I'd suggest it work like the pufferret:
Pseudo-Avian Respiration. Birds' bodies have both lungs, where gas exchange takes place, and also air sacs. A bird's air sacs are located within the thoraco-abdominal cavity. This creature's air sacs, however, will be located outside the body cavity, but will function in a way broadly analogous to that of a bird's breathing.
Description. Surrounding the thoraco-abdominal cavities of this animal are sub-cutaneous air sacs. At both the cepahalic & caudal ends of the air sacs are sphincters that will close to trap air and open to receive or expel air. A pair of paratracheas branches off the bronchus above the clavicles and runs down either side of the dorsal aspect of the spine. The air sacs are thin walled and stretchy and thus can be inflated to a relatively high volume. Rapidly contractile muscles allow for variable speed ejection of air in peristaltic fashion. Each sac thus encompasses the animal's body and along the ventral aspect empties into a common effluent conduit. This is a muscular structure that, much like the urethra, can expel air in short bursts or open wide for a forceful blast. This conduit in turn runs down along the animal's belly and terminates in a bifurcation on either side of the uro-genital tract.
Mode of action. The animal always breathes as per normal in order to perfuse its lungs. A flap along the ventral aspect of the deep trachea is normally closed, allowing for air to escape through the mouth (normal breathing, hissing, vocalisations, etc). However, when the flap opens, spent air is redirected through the paratracheas and into the air sacs. It is from these paratracheas that the air sacs receive spent air from the lungs. Whilst the animal in breathing in, the sphincters close off the air sacs, trapping air. During the puffing up behaviour, the animal appears to be in distress, rapidly heaving and huffing and hyperventilating. This is normal, as it's simply using its lungs as bellows to fill its air sacs.
Functionality. these animals use this capacity for a variety of reasons. First and foremost, the forceful expulsion of pheromones during the mating season; secondarily, the forceful expulsion of noxious chemicals as either defense or attack against potential predators. (Think of this critter as a skunk whose stink just keeps on giving all on a 10 fps wind!) Third, less forceful winds can be generated by this creature that, when passing through the posterior openings, can excite the skin flaps to vibrate. This results in a number of long, loud (and thankfully non-stinky) calls of varying duration, timbre and percussivity.
It is interesting to note that this animal never uses this capability whilst in the water as a flotation device. Presumably because there is no way to control bodily attitude, to say nothing of locomotion! Also, this capability can not be used to "store" air for later rebreathing, as it comprises spent air.
