Bronchiole

Bronchiole
Bronchiole
	Diagram of the alveoli with both cross-section and external view.
Details
System	Respiratory system
Identifiers
MeSH	D055745
TA98	A06.5.02.026
TA2	3282
TH	H3.05.02.0.00005
FMA	7410
	Anatomical terminology [edit on Wikidata]

The bronchioles or bronchioli (pronounced bron-kee-oh-lee) are the smaller branches of the

respiratory zone delivering air to the gas exchanging units of the alveoli. The bronchioles no longer contain the cartilage that is found in the bronchi, or glands in their submucosa.^[1]

Structure

A lobule of the lung enclosed in septa and supplied by a terminal bronchiole that branches into the respiratory bronchioles. Each respiratory bronchiole supplies the alveoli held in each acinus accompanied by a pulmonary artery branch.

The

ciliated cuboidal epithelium and a layer of smooth muscle

. Bronchioles divide into even smaller bronchioles, called terminal, which are 0.5 mm or less in diameter. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide into

alveolar ducts. Terminal bronchioles mark the end of the conducting division of air flow in the respiratory system while respiratory bronchioles are the beginning of the respiratory division where gas exchange

takes place.

The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either increase or reduce air flow. An increase in diameter is called

sympathetic nerves to increase air flow. A decrease in diameter is called bronchoconstriction, which is the tightening of the smooth muscle surrounding the bronchi and bronchioles due to and stimulated by histamine, parasympathetic nerves, cold air, chemical irritants, excess mucus production, viral infections, and other factors to decrease air flow. Bronchoconstriction can result in clinical symptoms such as wheezing, chest tightness, and dyspnea, which are common features of asthma, chronic obstructive pulmonary disease (COPD), and chronic bronchitis. ^[2]

Bronchioles

The

respiratory zone

, from the sixteenth through the twenty-third division of the tract.

Terminal bronchioles

The terminal bronchioles are the most distal segment of the conducting zone. They branch off the lesser bronchioles. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles are lined with simple ciliated cuboidal epithelium containing club cells. Club cells are non-ciliated, rounded protein-secreting cells. Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles. The secretion, called pulmonary surfactant, reduces surface tension, allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration. Club cells are a stem cell of the respiratory system, and also produce enzymes that detoxify substances dissolved in the respiratory fluid.

Respiratory bronchioles

The respiratory bronchioles are the narrowest airways of the lungs, 0.5 mm across.

bronchi

divide many times before evolving into the bronchioles. The respiratory bronchioles deliver air to the exchange surfaces of the lungs.^[5] They are interrupted by

club cells.^[6]