Anatomy of the Gastrointestinal System

THE DIGESTIVE / GASTROINTESTINAL TRACT

  • Instructor: Dr. Emma Bailey

  • Location: Room 344, Thomson Building

  • Contact: Emma.Bailey@glasgow.ac.uk

  • Institution: University of Glasgow

INTRODUCTION TO GI TRACT

  • The GI tract consists of mucosa, sub-mucosa, muscle, and serosa across its length.

  • Objectives:

    • Recognize GI tract composition.

    • Identify structures and functions in specific regions (mouth, oesophagus, stomach, intestines, rectum, anus).

OVERVIEW OF THE DIGESTIVE SYSTEM

  • Components:

    • Digestive tract: organ system for food processing

    • Accessory organs: liver, pancreas, gallbladder

  • Primary regions of digestive tract:

    • Oral cavity

    • Pharynx

    • Oesophagus

    • Stomach

    • Small intestine

    • Large intestine

    • Rectum / anus

HISTOLOGY OF GI TRACT

  • Key Layers of the GI Tract:

    • Mucosa:

      • Mucous membrane lining GI tract, secretes mucus for lubrication and protection.

      • Epithelium varies by location.

    • Submucosa:

      • Connective tissue layer containing blood and lymph vessels, and nerves.

    • Muscularis:

      • Typically consists of two layers of smooth muscle: circular and longitudinal.

    • Serosa:

      • Connective tissue covering that secretes fluid to lubricate the exterior of the GI tract.

ORAL CAVITY

  • Boundaries:

    • Lips (anterior)

    • Cheeks (laterally)

    • Palate (superior)

    • Tongue (inferior)

  • Structural Components:

    • Anterior 2/3: Freely movable with a lingual frenulum attachment.

    • Posterior 1/3: Connected to hyoid bone and oropharynx.

  • Pharyngeal arches:

    • Palatoglossal & palatopharyngeal arches house palatine tonsils.

PHARYNX

  • A common passageway for food and air.

  • Connects to the:

    • Larynx (respiratory)

    • Oesophagus (digestive)

  • Three regions:

    • Nasopharynx

    • Oropharynx

    • Laryngopharynx

  • Lining:

    • Nasopharynx: ciliated pseudostratified columnar epithelium

    • Oropharynx & laryngopharynx: moist stratified squamous epithelium

OESOPHAGUS

  • Muscular tube approximately 25cm long.

  • Connects laryngopharynx to stomach.

  • Passes through diaphragm at oesophageal hiatus (T10).

  • Lining:

    • Moist stratified squamous epithelium

  • Muscle Layers:

    • Inner circular and outer longitudinal layers.

  • Sphincters:

    • Upper oesophageal sphincter

    • Lower oesophageal (cardiac) sphincter

STOMACH

  • Shape & Location:

    • J-shaped, located in upper left abdominal quadrant beneath diaphragm.

  • Regions:

    • Fundus

    • Body

    • Pyloric region

  • Sphincters:

    • Cardiac sphincter

    • Pyloric sphincter

  • Lining:

    • Simple columnar epithelium with gastric pits leading to gastric glands.

PERITONEUM

  • Sheet of serous membrane:

    • Parietal peritoneum lines cavities

    • Visceral peritoneum covers organs

  • Cavities:

    • Potential space between parietal and visceral layers

  • Types:

    • Retroperitoneal: organs behind peritoneum

    • Intraperitoneal: organs within peritoneum

SMALL INTESTINE

  • Site of 90% nutrient absorption.

  • Length: >6 meters from pyloric sphincter to ileocecal junction.

  • Subdivisions:

    • Duodenum: ~30cm

    • Jejunum: ~2.5m

    • Ileum: ~3.5m

HISTOLOGY OF SMALL INTESTINE

  • Features:

    • Mucosa and submucosa form circular folds (plicae circularis) which are permanent.

    • Villi increase surface area for absorption.

    • Duodenum has Brunner’s glands (mucus secretion), ileum has lymphoid follicles (Peyer patches).

LARGE INTESTINE

  • Functions:

    • Reabsorbs water

    • Compacts contents into faeces

    • Absorbs vitamins

    • Houses >700 species of bacteria

    • Storage of faecal material

  • Structure:

    • Extends from ileocecal junction to anus

    • Comprised of ascending, transverse, descending colon, rectum, anal canal, and sigmoid colon.

HISTOLOGY OF LARGE INTESTINE

  • Mucosa:

    • Simple columnar epithelium without villi or plicae, featuring tubular glands (crypts).

  • Muscularis:

    • Made up of outer longitudinal (taeniae coli) and inner circular layers to create haustra (pouches for expansion).

RECTUM AND ANAL CANAL

  • Rectum:

    • Terminal part of large intestine, with transverse rectal folds.

  • Anal canal:

    • Composed of superior simple columnar epithelium and inferior stratified squamous epithelium, with anal columns and sinuses.

    • Contains internal (involuntary) and external (voluntary) anal sphincters.

MAJOR SPHINCTERS OF THE GI TRACT

  • Formed by thickenings of the inner circular muscle layer.

    • Upper oesophageal sphincter

    • Pyloric sphincter

    • Ileocaecal valve

    • Internal anal sphincter

SUMMARY

  • Comprehensive overview of oral cavity, pharynx, oesophagus, stomach, peritoneum, small intestine, and large intestine.

  • Understanding of structure, histology, and functions across the GI tract.