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.
Tramadol can be administered orally at a dose of 1 to 4 mg/kg twice daily. Tramadol has the advantage of being an oral preparation that is not a controlled drug. However, some cats find the taste unpalatable, and both dysphoria and mydriasis have been reported as side effects in some cats, particularly at higher doses. Buprenorphine can also be used for chronic pain and is a good option in cats that are difficult to medicate or those that do not respond well to tramadol.