The pulmonary system consists of the airways (nasal passages, mouth, nasopharynx, larynx, and trachea), lungs (two lobes on the left, three on the right), diaphragm, and pulmonary blood vessels. Turbinates are tissue protrusions in each nostril that create air turbulence. Large pollutants, such as dust, are trapped by nasal hairs, whereas the tiny capillaries of the nares warm and humidify the air. Stimulation of nasal irritant receptors by triggers such as pollen activates the sneeze reflex and clears the nares. Insensible water loss of approximately 1 pint per day occurs as we provide humidity to the air we breathe. The frontal, maxillary, and ethmoid sinuses are air-filled spaces that provide resonance to the voice. The larynx contains the vocal cords. Aspiration is prevented by the epiglottis, a tissue flap that closes over the tracheal opening during swallowing.

The trachea branches into the right and left bronchus and then further divides 16 times, finally ending in terminal bronchioles. Mucus and cilia, which line the airway, trap and remove inspired foreign particles by an escalator motion. When the mucus reaches the pharynx it is either swallowed or expectorated. If tracheal or large airway irritant sensors are triggered, the cough reflex is activated and the lower airways are cleared. The ability to clean and humidify the airway and prevent an environment where bacteria can flourish is impaired by anything that damages the mucociliary system, such as dehydration, smoking, dry air, mouth breathing, or a tracheostomy that bypasses the nares.