Digestive system (Inter) 2025

Digestive System

Overview

  • The digestive system consists of the digestive tract and accessory digestive glands.
  • Digestion involves both mechanical and chemical processes to break down food into smaller pieces and molecules.
  • The digestive tract includes the oral cavity, pharynx, esophagus, stomach, intestine, anal canal, and anus.
  • Accessory digestive glands include salivary glands, liver & gallbladder, and pancreas.

Digestive Wall Layers

  • The digestive wall is composed of 4 layers:
    1. Mucosa
    2. Submucosa
    3. Muscularis externa
    4. Serosa/Adventitia

Mucosa

  • Composed of 3 parts:
    • Epithelium:
      • The lumen is mainly lined by simple columnar epithelium, except in the mouth, esophagus, and anal canal, which are lined by nonkeratinized stratified squamous epithelium.
      • Functions to secrete mucus and enzymes, protect underlying tissue, and carry out secretion and absorption.
    • Lamina propria: Loose connective tissue
    • Muscularis mucosae: Thin smooth muscle layer

Submucosa

  • Thick layer of connective tissue containing:
    • Submucosal plexus (Meissner’s plexus): Controls secretion of glands in the digestive tract.
    • Blood and lymph vessels.
    • Submucosal glands.
    • Functions to nourish surrounding tissues and carry away absorbed material.

Muscularis Externa

  • Smooth muscle with 2 layers (except in the stomach, which has 3 layers):
    • Inner circular layer.
    • Outer longitudinal layer.
    • Myenteric or Auerbach’s plexus located between the 2 muscle layers.
    • Functions to regulate the movement of the digestive tract.

Serosa / Adventitia

  • Outermost layer.
    • Serosa: Visceral peritoneum, consisting of simple squamous epithelium and connective tissue.
    • Adventitia: Only connective tissue layer, found in the pharynx, esophagus, and rectum.
    • Functions to protect the underlying tissue and secrete serous fluid to moisten & lubricate the organ.

Peritoneum

  • Body wall & organ of abdominal cavity are covered with serous membrane, peritoneum.
  • Composed of 3 parts:
    1. Parietal peritoneum
    2. Visceral peritoneum or serosa
    3. Part attaching between 1 and 2 (Peritoneal ligaments)
  • The cavity between the parietal and visceral peritoneum is called the “Peritoneal cavity,” containing serous fluid for lubrication and reduced resistance of intestinal movement during digestion.

Peritoneum Attachments

  • Falciform ligament: Connects the anterior wall to the liver and diaphragm.
  • Greater omentum: Connects the greater curvature of the stomach to the transverse colon.
  • Lesser omentum: Connects the lesser curvature of the stomach to the liver.
  • Mesentery: Connects the small intestine to the posterior wall.
  • Mesocolon: Connects the large intestine to the posterior wall.
  • Many organs of the abdominal cavity are held in place by a double layer of peritoneum , to allow the passage of blood vessels and nerves to supply the organs.

Omentum and Ligaments Details

  • Greater omentum: Connects the greater curvature of the stomach to the transverse colon.
  • Lesser Omentum: Connects the lesser curvature of the stomach and duodenum to the liver.
  • Falciform ligament: Connects the anterior abdominal wall to the liver and diaphragm; its inferior part contains the round ligament of the liver (fetal umbilical vein).
  • Mesentery: Connects the small intestine (jejunum & ileum) to the posterior abdominal wall.
  • Mesocolon: Connects the large intestine to the posterior abdominal wall.

Oral Cavity or Mouth

  • Divided into 2 parts based on location, anterior or posterior to the teeth.
    1. Oral cavity (proper): Posterior to the teeth.
      • Superior: Palate (hard palate - bone; soft palate – skeletal muscle that extends inferiorly in the midline, “uvula”).
      • Inferior: Tongue.
      • Posterior: Oropharynx
    2. Oral Vestibule: Anterior to the teeth, between the cheek and lip and external surface of the tooth arch.
  • Oral cavity and vestibule communicate posterior to the 3rd molar teeth.

Tongue

  • Skeletal muscular organ to manipulate food.
  • The dorsal surface is covered by stratified squamous epithelium.
  • The lingual frenulum attaches the mid-inferior part to the floor to prevent excess movement.
  • Lingual papillae:
    • Filiform papilla: Without taste buds, functions to attach food during chewing.
    • Papillae with taste buds:
      • Fungiform: Mushroom shape
      • Circumvallate: Each surrounded by a narrow channel, located in a V-shaped line
  • The dorsal surface of the tongue is divided into an anterior two-thirds and a posterior one-third. The posterior one-third has lymphoid tissue (lingual tonsil).

Salivary Glands

  • Three pairs of salivary glands secrete saliva into the oral cavity.
  • Saliva: A mixture of serous and mucus fluid containing:
    • Amylase
    • Immunoglobulin A (a weak antibacterial action)
  • Gland locations and ducts:
    • Parotid gland: Largest gland situated below and in front of the pinna; the parotid duct opens into the oral vestibule at the level of the second upper 2nd molar tooth; Mumps impact this gland.
    • Each duct opens lateral to the lingual frenulum
    • Numerous ducts that open inferior to the tongue