Digestive Anatomy Notes

Digestive Anatomy

Functional Categories

  • Digestive System is divided into two functional categories:
    • Organs involved in digesting indicated by the blue box of digestion or gastrointestinal tract.
    • Organs that don't play a direct role in digesting and absorbing of nutrients, but help with the digestion and absorption.

Peritoneal Cavity

  • Serous membrane in the abdominal pelvic cavity.
  • Two layers:
    • Visceral peritoneum: covers the organs.
    • Parietal peritoneum: covers the inside of the cavity.
  • Mesenteries: made from visceral peritoneum.
    • Greater omentum.
    • Mesocolon: holds the large intestine in place.
    • Lesser omentum.
    • Mesentery proper: holds the small intestines in place.

Retroperitoneal Organs

  • Organs covered by the parietal peritoneum.
    • Pancreas
    • Duodenum (partially)
    • Rectum
    • Uterus
    • Bladder
  • Intraperitoneal organs: covered by the visceral peritoneum (e.g., stomach and intestines).

Greater Omentum

  • Hangs further down than depicted in some diagrams.
  • Apron of fat coming off the greater curve of the stomach.

Mesentery Proper

  • Keeps internal organs in place and prevents twisting.

Basic Layers of the Digestive Tract

  • Four principal layers from mouth to anus:
    • Mucosa: innermost layer.
    • Submucosa: layer below the mucosa.
    • Muscularis externa.
    • Serosa: outermost layer, also known as the visceral peritoneum.
  • Food travels through the digestive tract against the mucosa layer.
Mucosa
  • Epithelium:
    • Varies depending on the location in the digestive tract.
      • Keratinized stratified squamous in the mouth.
      • Simple columnar in the stomach and intestines.
      • Stratified squamous in the anus.
    • Enteroendocrine cells: hormone-producing cells located in the epithelium of the mucosa.
    • May have folds (e.g., rugae of the stomach).
  • Lamina Propria:
    • Connective tissue (areolar).
    • Contains blood vessels, nerve supplies, and lymphatic vessels.
    • May contain mucus glands (e.g., mouth, pharynx, esophagus, duodenum).
  • Muscularis Mucosa:
    • Bottom portion of the mucosa.
    • Inner layer (circular).
    • Outer layer (longitudinal).
Submucosa
  • Contains larger blood vessels (arterioles and venules).
  • Contains nerve fibers (plexus of Meissner) with parasympathetic and sympathetic nerve fibers.
  • Contains exocrine glands that produce buffers and enzymes.
Muscularis Externa
  • Thick region of smooth muscle.
  • Two layers:
    • Inner layer (circular).
    • Outer layer (longitudinal).
  • Myenteric plexus: located between the circular and longitudinal layers.
    • Controls the contraction and relaxation of the smooth muscle layers.
  • Peristalsis and Segmentation:
    • Processes that move food along the digestive tract.
Peristalsis
  • Moves food from place to place (A to B).
  • Contraction of circular muscles behind bolus, relaxation ahead.
  • Contraction of longitudinal muscles to push food along the tube.
Segmentation
  • Multiple portions of the circular muscularis externa contract.
  • Creating segments of bolus.
  • Increases the surface area of the material in the digestive tract.

Oral Cavity (Buccal Cavity, Mouth)

  • Hard Palate:
    • Comprised of maxilla and palatine bone.
    • Lined with keratinized stratified squamous epithelium.
  • Soft Palate:
    • No bone tissue.
    • Not keratinized.
  • Mechanical Digestion:
    • Biting and chewing to increase surface area.
  • Chemical Digestion:
    • Salivary amylase and lingual lipase.
  • Salivary Glands:
    • Provide lubrication.
  • Tonsils:
    • Pharyngeal, palatine, and lingual tonsils.
Salivary Glands
  • Three pairs:
    • Parotid: source of salivary amylase; enters the mouth through the parotid duct.
    • Sublingual: below the tongue; provides buffers and mucin through multiple ducts.
    • Submandibular: provides the majority of saliva (70%); contains salivary amylase and buffers.
  • Produce about 1.5 liters of saliva a day.

Teeth

  • Four Major Zones:
    • Crown: covered in enamel (hardest substance in the body).
    • Neck.
    • Root.
    • Pulp Cavity: contains blood vessels and nerve endings.
  • Dentin: below the enamel, similar to spongy bone.
  • Cementum and Periodontal Ligaments: anchor the tooth to the socket.
  • Types of Teeth:
    • Incisors.
    • Cuspids (canines).
    • Bicuspids (premolars).
    • Molars.
  • Primary Teeth: 20 teeth (five per quadrant) in toddlers.
  • Secondary Teeth: 32 teeth (eight per quadrant) in adults.
  • Dental Formula: indicates the pattern of teeth (e.g., 2-1-2-3 for humans).

Pharynx

  • Nasopharynx, oropharynx, and laryngopharynx.
  • Epiglottis: prevents food from entering the trachea.

Esophagus

  • Highly folded mucosa.
  • Comprised of non-keratinized stratified squamous.
  • First third is pure skeletal muscle tissue.
  • Second third is a combination of skeletal and smooth muscle.
  • Third third is pure smooth muscle.
  • Last inch is retroperitoneal.
  • Allows for expansion to accommodate big chunks of food.

Stomach

  • Four Major Regions:
    • Cardia.
    • Fundus.
    • Body.
    • Pylorus (divided into antrum and pyloric canal).
  • Greater and Lesser Omentums:
    • Greater curvature: greater omentum.
    • Lesser curvature: lesser omentum.
  • Muscularis Externa:
    • Three layers: oblique, circular, and longitudinal.
  • Cardia: many goblet cells, lots of mucus, not a lot of enzyme and acid producing cells.
  • Body and Fundus: chief and parietal cells.
  • Pylorus: G cells and D cells (enteroendocrine cells).
  • Pyloric Sphincter: circular band of smooth muscle that blocks the entrance into the duodenum.
  • Rugae: folds of mucosa to allow for the expansion of the stomach.
  • Mucous Epithelium: simple columnar epithelium covered by a layer of mucus.