TRACHEA; BRONCHUS/BRONCHI; BRONCHIOLES; ALVEOLI
When we breathe, air enters our mouth or nose and travels down our windpipe. The medical name for the windpipe is the trachea. The air that travels down the trachea will then flow toward your two lungs by dividing into a right and left main airways. These airways are called the right and left bronchus (bronchi is the pleural form of bronchus).
Each bronchus branches into many smaller tubes or airways called bronchioles. Each bronchiole eventually ends with a cluster of small air-filled sacs called alveoli. Alveoli is the pleural form for these units; a single air-filled sac is called an alveolus. Alveoli are often described as being similar to a cluster of microscopic grapes. These "grapes" have openings that allow air to flow across the cluster. While air can enter an alveolus from a bronchiole, it can also enter from other alveoli that are beside an alveolus. This entire air flow pathway that begins with the nose or mouth and ends with the alveoli is referred to as the airway.