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The division of the trachea forms the right and left bronchi. Each of the bronchi runs obliquely, before entering the hilius of the lungs. The right primary bronchi is wider, straighter, and shorter that the left primary bronchi. When air enters the bronchi, it is warm, humidified, and cleansed of all pathogens.

The lungs are the primary organs of the respiratory system. The lungs occupy the thoracic cavity. The apex, the superior portion of the lung, is located below the clavicle. The base of the lungs is the section that rest upon the diaphragm. Each lung is subdivided into lobes. Three lobes characterize the right lung; two lobes characterize the left lung.

The surface of the lungs is covered by a visceral serosa known as the visceral, or pulmonary, pleura. Each wall of the thoracic cavity is covered by the parietal pleura. Each pleural membrane forms a secretion known as the pleural fluid. The pleural fluid allows the lungs to slide over the thoracic wall during respiration. The pleural mucosae can slide from one another and side to side, but they resist being pulled apart. Due to the characteristics of the pleural mucosae, the lungs rest quite tightly in the thorax wall.

When the primary bronchi enter the lungs, they divide into smaller branches. After the continuous shortening of size, the primary bronchi become the bronchioles. The terminal bronchioles divide to become the respiratory zone structures. The respiratory zone structures eventually divide to become the alveoli. The respiratory zone houses the transfusion of air to blood. The respiratory zone also contains the alveoli, the alveolar sacs, the alveolar ducts, and the respiratory bronchioles. The remaining area of the lungs are known as the conducting zone structures. The conducting zone structures contain all the passages to and from the respiratory zone. Within the lung there are millions of alveoli. The alveoli are responsible for the transfusion of oxygen to blood.