Alveolus

Definition
noun, plural: alveoli
(1) pulmonary alveolus; air sac
(2) (general) A concave cavity or pit
alveolar, adjective
Of, pertaining to, or resembling, alveoli or little cells, sacs, or sockets
Supplement
In general, the term alveolus is used to pertain to a concave cavity or pit, such as that of a honeycomb. It is however also used commonly as a synonym or shortened term for the pulmonary alveolus. The pulmonary alveolus is the thin-walled saclike terminal dilation of the respiratory bronchioles, alveolar ducts, and alveolar sacs, and from where gas diffusion between the air and blood occurs. They occur in mammalian lungs since other vertebrates have different structures that are involved in gas exchange.¹ In human lungs, there are about 700 million alveoli that produce 70m² of surface area.² The pulmonary alveoli consist of two major cell types. They are the alveolar cells (i.e. type 1 alveolar cells and type 2 alveolar cells) and the alveolar macrophages. Type 1 and type 2 alveolar cells are involved in the gas exchange whereas the alveolar macrophages are associated with phagocytosis, homeostasis, and tissue remodeling. The type 1 alveolar cells (or the squamous alveolar cells) cover most of the alveolar surface, i.e. about 90-95%. The type 2 alveolar cells (or the great alveolar cells) are cells secreting pulmonary surfactant. The surfactant is essential in decreasing the surface tension of water within the alveoli. It allows the membrane to separate, thus, improving gas exchange.
Word origin: Latin (a little hollow)
See also:

• Alveolar angle
• Alveoalgia
• Alveolotomy
• Alveolar air
• Alveolar process
• Inferior alveolar nerve
• Alveolar macrophage
• alveolar cell

Reference(s):

¹ Daniels, Christopher B. and Orgeig, Sandra (2003). “Pulmonary Surfactant: The Key to the Evolution of Air Breathing”. News in Physiological Sciences 18 (4): 151–157.

² Roberts, M., Reiss, M., Monger, G. (2000) “Gaseous exchange.” Advanced Biology. Surrey, Nelson. p. 167.