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.
Like morphine, buprenorphine has a relatively short half-life. In most people, it will be undetectable in urine after 2-4 days of complete abstinence. For saliva, it's slightly less at 1-3 days. Hair drug tests, unfortunately, usually have a longer detection time of 90 days. For a urine or saliva test, it's probably best to stop taking your medication four days before a drug test.