The to less than half a millimeter. The

The trachea, commonly known as the ‘windpipe,’ branches off to allow air to go into each of our two lungs. Once there, the branching doesn’t stop. Progressively smaller tubes called bronchioles ensure that air reaches all different parts of the lungs.

What are Bronchioles?

The respiratory system consists of a pair of lungs and a trachea through which we draw in air.

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Air pressure of the atmosphere forces the air inside. For this to happen, the body must lower air pressure inside the chest cavity. It does this by expanding the cavity, using muscles in the rib cage and a special muscle called a diaphragm located under the lungs.When we breathe, the air is first drawn in through the mouth and nose. Moving downward, it goes through the pharynx (or the throat) and through the larynx (or the voice box). From there, it enters the main airway called the trachea.

The trachea has rings of cartilage supporting it. The trachea branches off into two bronchi, one for each lung.The bronchi themselves branch several times into smaller divisions.

After several branches, these airways are no longer supported by rings of cartilage, and are called bronchioles.


The lungs are filled with millions of microscopic alveoli to allow for a high rate of exchange of gases with the atmosphere. Each air sac provides surface area for this gas exchange.

In order to get air to all of the alveoli, the bronchioles have to branch smaller and smaller. Bronchioles range in diameter from several millimeters to less than half a millimeter. The tip of each bronchiole, called a terminal bronchiole, ends at a cluster of alveoli that it feeds.

The function of the bronchioles is to ensure that incoming air is supplied to each alveolus.The alveoli are surrounded by tiny blood vessels called capillaries. It is through the thin capillary walls that oxygen enters the blood, and carbon dioxide and water leave the blood.

The carbon dioxide and water vapor are then expelled from the lungs via the trachea, and into the air.Oxygen is used for cellular respiration, which is when our cells break down simple molecules, like glucose, into energy compounds. Water and carbon dioxide are waste products of cellular respiration. Other atmospheric gases, such as nitrogen, also enter and leave the bloodstream, but the levels stay relatively constant since our bodies do not use or produce them.

The trachea, bronchi and larger bronchioles have an inner lining of mucus that traps antigens, such as dust and invading microorganisms. Hair-like structures, called cilia, move this mucus along toward the throat, where we cough to remove it from our lungs. It tends to go down into the esophagus where it is swallowed, though strong coughing can expel this mucus from the mouth and into the air.


Certain conditions like asthma can cause bronchioles to contract, making breathing difficult. Bronchitis is an inflammation of the bronchioles caused by bacterial infection. Other diseases can affect other parts of the lungs.

Lesson Summary

The bronchioles function as part of a larger system, connecting the major air pathways to the microscopic alveoli, where gas exchange takes place.

The combination of the alveoli, providing surface area for gas exchange, and branching bronchioles, providing air to the alveoli, allows our lungs to maximize space inside the body for respiration and increase the rate of gas exchange. Conditions that can affect the bronchioles are asthma and bronchitis.

Key Terms

  • diaphragm: special muscle in the rib cage under the lungs that is used to expand the chest cavity to lower air pressure and draw in air
  • trachea: main airway into which air is drawn downward to the lungs
  • bronchi: two main branches from each lung into which air is brought from the trachea
  • bronchioles: smaller airway branch divisions from the bronchi that are no longer supported by rings of cartilage
  • terminal bronchiole: tip of each bronchiole that ends at a cluster of alveoli
  • alveoli: millions of microscopic air sacs in the lungs that provide surface area for gas exchange
  • capillaries: tiny blood vessels that surround the alveoli
  • cilia: hair-like structures in the trachea, bronchi, and larger bronchioles that aid in the movement of mucus toward the throat
  • asthma: respiratory condition that cause bronchioles to contract, making breathing difficult
  • bronchitis: respiratory condition that is an inflammation of the bronchioles caused by bacterial infection
The respiratory system is responsible for the exchange of oxygen and other gases in the lungs.
Illustration of respiratory system

Learning Outcomes

While studying the lesson on the bronchioles, you can prepare to:

  • Describe the ways in which bronchioles assist with the exchange of oxygen and other gases in the lungs
  • Name conditions that affect the bronchioles

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