Pharynx

The pharynx divides into three sections: the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx is the upper segment behind the nasal cavity, lined with ciliated pseudostratified columnar epithelium. The oropharynx is the middle section serving as a shared pathway for air and food. The laryngopharynx is the lowest part, opening into both the esophagus and larynx.

The nasopharynx contains pharyngeal tonsils, or adenoids, which trap dust and pathogens from inhaled air. These lymphoid tissue clusters act as a reservoir of lymphocytes and are covered with ciliated epithelium that captures and eliminates pathogens. The ciliated pseudostratified columnar epithelium also propels mucus downward to remove trapped particles.

The oropharynx and laryngopharynx are lined with nonkeratinized stratified squamous epithelium to withstand abrasion from food particles, unlike the nasopharynx's ciliated columnar epithelium. This epithelial shift occurs as the nasopharynx transitions into the oropharynx at the fauces. The stratified squamous tissue provides protection in areas where food and liquid pass through.

The oropharynx houses two sets of tonsils—palatine tonsils embedded in lateral walls and lingual tonsils posterior to the oral cavity. Both are composed of lymphoid tissue that captures and eliminates pathogens entering via oral or nasal cavities. These tonsils function as part of the immune system's first line of defense in the respiratory and digestive pathways.

The uvula, a small teardrop-shaped structure at the soft palate's peak, swings upward during swallowing along with the soft palate. This movement seals off the nasopharynx, preventing ingestion of materials into the nasal cavity. This reflex action ensures food and liquid travel only into the esophagus rather than the respiratory pathway.

The nasopharynx is linked to each middle ear cavity by auditory, or Eustachian, tubes. When pathogens from a common cold infect the nasopharynx, they can travel through these tubes and cause middle ear infections. This anatomical connection explains the frequent association between upper respiratory infections and ear complications.

The pharynx is approximately 13 centimeters long and extends from the internal nares, or nasal openings, down to the cricoid cartilage of the larynx. It serves as a continuous conduit from the nasal cavities through the three pharyngeal regions. At its lower end, the laryngopharynx opens anteriorly into the larynx while posteriorly leading into the esophagus.