Lower Respiratory Tract Anatomy

Overview

This lecture covers the anatomy and key features of the lower respiratory tract, focusing on the larynx, trachea, bronchi, lungs, and key differences between the right and left lungs.

Anatomy of the Larynx

• The lower respiratory tract begins at the larynx, suspended by the hyoid bone.
• The thyroid cartilage forms the main structure but does not completely encircle the larynx.
• The cricoid cartilage lies below the thyroid cartilage and forms a complete ring.
• Above the cricoid cartilage are the paired arytenoid cartilages, which move to adjust vocal ligaments.
• The epiglottis closes over the laryngeal inlet during swallowing to prevent food entry into the airway.
• Vocal ligaments (true vocal cords) vibrate to produce sound; above them are the false vocal cords (vestibular folds).

Trachea and Cough Reflex

• Tracheal cartilages are C-shaped and incomplete posteriorly; the trachea transitions into the bronchi at the carina.
• The carina is a sensitive ridge that triggers the cough reflex if stimulated by foreign objects.
• Ciliated pseudostratified columnar epithelium lines the airway below the vocal cords, moving debris upward (ciliary escalator).
• Smoking paralyzes cilia; thus, suppressing the cough reflex in smokers is dangerous.

Epithelium and Tissue Types

• Stratified squamous epithelium is found in the oral cavity, oropharynx, laryngopharynx, and parts of the larynx exposed to friction.
• Respiratory epithelium (ciliated pseudostratified columnar) is found in the nasal cavity, trachea, and lower airways.
• Finding the wrong epithelium type in a region may indicate metastatic cancer.

Bronchi and Lung Anatomy

• The right main bronchus is wider and more vertical than the more narrow and horizontal left main bronchus; aspirated objects commonly lodge in the right.
• No particles larger than 7 microns should reach below the vocal cords; this is the size of a red blood cell.
• The right lung is shorter, wider, and has a larger volume than the taller, narrower left lung.
• The right lung has three lobes (superior, middle, inferior) separated by horizontal and oblique fissures.
• The left lung has two lobes (superior, inferior) separated by an oblique fissure and has the lingula, a remnant of a possible third lobe.

Hilum and Bronchial Anatomy

• The hilum is where bronchi, arteries, and veins enter/exit the lung.
• "Right Anterior, Left Superior" (RALS) describes the relationship of the pulmonary artery to the bronchus at the hilum: right lung - artery anterior; left lung - artery superior.
• The bronchial tree branches from primary (main) to secondary (lobar) to tertiary (segmental) bronchi.

Key Terms & Definitions

• Larynx — voice box, starts the lower respiratory tract.
• Thyroid Cartilage — main cartilage, partial ring in larynx.
• Cricoid Cartilage — complete ring cartilage under thyroid cartilage.
• Arytenoid Cartilages — paired cartilages controlling vocal cords.
• Vocal Ligaments (True Cords) — ligaments that vibrate to produce sound.
• False Vocal Cords — mucosal folds above the true vocal cords.
• Carina — cartilaginous ridge at bronchial split, triggers cough reflex.
• Ciliary Escalator — system where cilia move debris up and out of airway.
• Hilum — lung region where vessels and bronchi enter or leave.
• RALS — mnemonic for pulmonary artery position: Right - Anterior to bronchus; Left - Superior.

Action Items / Next Steps

• Review diagrams and images of the larynx, trachea, and lungs to reinforce anatomical orientation.
• Memorize the RALS mnemonic for hilum identification in lung anatomy.
• Read about tissue types and their normal locations within the respiratory tract.