air bladder

/EH1 R B L AE1 D ER0/
noun
  1. 1

    An internal organ that fish use to control their buoyancy, allowing them to maintain or change depth by changing their density.

  2. 2

    A type of packing material that consists of an airtight bag full of air, used to cushion fragile items.

Source: https://en.wiktionary.org/wiki/air%20bladder