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.
Buprenorphine can be detected in the urine for 1-3 days. There is no data currently available on blood testing for buprenorphine. A saliva test can detect Buprenorphine for up to 1-3 days. Buprenorphine, like like many other drugs, can be detected with a hair follicle drug test for up to 90 days.