The end of the trachea divides and forms two branches. Right and left branches. Both branches are called bronchus. The point where the trachea divides is called the carina. The right bronchus is surrounded by 9-12 and the left bronchus by 6-8 cartilaginous rings. The right branch divides into 3 parts and enters the right lung and the left branch divides into 2 parts and enters the left lung. Each branch divides repeatedly to form numerous bronchioles. Each bronchiole contains numerous tiny air chambers. These are called alveoli. Alveolus is the structural and functional unit in the lungs. The entire trachea looks like an inverted tree and is called the respiratory tree.
Function: Air enters the lungs from the trachea through the bronchus and air containing CO2 leaves the lungs through the trachea.