The trachea, or windpipe, helps supply air to the lungs by providing a passage way from the mouth. It is about 4 to 5 inches long and 1 inch in diameter in the average adult.

It is made of smooth muscle and several c-shaped rings of cartilage. Those rings provide stability and help prevent the trachea from collapsing and shutting off the airway.

The trachea extends from the neck and divides into two main bronchi that divide to the lungs. They are like the trunk of the bronchial tree.

The bronchi are structurally similar to the trachea, as they are divisions from it. The bronchi are lined with the same type of mucus that lines the rest of the respiratory tract.

Foreign objects breathed into the lungs often end up in the right bronchus, as it is larger than the left.

Once inside 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 (tiny air sacs where the exchange of carbon dioxide and oxygen occurs) by way of alveolar ducts.

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 finally 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 primarily responsible for exchanging carbon dioxide and oxygen, which occurs in the lungs.

A layer of protective mucus, called a mucus blanket, covers a large portion of the membrane lining the bronchial tree and is an important air purifier. Your body produces about a half cup of mucus daily. Microscopic, hair-like cilia move the cleansing mucus up to the pharynx (a membrane-lined cavity that connects the nose and mouth to the esophagus) from the lower part of the bronchial tree.