The nose and nasal cavity as well as the pharynx are the major conduits of air into the body. The nose is a cartilagenous, bony structure with 2 openings to the outside called nares.
The three parts of the pharynx listed above are also conduits for movement of air into the larynx and then into the lower recesses of the human respiratory system. The oropharynx is where chewing of food (mastication) occurs and is a conduit for swallowing (deglutition) of food into the esophagus and down the gastrointestinal tract. When food and air are both present during chewing and breathing, a very complex network of nerves and muscles must act to prevent movement of food into the larynx and further down into the respiratory tract (aspiration).
From the larynx the air moves into the trachea which is an air conduit composed of a semicircular cartilagenous wall. The trachea enters the thoracic cavity and therein divides (bifurcates) into the left and right mainstem bronchi which further divide and eventually lose their cartiagenous structure, becoming bronchioles.
These bronchioles further divide and eventually the terminal respiratory unit is reached where gas exchange can occur. The terminal respiratory unit consists of a single terminal bronchiole from which arise a branching chain (probably 2 or 3 divisions) of respiratory bronchioles which are studded with individual alveoli for gas exchange. Beyond the respiratory bronchioles are the branchings of the alveolar ducts, sacs and alveoli.
The alveoli are separated from the pulmonary capillaries by a thin membrane known as the alveolocapillary membrane. It is at this point that actual gas exchange transpires.