Dear micbob,
B.U.N. is short for "Blood Urea Nitrogen". It is the measure of urea nitrogen in the blood which has not been processed or cannot be processed as usual by the kidneys.
Now....for the hard part. The kidneys are unable to regenerate -as the liver can- from damage or failure. Usually, by the time kidney failure is diagnosed, 75% of function is already lost. (This of course can vary, depending on the 'staging' of the disease.)
The only thing which can be done, is to try to keep the kidneys functioning at 25% or whatever level of function there is. That is, to stop it getting worse, for as long as possible.
That is feasible, with the right treatment. The prognosis is varied and depends on the treatment and diet plan....the dog's general health/age etc....and the stage of kidney failure when diagnosed.
I wish you and your dog well, and hope things will stabilise for as long as possible.
I am so sorry to read this. Please look to the right of this page and you can click on Groups and then Chronic Kidney Failure in Dogs. You may certainly find the information and support you need.