Dilation and Constriction of Bronchioles

Bronchioles have a much smaller diameter than the bronchi (about 0.3-0.5 mm). The walls of the bronchioles lack cartilage rings, but contain smooth muscle cells which can increase or decrease the diameter of the bronchioles in response to signals sent from the autonomic nervous system.

Smooth muscle contraction leads to bronchoconstriction (decrease in the diameter of the bronchioles) while smooth muscle relaxation leads to bronchodilation (increase in the diameter of the bronchioles).

Parasympathetic stimulation causes bronchoconstriction while sympathetic stimulation causes bronchodilation.

During an asthma attack, the muscles that encircle the airway tighten or constrict, limiting the flow of air to and from the lungs. The airway may also become further inflamed and plugged with mucus. As the person with asthma tries to breathe, symptoms of chest tightness, breathlessness, and wheezing can occur.

Bronchoconstriction during an asthma attack


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