Respiratory Bronchioles and the Alveoli

The respiratory portion starts at the respiratory bronchioles that are the continuation of the terminal bronchi. The respiratory bronchioles in turn branch into a number of alveolar ducts that lead into the alveoli. Gas exchange takes place in the respiratory portion.

The alveoli are the functional units of the lungs and form grape-like clusters. A single alveolus looks like a small bubble lined with a single layer of squamous epithelium supported on an elastic basement membrane and surrounded by capillaries.

A group of two or more alveoli opening into one alveolar duct is called an alveolar sac. There are over 350 million alveoli in each lung providing a very large surface area (65m2) for adequate gas exchange.