Final answer:
The bronchial tree is the highly branched system consisting of the bronchi and bronchioles that conduct air through the lungs, leading to the alveoli where gas exchange occurs.
Step-by-step explanation:
The highly branched system of air-conducting passages you are asking about that originate from the left and right primary bronchi and progressively branch into narrower tubes are called the bronchial tree. Starting with the primary bronchi, which enter the lungs, they divide into secondary bronchi, then into tertiary bronchi, and continue branching into even smaller tubes known as bronchioles. These bronchioles further subdivide until they become tiny terminal bronchioles, which ultimately end in clusters of alveoli, the site of gas exchange.
The structure of these bronchi and bronchioles ensure that air is efficiently conducted through the lungs. Cartilage and smooth muscle support the passageways early in the branching, but as they narrow into bronchioles, the walls consist primarily of smooth muscle with elastic fibers because of the absence of cartilage. The entire assembly resembles a tree, with each branch getting progressively smaller as it reaches different parts of the lung. This network is intrinsic to the respiratory zone, controlling air flow and facilitating the vital process of gas exchange between the lungs and the bloodstream.