  • Specific structure: Myenteric Plexus, which controls motility throughout the GI tract.

Serosa / Adventitia

  • Definition: The outermost layer.

  • Classification:

    • Adventitia:

    • The outermost layer of the esophagus, which lacks serosa because the esophagus is situated outside the abdominal cavity.

    • Serosa:

    • The outer covering of organs within the abdominal-pelvic cavity, also known as visceral peritoneum.

  • Function: Secretes fluid to lubricate the external GI tract and reduce friction during organ movement.

Enteric Nervous System

  • The enteric nervous system (ENS) is crucial for controlling activities within the digestive tract. It includes:

    • Neurons extending from esophagus to anus.

    • Plexuses:

    • Myenteric Plexus: Controls GI tract motility (movement).

    • Submucosal Plexus: Controls GI tract secretions.

  • Sympathetic vs. Parasympathetic Influence:

    • Parasympathetic Division: Stimulates secretions and motility through the vagus nerve (cranial nerve number 10). This division promotes:

    • Flow of saliva

    • Peristalsis and secretions

    • Release of bile

    • Sympathetic Division: Inhibits secretions and peristalsis, appropriate during stressful situations.

Peritoneum

  • Definition: The largest serous membrane of the body.

  • Importance: Supports freedom of movement for the stomach and intestines during contractions while holding the organs in place.

  • Folds of Peritoneum:

    • Mesentery: Provides structure for blood vessels and nerves leading to and from GI organs. Holds abdominal viscera in proper relationships, preventing twisting and tangling of small intestine from contractions.

  • Layers of Peritoneum:

    • Parietal Peritoneum: Outer layer lining the abdominal-pelvic cavity.

    • Visceral Peritoneum: Inner layer that covers certain organs (synonymous with serosa).

  • Peritoneal Cavity: The space between the two peritoneal layers that contains lubricating serous fluid.

Types of Peritoneal Organs

  • Intraperitoneal Organs: Covered by mesentery or serosa on both sides (sandwiched).

  • Retroperitoneal Organs: Located against the posterior body wall, covered by peritoneum only on the anterior side. The following organs are retroperitoneal:

    • Kidneys

    • Adrenal glands

    • Duodenum

    • Most of the pancreas

    • Parts of the large intestine

Conclusion

  • The GI tract has intricate structures and systems, critical for proper digestion and overall health. Understanding these foundational concepts is essential for further study in digestive physiology and anatomy.