Respiratory tract
The trachea then splits into the two main bronchi.  This split happens at a place called the carina, which is behind the middle of the sternum (breastbone).  The bronchi are two tubes which carry air into the lungs.  The main bronchi then split into smaller and smaller bronchi, which reach out into all the parts of the lungs like tree branches.  Finally, the smallest bronchi, called bronchioles, end in the alveoli.