The difference between bronchi and bronchioles is cartilage.
Bronchi have cartilaginous support in the connective tissue that helps keep them open even if the smooth muscle contracts in response to an irritant.
Bronchioles, by contrast, are smaller, have no cartilaginous support, and are at the end of the respiratory conducting system, the tubes that lead directly into alveoli, the gas-exchange epithelium. If the smooth muscle contracts, these tubules can close almost completely, making it very difficult for air to reach the alveoli.
This is what commonly happens in asthma, and it can be a life-threatening condition. Beta-adrenergic agonists of the sort found in “rescue inhalers” can help relax bronciolar smooth muscle and relieve the respiratory symptoms.
