Lungs are responsible for breathing, which provides oxygen to the blood and in exchange rid the blood of carbon dioxide. The inhaled air passes through a tree of respiratory tubes: first through the trachea that branch into two bronchi, each supplying one of the two lobes of the lung. In the lung, each bronchus (plur. bronchi) branches into narrower bronchi that eventually terminate in a bronchiole.

The lung can be histologically divided into these main components: bronchi, bronchioles, respiratory zone, and blood vessels. These are intermixed and surrounded by a pleura, a membrane that protects the lung from friction that occurs when the lung moves during breathing.

The respiratory zone is where the gas-exchange takes place and it consists of respiratory bronchioles, alveolar ducts and alveoli. The respiratory bronchiole is the last part of the respiratory tree which delivers air to the alveoli. The alveolar duct is an air-filled corridor connecting the respiratory bronchiole to several alveoli that protrude from the alveolar duct.

The alveoli are air-filled spaces surrounded by alveolar walls and consist of:

  • Alveolar cells type 1, covering 95% of the alveolar wall surfaces and involved in the exchange of carbon dioxide and oxygen between the blood and air in the alveoli.
  • Alveolar cells type 2, located mainly at the junctions between alveolar walls and produce surfactants that are essential for the normal function of alveoli.
  • Capillaries, smooth muscle and elastic fibers in the alveolar walls.

Macrophages, solitary or in groups, are spread out in the alveolar ducts and alveoli. They protect the lung from pathogens and other harmful particles.