The third lung heaves and sighs.
It gasps and writhes,
Filling with blood
That it quickly releases
To prevent a watery grave.

It is never empty,
And never full.
With walls of elastic,
It shrinks and grows
To accommodate.

As it ages,
The elastic becomes hard and brittle,
Cracking a little more every day,
Retaining less, aching more,
Until it has no choice but to cease.