This occurs when a substance passes from the fluid in the tubular lumen through the apical membrane, across the cytosol , and then into the interstitial fluid.
Yes, it's potent stuff. Cat receive it either as an injection or orally. It takes effect in roughly half an hour, lasting about 8 hours before another administration is necessary. Cats usually react negatively to opiates, but buprenorphine is an exception. That's one reason vets use it if they can receive the necessary special certification.
Suboxone (Buprenorphine) has a half-life on average of 37 hours. So it'll be completely out of your system in about, oh, 24 days. 30 days @ 8 mg day = 32 mg total in system. 32 mg/37 hrs=576hrs for .000122 mg in system. 576 hours /24 hrs = 24 days. I might not always be right, but I'm never wrong.