> So how come that the trips lasts 5-12 hs, with negligible blood concentrations?
Some drugs are very tightly bound to the receptors they target. (A few are even permanent, so the target protein has to be recycled to turn off the drug.) The blood-brain barrier serves as a one way gate that retains some drugs despite falling blood levels.
A "more than recreative" dose of 200 micrograms is still a very small dose compared with most drugs. And it will blow your head up.
According to various sources (e.g. erowid) LSD is eliminated by the liver very quickly.
So how come that the trips lasts 5-12 hs, with negligible blood concentrations?
I may very well behave as Tim Leary said a "chemical key" for certain brain circuits.