Each lobe is further divided into segments. As the two main bronchi enter the lungs, they divide into narrower bronchi. These bronchi then divide and subdivide into narrower and narrower tubes, called bronchioles. The bronchioles terminate in tiny and extremely thin-walled air sacs called alveoli. The oxygen-carbon dioxide exchange takes place here through the moist walls of the alveoli.
Oxygen passes from the alveoli into the surrounding fine blood vessels, called capillaries.
Carbon dioxide passes from these capillaries back into the alveoli and is breathed out. The lungs and the inner surface of the thorax are covered by a thin membrane called the pleura. A small amount of lubricating fluid on the pleura permits the lungs and ribcage to move against each other without friction. The bronchi and bronchioles are lined with cells that keep them moist. These cells have small hair-like projections that sweep mucus and debris up toward the trachea and eventually to the throat.