The trachea is a tube about 4 inches long and less than an inch in diameter. The trachea begins just under the larynx and runs down behind the breastbone. The trachea then divides into two smaller tubes called bronchi one bronchus for each lung.
The trachea is composed of about 20 rings of tough cartilage. The back part of each ring is made of muscle and connective tissue. Moist, smooth tissue called mucosa lines the inside of the trachea. The trachea widens and lengthens slightly with each breath in, returning to its resting size with each breath out.