Suatu struktur sangat kecil dan menyerupai kantung yang terdapat pada ujung-ujung beronkiolus. Pada permukaannya terdapat banyak pembuluh kapiler. Udara dalam alveolus memberikan oksigen pada darah dan sebaliknya menerima karbon dioksida dari darah. Pertukaran gas tersebut terjadi pada permukaan alveolus yang sangat tipis dan dinding kapiler.

Merriam-Webster Online Dictionary
alveolus (noun)
a small cavity or pit as
a) a socket in the jaw for a tooth
b) a small air-containing compartment of the lungs in which the bronchioles terminate and from which respiratory gases are exchanged with the pulmonary capillaries
c) an acinus of a compound gland
d) a cell of a honeycomb
- teethridge
Alveolus (Wikipedia)
"Alveolus" redirects here. For other uses, see Alveolus (disambiguation).
"Alveolar" redirects here. For the consonant class, see Alveolar consonant.
Pulmonary alveolus
Alveolus diagram.svg
The alveoli
System Respiratory system
Latin alveolus pulmonis
Code TH H3.
Anatomical terminology

An alveolus (plural: alveoli, from Latin alveolus, "little cavity") is a hollow cavity found in the lung parenchyma, and is the basic unit of respiration. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas-exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the alveolar blood vessels where, through diffusion, it releases its carbon dioxide and absorbs oxygen.