Answer
15.d
Bronchioles
Work Step by Step
Bronchioles are very small branches of the respiratory tree of passages that begins with the trachea. There are terminal bronchioles and respiratory bronchioles. Bronchioles are end branches of bronchi. Unlike the more proximal branches of the respiratory tree, bronchioles have no cartilage and are really tiny tubes of involuntary muscles, with ciliated or non-ciliated epithelial cells, mucus secreting cells, and some collagen and elastic fibers. Smooth muscles of bronchioles contract and expand to regulate the flow of air in and out of the air sacs or alveoli.
