Drawn into the respiratory system through the nostrils or external nares, air enters the nasal cavities, where it is warmed and moistened and where dust particles are filtered out. While the bony ridges in the cavities cause the incoming air to swirl, insuring that all the air is exposed to these processes, the mucous lining and the cilia of the cavities' epithelial cells increase the efficiency of these processes. The air passes from the cavities through the internal nares into the pharynx, a passage through which food also must travel on its way to the esophagus. The mixing of the respiratory and digestive systems is the result of the fact that the nasal passages and mouth arose evolutionarily first as organs of smell and taste and only later as organs for breathing. To prevent food from entering the respiratory system during swallowing, the soft palate at the back of the mouth covers the internal nares while the epiglottis covers the glottis, the opening through which the air leaves the pharynx and enters the larynx. The cartilage-encased larynx, also known as the Adam's apple, functions as the voice box. When air passes through the larynx, the air causes the two elastic ridges or vocal chords that stretch across the larynx to vibrate, producing sound. Changes in the tension of the chords effect changes in the pitch of the sounds produced.
From the larynx, the air passes into the trachea, the duct that leads to the thoracic cavity. The cilia of the trachea's epithelial lining prevents dust and mucus from reaching the lungs. The cartilage rings of the tracheal wall prevent the trachea
