Which structures lack cartilage and have smooth muscle?

The structures that lack cartilage and contain smooth muscle are indeed the bronchioles. As the respiratory system branches out from the trachea to the lungs, the airways undergo significant changes in their structure.

Bronchioles are the smaller air passages that arise from the tertiary bronchi and continue to divide into even smaller terminal bronchioles. Unlike the primary, secondary, and tertiary bronchi, which are composed of cartilage that provides structure and prevents collapse, bronchioles have walls made up predominantly of smooth muscle and elastic fibers. This muscular layer allows bronchioles to regulate airflow by constricting and dilating, a process known as bronchodilation and bronchoconstriction.

The absence of cartilage in bronchioles helps to make them more flexible, which is crucial for their function in airflow regulation in response to changing oxygen demands in the lungs. The presence of smooth muscle in bronchioles also enables them to respond to various stimuli, such as allergens or irritants, making them integral in the body's respiratory defense mechanisms.

In contrast, primary, secondary, and tertiary bronchi maintain cartilage support throughout their structure, which aids in keeping these larger airways open as air passes through during breathing.