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.
