What are the smallest air passages within the lungs called?

The smallest air passages within the lungs are indeed known as bronchioles. They branch off from the larger bronchi and lead into the alveoli, which are the tiny air sacs where gas exchange occurs. Bronchioles have a diameter of less than a millimeter and lack cartilage in their walls, which distinguishes them from the bronchi that are larger and supported by cartilage. This structure allows bronchioles to be more flexible and responsive to the changes in airflow and resistance within the lungs. As they branch out into the alveolar ducts and finally reach the alveoli, they play a critical role in directing air to the sites where oxygen and carbon dioxide exchange happens, making them essential for efficient respiratory function.