The trachea, also called the windpipe, is part of the passageway that supplies air to the lungs. Any prolonged blockage, even for a few minutes, can cause death.

The trachea is about 4.5 inches long and 1 inch in diameter, and is comprised of smooth muscle and several c-shaped rings of cartilage. The rings of cartilage provide stability and help to prevent the trachea from collapsing and blocking off the airways. The trachea extends from the neck and divides into two main bronchi.


Structurally similar to the trachea, the two primary bronchi are located inside the lungs. The right bronchus is slightly larger than the left one. Because of this, foreign objects breathed into the lungs often end up in the right bronchus. The bronchi are lined with the same type of mucus that lines the rest of the respiratory tract.

Deeper into the lungs, each bronchus is further divided into five smaller, secondary bronchi, which provide air to the lobes of the lungs. The secondary bronchi continue to branch off to form the tertiary bronchi, which are further divided into terminal bronchioles. There are as many as 30,000 tiny bronchioles in each lung. They lead to the alveoli by way of alveolar ducts.

Bronchial tree

Together, the trachea and the two primary bronchi are referred to as the bronchial tree. At the end of the bronchial tree lie the alveolar ducts, the alveolar sacs, and the alveoli. 

The tubes that make up the bronchial tree perform the same function as the trachea: they distribute air to the lungs. The alveoli are responsible for the primary function of the lungs, which is exchanging carbon dioxide and oxygen. 

A layer of protective mucus, called a mucus blanket, covers a large portion of the membrane lining the bronchial tree. The mucus is an important air purifier.

The average adult produces about 125 milliliters of mucus daily, which is slightly more than half a cup. Microscopic, hair-like cilia move the cleansing mucus up to the pharynx—part of the throat between the mouth and esophagus—from the lower part of the bronchial tree. Cigarette smoke paralyzes the cilia, which allows mucus to accumulate and leads to what is called smoker’s cough.