acinus

The acinus is the lung’s basic gas-exchange neighbourhood — everything fed by one terminal bronchiole. Picture that final conducting twig opening into a whole cluster of respiratory bronchioles, alveolar ducts and air sacs; that entire bunch is one acinus.

It is the functional unit of the lung: all of its respiratory zone — the parts that actually exchange oxygen and carbon dioxide — is organised acinus by acinus. A handful of neighbouring acini grouped together makes up a secondary pulmonary lobule, the unit often seen as a small block on detailed CT scans.

Thinking in acini helps make sense of disease patterns. Emphysema is often described by where within the acinus the damage falls — centred on the proximal respiratory bronchioles (centriacinar) or spread across the whole unit (panacinar) — which in turn hints at the likely cause.