bronchiole

Bronchioles are the lungs’ smallest air-conducting twigs — the fine branches that take over after the bronchi have divided many times, each typically under a millimetre wide. If the airways are a tree, these are the slender outermost shoots before the leaves.

Their defining feature is what they lack: no cartilage in the wall. Instead their calibre is controlled almost entirely by a sleeve of smooth muscle, so when that muscle tightens these tiny tubes narrow markedly. They still carry only air and do not exchange gas, which is why they are called conducting bronchioles.

Because they are narrow and have no cartilage to splint them open, bronchioles are easily squeezed by inflammation, mucus or muscle spasm. The collective resistance of these small airways is where much of the airflow limitation in asthma and COPD actually occurs.