the.com/sac
A biological pouch, doing the quiet work of holding things you'd rather not spill.
means A pouch-like structure in a plant or animal, often containing fluid, air, or other contents.
from From Latin saccus, meaning bag or sack — the same root that gave us sack, satchel, and the financial sense of being sacked.
Air handlerYour lungs end in tiny air sacs called alveoli.
Egg vaultSpiders bundle eggs in silken sacs for protection.
Spelling trapAnatomy prefers sac; everyday English prefers sack.