The trachea and primary bronchi are supported by rings of cartilage. Cartilage is absent in bronchioles and their walls gradually thin to simple simple cuboidal epithelium in the terminal and respiratory bronchioles. The respiratory zone is that part of the respiratory system that includes the terminal bronchioles and alveoli within the lungs. It is the site of gas exchange.
Bronchioles branch until they form respiratory bronchioles. From the respiratory bronchioles, small tubes (alveolar ducts) lead to alveolar sacs containing multiple alveoli. Together, these form the respiratory unit where gas exchange takes place. Alveoli are lined by simple squamous epithelia and are highly vascularized. Lung (pulmonary) surfactant coats the interior of alveoli to keep them from collapsing.
Making a Difference in the World 