Organs of the Upper Respiratory Tract

Complete, exhaustive anatomical study covering the structural organization, mucous membranes, regional compartments, and clinical pathologies of the upper airway. Prepared for Diploma-Level Clinical Medicine Curriculum.

1. Introduction

The upper respiratory tract consists of the nose, nasal cavity, paranasal sinuses, pharynx, and larynx. These structures serve as the entry portal for air, conditioning it through warming, humidification, and filtration before it reaches the lower respiratory tract. The upper airway also houses the organs of olfaction (smell) and phonation (voice production).

2. The Nose and Nasal Cavity

2.1 External Nose

The external nose is a visible part of the respiratory system that projects from the face. It is composed of:

• Bone: Nasal bones, frontal processes of the maxillae, and the nasal part of the frontal bone.
• Cartilage: Septal, lateral, alar, and major/minor cartilages.
• Nares: The anterior openings (nostrils), bounded laterally by the alae.
• The bridge is supported by bone, while the tip and alae are cartilaginous and mobile.

2.2 Nasal Cavity

The nasal cavity extends from the nostrils anteriorly to the choanae posteriorly, where it communicates with the nasopharynx. It is divided into right and left halves by the nasal septum, which is composed of the perpendicular plate of the ethmoid, the vomer, and the septal cartilage.

Boundaries:

• Floor: Palatine process of the maxilla and horizontal plate of the palatine bone (hard palate).
• Roof: Nasal bones, cribriform plate of the ethmoid, and the body of the sphenoid.
• Lateral wall: Composed of several bones and characterized by the presence of conchae.

2.3 Nasal Conchae (Turbinates)

The lateral wall contains three bony projections: superior, middle, and inferior conchae. These structures increase the surface area for warming and humidifying inspired air. Beneath each concha is a passage called a meatus:

• Superior meatus: Receives drainage from the posterior ethmoidal air cells.
• Middle meatus: Receives drainage from the frontal sinus, maxillary sinus, and anterior/middle ethmoidal air cells.
• Inferior meatus: Receives drainage from the nasolacrimal duct.

2.4 Nasal Mucosa

• Respiratory region: Lined with ciliated pseudostratified columnar epithelium with goblet cells. It covers most of the cavity. Mucus traps particles, and cilia move the mucus toward the pharynx (the mucociliary escalator).
• Olfactory region: Located in the roof and superior concha. It contains olfactory receptor neurons with cilia that bind odorant molecules. Axons pass through the cribriform plate to the olfactory bulb.
• A rich vascular plexus (cavernous tissue) warms inspired air.

2.5 Paranasal Sinuses

These are air-filled cavities within cranial bones that communicate with the nasal cavity:

• Frontal sinuses: Within the frontal bone; drain into the middle meatus via the frontonasal duct.
• Maxillary sinuses: The largest; located within the body of the maxilla. Their floor is near the alveolar process of the maxilla, making them vulnerable to dental infection.
• Ethmoidal sinuses: Multiple small air cells within the ethmoid bone.
• Sphenoidal sinuses: Within the body of the sphenoid; drain into the sphenoethmoidal recess.

Functions: Lighten the skull bones, resonate the voice, and produce mucus. Inflammation is known as sinusitis.

3. The Pharynx

The pharynx is a muscular tube (12–14 cm) extending from the base of the skull to the esophagus. It serves as a common passage for air and food.

3.1 Nasopharynx

• Posterior to the nasal cavity and above the soft palate.
• Lined by respiratory epithelium.
• Contains the pharyngeal tonsil (adenoids) on the posterior wall.
• The auditory (Eustachian) tube opens into the lateral wall, equalizing middle ear pressure.
• During swallowing, the soft palate elevates to seal the nasopharynx, preventing nasal regurgitation.

3.2 Oropharynx

• Posterior to the oral cavity, between the soft palate and the epiglottis.
• Contains the palatine tonsils in the tonsillar fossae.
• Contains the lingual tonsil at the base of the tongue.
• Waldeyer ring: A ring of lymphoid tissue (pharyngeal, palatine, and lingual tonsils) surrounding the oropharyngeal inlet for immune defense.

3.3 Laryngopharynx (Hypopharynx)

• Extends from the tip of the epiglottis to the inferior border of the cricoid cartilage (C6), where it becomes the esophagus.
• The piriform fossae are recesses on either side of the laryngeal inlet; foreign bodies may lodge here.
• During swallowing, the larynx elevates and the epiglottis tilts downward to cover the laryngeal inlet, directing food into the esophagus.

4. The Larynx

The larynx (voice box) is a cartilaginous structure located at the C3–C6 vertebral levels. It connects the pharynx to the trachea and serves three functions: airway patency, phonation, and protection of the lower airway.

4.1 Laryngeal Cartilages

• Thyroid cartilage: The largest; forms the laryngeal prominence (Adam's apple). Composed of two laminae joined anteriorly.
• Cricoid cartilage: Signet-ring shaped; the only complete cartilaginous ring of the airway.
• Epiglottis: Leaf-shaped elastic cartilage attached to the thyroid cartilage. During swallowing, it folds down to cover the laryngeal inlet.
• Arytenoid cartilages: Paired pyramidal cartilages on the cricoid; articulate with the cricoid and move the vocal cords.
• Corniculate and cuneiform cartilages: Small paired cartilages within the aryepiglottic folds.

4.2 Laryngeal Cavity

• Vestibule: Space above the vestibular (false) vocal folds.
• Vestibular folds (false cords): Superior mucosal folds containing vestibular ligaments; no role in phonation.
• Vocal folds (true cords): Inferior mucosal folds containing vocal ligaments and the vocalis muscle. The space between them is the rima glottidis.
• Glottis: Comprises the vocal folds and the rima glottidis; the narrowest part of the larynx in adults.
• Subglottis: Region below the vocal folds extending to the inferior border of the cricoid.

4.3 Intrinsic Muscles

4.4 Nerve Supply

• Superior laryngeal nerve (CN X): Internal branch provides sensory innervation above the cords; external branch supplies the cricothyroid muscle.
• Recurrent laryngeal nerve (CN X): Motor to all intrinsic muscles except cricothyroid; sensory below the vocal cords.

5. Clinical Correlations

6. Key Points Summary

• The nasal cavity warms, humidifies, and filters air; the olfactory region detects odors.
• Paranasal sinuses lighten the skull, resonate voice, and drain into specific nasal meatuses.
• The pharynx has three regions: nasopharynx (air only), oropharynx (air and food), and laryngopharynx (air and food).
• The larynx contains vocal folds for phonation and the epiglottis for airway protection.
• The recurrent laryngeal nerve innervates most intrinsic laryngeal muscles; injury results in voice changes or airway distress.