The Frontal Kidney has only one function and that is to filter all the bad junk out before inflating the bladder.  It is connected to the frontal kidney and appears that this is it's only function as it is connected no where else.
    If the fish has been exposed to toxins on a constant basis the fish cannot keep the nasal flairs closed all the time so these toxins get in and create problems that some people commonly call swim bladder disease, which by the way is very rare. Nine out of ten times floating is caused by overfeeding or constipation or feeding floating foods without soaking it.    Jo Ann

Cranial Kidney Theory by Jo Ann Burke
Since so many have asked, I will try to take my very complicated notes and try to give you to the best of my knowledge, a condensed version of my theory.

We should probably start with the bladder.  The bladder is divided into two divided sections, one is called the cranial, which is partially elastic and can expand, and the caudal which is inelastic. It is controlled by gases to allow the fish to float or sink.  A gas gland (rete mirabile)  one of three forms, torus (round), stellate (star shaped), or dendriform (tree like) controls the flow of these gases.  The muscle (a sphincter) can contract or expand to allow the gases to escape or build up. Occassionally when to much gas escapes you will find your fish headstanding on the bottom or top.  Also cold weather seems to affect the way these gases are distributed. These gases are introduced through the system by osmosis across or absorbtion through the lining.  This vascular organ consists of parallel venous and arterial rete mirabile that are arranged in a counter current way. On scrutiny of  a thorough necropsy I found the Cranial Kidney is connected to the cranial part of the bladder. The cranial kidney has no function in the urinary system.  Lymphoid tissue is apparent, and so are hemotopoietic blast cells.  The mitotic index is very high, given to the theory some sort of cleaning or filtering is taking place.

The nasal flare are not connected to the oral cavity.  No information is available on function other then they lead to a blind sac. Upon closer scrutiny I have observed that when toxins are present at a lethal rate this cavity is filled and through the process of either absorbtion or osmosis are also apparent and present in the cranial kidney.  The ability of the fish to contract the flair in the case of an accidental introduction of a toxin is apparent when they are removed from that environment as no trace elements of the toxins are apparent. This leads me to believe there is a direct correlation between the nasal flair and the cranial kidney.  When the fish is not removed the nasal cavity and cranial kidney  clearly contain lethal doses of toxins as does the cranial bladder.

The weberian ossicles consisting of modified cranial vertebrae, form a bridge the connects the bladder to the inner ear canals.  GF have a large amount of sesamoid bones, representing ossfied myuoseptal connective tissue, making absorbtion or osmosis from nasal flares a distinct probability.

Using a TSA 5% blood agar and culturing the cranial kidney, all findings have been gram positive, I am not sure what this signifies other than to prove that most kidney and bladder infections appear to be gram positive.  HPLC's performed on tissue samples have proven toxicity in all of the above
organs.  Therefore making me suspect of the purpose of the frontal kidney's failure to preform the processes intended to assist the gas gland in filtering the toxins and other harmful substances or pathogens to invade the cranial kidney, hence the blown kidney theory.

This is a condensed as I can get this theory with what time I have to devote to this particular problem. Jo Ann aka/Lazulifawn