Bang!

Not quite. They're in your blood because that is where they are being used as fuel. The ashes are what's left over after the burning, they spill over into your lungs and urine as you get rid of them. Drinking water dilutes the concentration and keeps you hydrated and your cells happy and it makes you pee. This is how you get rid of them. Exercise burns them up too. Ever notice that with a really hot fire there are fewer ashes left behind? Less to clean up in the fireplace? Exercise requires fuel, more ketone bodies are used and used more completely, there are less floating around in your system to spill over. After exercising I barely register "trace" if I test, regardless of the time of day.

So if you've used up most of your fuel exercising or being active and you've diluted the concentration of what is left in your system, then your body needs to make more from the fat you've got stored away.

This of course is simplified I'm sure I can find a much more detailed explanation if you're intested
Nat