Position & Connections
Superiorly opens to the nasal cavity
Anteriorly / centrally opens to oral cavity
Inferiorly bifurcates into:
Esophagus (posterior)
Larynx → trachea (anterior)
Epithelial lining
Moist stratified squamous epithelium (mucous membrane)
“Moist” = mucous-secreting; far fewer keratinized dead cells than epidermis
Function: protection against abrasion from food bolus
Same epithelium continues through the entire esophagus; switches to simple epithelium in the stomach
Rule of thumb
Wherever you see a stratified epithelium in the body, its purpose is protection
Nasopharynx
Contains pharyngeal tonsils (R & L)
Houses openings of the auditory (Eustachian) tubes
Air-filled passage to the middle ear (temporal bone)
Equalises pressure across the tympanic membrane
Example: cabin pressure changes during airplane take-off/landing → muffled hearing
Oropharynx
Adjacent to oral cavity
Contains palatine tonsils & lingual tonsils
Laryngopharynx
Opens to both larynx and esophagus
Spatial note: esophagus lies posterior to larynx/trachea → digestive tract runs posterior to respiratory tract from this point downward
Acts as a common passageway for both swallowed material and inhaled/exhaled air
Performs no mechanical or chemical digestion
Muscular tube (~25 cm) running through thorax
Passes through diaphragm at the esophageal hiatus to enter stomach
Upper Esophageal Sphincter (UES)
Junction of pharynx & esophagus
Tightly contracted except during swallowing
Lower Esophageal Sphincter (LES)
Junction of esophagus & stomach
Prevents reflux of gastric contents
Failure ➞ esophageal reflux / heartburn; stomach contents have \text{pH}=1\text{–}2 and damage unprotected esophageal mucosa
General principle: Every digestive-tract junction is guarded by a sphincter
Mucosa
Stratified squamous epithelium
Lamina propria (loose CT)
Muscularis mucosae (thin smooth muscle)
Submucosa (loose/areolar CT)
Blood/lymph vessels, nerves, glands
Muscularis externa
Inner circular layer
Outer longitudinal layer
Produces peristalsis
Adventitia (outermost)
Dense CT anchoring esophagus to surrounding tissues (unlike serosa found in abdominal GI organs)
Lumen morphology
Soft-tissue walls → lumen is collapsed & irregular when empty
Expands & conforms to bolus shape during swallowing
Definition: coordinated reflex that moves a bolus from oral cavity to stomach
Tongue pushes bolus posteriorly against oropharynx → activates stretch/pressure receptors in pharyngeal wall
Triggered by sensory input from pharynx
Simultaneous events:
Upper esophageal sphincter relaxes (opens)
Epiglottis pulled downward to cover glottis (laryngeal opening)
Epiglottis = cartilaginous flap; "epi" = above, "glottis" = vocal opening
Prevents aspiration; failure ➞ choking → cough reflex
Additional safety: vestibular folds/membranes above vocal cords can trap stray material
Soft palate elevates → seals nasopharynx, preventing bolus entry into nasal cavity
Pharyngeal (superior, middle, inferior) constrictor muscles contract sequentially behind bolus → peristaltic wave toward esophagus
UES re-closes once bolus passes
Esophageal peristalsis propels bolus toward stomach
LES reflexively relaxes on approach, then re-contracts after entry to maintain barrier against reflux
Acid reflux (GERD)
LES incompetence → gastric acid backflow → mucosal irritation ("heartburn")
Protective adaptation
Stratified epithelium in mouth, pharynx, esophagus shields against abrasive solid food
Stomach switches to simple columnar epithelium specialized to withstand acidity and secrete mucus
Respiratory interaction
Pharynx serves both digestive & respiratory tracts → coordination via epiglottis is critical
Swallow–breath coordination essential; dysphagia increases aspiration risk
Equalisation of middle-ear pressure (auditory tube)
Demonstrates GI/respiratory structures’ role in auditory function & real-world phenomena (air travel pressure changes)