Terminal bronchiole
A terminal bronchiole is a bronchiole
The bronchioles or bronchioli are the first airway branches that no longer contain cartilage or glands in their submucosa. They are branches of the bronchi.The bronchioles terminate by entering the circular sacs called alveoli.- Structure :...

 at the end of the conducting zone
Conducting zone
The conducting zone of the respiratory system is made up of the nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles; their function is to filter, warm, and moisten air and conduct it into the lungs. It consists of a series of interconnecting cavities and tubes both...

. At the transition into the respiratory zone
Respiratory zone
The respiratory zone is the site of O2 and CO2 exchange with the blood.The respiratory bronchioles and the alveolar ducts are responsible for 10% of the gas exchange. The alveoli are responsible for the other 90%....

, alveoli
Pulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...

 become present.

The terminal bronchiole is the most distal segment of the conducting zone. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles are lined with simple cuboidal epithelium containing Clara cell
Clara cell
Clara cells are dome-shaped cells with short microvilli found in the small airways of the lungs.Clara cells are found in the ciliated simple epithelium...

s. Terminal bronchioles contain a limited number of ciliated cells and no goblet cells. Clara cells are non-ciliated, rounded protein secreting cells. Their secretions are a nonsticky, proteinaceous compound to maintain the airway in the smallest bronchioles. The secretion, called surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...

, reduces surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

, allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration. Clara cells, a stem cell of the respiratory system, produce enzymes that detoxify substances dissolved in the respiratory fluid.

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