In birds*, rather than have these terminal alveolar sacs, they have structures in their lungs called parabronchi. These are tube-shaped rather than little cul-de-sacs. They also have air sacs at the front and back of their lungs that can hold air. Here’s what it looks like.
Having these parabronchi (tubes) means that air passes over the semipermeable membrane going only one direction, so their breathing is termed “unidirectional” rather than “tidal”. It allows oxygen to diffuse over that membrane for longer, too. It can be broken down like this:
1) Inhale 1: air enters the trachea and moves into posterior air sacs.
2) Exhale 1: air moves from posterior air sacs into the lungs (and therefore parabronchi).
3) Inhale 2: air moves from the lungs into the anterior air sacs.
4) Exhale 2: air moves from the anterior air sacs out through the trachea.
Having these parabronchi (tubes) means that air passes over the semipermeable membrane going only one direction, so their breathing is termed “unidirectional” rather than “tidal”. It allows oxygen to diffuse over that membrane for longer, too. It can be broken down like this:
1) Inhale 1: air enters the trachea and moves into posterior air sacs.
2) Exhale 1: air moves from posterior air sacs into the lungs (and therefore parabronchi).
3) Inhale 2: air moves from the lungs into the anterior air sacs.
4) Exhale 2: air moves from the anterior air sacs out through the trachea.