The term alveolus refers to a hollow cavity, basin or bowl in Latin. Consequently, there are different types of alveoli (plural) found throughout the human body. However, alveoli are most often used to describe the small air sacs of the lungs of mammals, and are therefore known more specifically as the pulmonary alveoli.
The pulmonary alveolus is a sac roughly 0.2 to 0.5 mm in diameter. These alveoli are located at the ends of air passageways in the lungs. Sometimes, people compare alveoli structures to the appearance of a raspberry or a “bunch of grapes.” In the average adult lung, there is an average of 480 million alveoli.
Things can be said about the alveolar shape and structure:
- Largely polyhedral shape
- Open at one end, like a cup
- Walls of the alveoli are composed of the pulmonary capillary sheet
- Alveolar surfaces are covered in a thin (200 nm) layer of surfactant which acts as the interface with the gas
Alveoli are the endpoint of the respiratory system which starts when we inhale air into the mouth or nose. The oxygen-rich air travels down the trachea and then into one of the two lungs via the right or left bronchus. Carbon dioxide molecules, a by-product of cellular respiration, are diffused back into alveolus where they are expelled out of the body through the nose or mouth. During inhalation, alveoli expand as the negative pressure in the chest is created by contraction of the diaphragm. During exhalation, the alveoli recoil (spring back) as the diaphragm relaxes.