List the structures that make up the pathway of air through the respiratory system starting with the external nares.

Nose: Air is inhaled through the nostrils (and sometimes through the mouth) where it is filtered by the hairs and cilia to remove dust particles and moistened. The nasal cavity also moderates the temperature of the inhaled air.

Pharynx: This is a common passage for food, water, and air. It leads from both the nose and the mouth and leads to both the trachea (windpipe) and the oesophagus (foodpipe).

Larynx: This is the voice box situated just over the trachea and has a flap called the epiglottis which closes during swallowing to prevent food from entering the trachea and opens during breathing.

Trachea: Air now enters the windpipe which is situated behind the sternum (breastbone) and between the two lungs.

Bronchus: From the trachea, two bronchi (one bronchus for each lung) enter the lungs and divide and subdivide into secondary and tertiary bronchi, getting narrower as they proceed into the lung.

Bronchioles: The tertiary bronchi branch into fine tubules called bronchioles, the last passageways for air.

Alveoli:. Air ends its pathway in the alveoli (sing. alveolus) which are tiny sacs with very fine singe-celled walls. These alveoli are surrounded by capillaries that connect the pulmonary arteries and veins and that enable transfer of oxygen and carbon dioxide.

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