respiratory tract

The upper respiratory tract includes the nose and nasal cavity, paranasal sinuses, pharynx (nasopharynx, oropharynx, and laryngopharynx), and larynx. The lower respiratory tract begins at the trachea and continues through the bronchi, bronchioles, and alveoli of the lungs. The division at the larynx/trachea boundary is not universally standardized, with some classifications placing the larynx in the upper tract and others in the lower, but functionally the upper tract conditions (inspired) air while the lower tract exchanges gases.

The nasal cavity performs three key conditioning functions on inspired air: warming (blood-rich mucosa raises air temperature toward body temperature), humidifying (mucous secretions add moisture to near 100% relative humidity), and filtering (turbinate bones create turbulent airflow that deposits particles onto mucus; cilia then sweep particles toward the pharynx). These functions protect the delicate alveolar surfaces from cold, dry, or contaminated air.

Respiratory tract infections are divided by location. Upper respiratory tract infections (URTIs) include the common cold (rhinovirus), sinusitis, pharyngitis (strep throat), and laryngitis. Lower respiratory tract infections (LRTIs) include bronchitis, bronchiolitis (common in infants), pneumonia (bacterial, viral, or atypical), and tuberculosis. LRTIs are generally more serious because they involve gas-exchange surfaces and can cause hypoxemia.