The bronchioles or bronchioli are the first airway

Airway

The pulmonary airway comprises those parts of the respiratory system through which air flows, conceptually beginning at the nose and mouth, and terminating in the alveoli...

 branches that no longer contain cartilage

Cartilage

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 or glands in their submucosa

Submucosa

In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle .-Contents:Blood vessels, lymphatic vessels, and nerves will run through...

. They are branches of the bronchi.
The bronchioles terminate by entering the circular sacs called alveoli.

Structure

A pulmonary lobule is the portion of the lung ventilated by one bronchiole. Bronchioles are approximately 1mm or less in diameter and their walls consist of ciliated cuboidal epithelium and a layer of smooth muscle.

Bronchioles divide into even smaller butt bronchioles that are 0.5mm or less in diameter. Terminal bronchioles in turn divide into 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 actual 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 bronchodilation and is stimulated by either epinephrine or sympathetic nerves to increase air flow. A decrease in diameter is called bronchoconstriction and is stimulated by Histamine, parasympathetic nerves, cold air, chemical irritants and other factors to decrease air flow.

Pathology

[the [Bronchospasm]], a life-threatening situation, occurs when the smooth muscular tissue of the bronchioles constricts, severely narrowing their diameter, the most common cause of this is asthma. Bronchospasm is commonly treated by oxygen therapy

Oxygen therapy

Oxygen therapy is the administration of oxygen as a medical intervention, which can be for a variety of purposes in both chronic and acute patient care...

 and bronchodilator

Bronchodilator

A bronchodilator is a substance that dilates the bronchi and bronchioles, decreasing resistance in the respiratory airway and increasing airflow to the lungs. Bronchodilators may be endogenous , or they may be medications administered for the treatment of breathing difficulties...

s.

The medical condition of inflammation of the bronchioles is termed bronchiolitis

Bronchiolitis

Bronchiolitis is inflammation of the bronchioles, the smallest air passages of the lungs. It usually occurs in children less than two years of age and presents with coughing, wheezing, and shortness of breath. This inflammation is usually caused by viruses...

. Diseases of the bronchioles include asthma

Asthma

Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...

, bronchiolitis obliterans

Bronchiolitis obliterans

Bronchiolitis obliterans , also called obliterative bronchiolitis and constrictive bronchiolitis , is a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles are compressed and narrowed by fibrosis and/or inflammation...

, respiratory syncytial virus infections, and influenza

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

.