well they reach the brain directly through the bloodstream without passing the liver and such.
in the oral route of administration case the metabolism of tramadol is liver-mediated demethylation and glucuronidation via CYP2D6 & CYP3A4. but you can find many interesting things online on this issue, but for now just don't shoot them if you have other questions related to the substance just PM me.
P.S:
1. I would not consider abusing tramadol for a long time because the withdrawal has two characteristics of different withdrawal symptoms, one from the opioid side ( diarheea,stomach ache, restless leg syndrome, nausea, muscle pain) and the other from SNRI/SRRI side ( anxiety, depresion, mad brain zaps).
2. i've been using it for 6 years already, I relapsed a lot of times, used other types of opiates but always came back to my first love to say so. it is just not worth it, very shitty drug to withdraw from.