Capillary Net of an Alveolus
showing the erythrocytes within the blood vessels.
CREDIT: Oliver Meckes (via FEI)
Scanning Electron Microscope
Alveolus (plural: alveoli) is from the Latin word for “little cavity” and refers to an anatomical structure that has the form of a hollow cavity.
Found in the lungs, the pulmonary alveoli are the terminal ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which 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 [X]