my macbook pro fan is running really loudly when i try to play games?

okay so i have a macbook pro with mountain lion installed and everything is up to date and i have lots of memory but I tried playing the demo of world of warcraft because i had the full game on my old imac which i don't have anymore, however when i played it after a few seconds the fan started and the noise was so loud it sounded as if it was going to explode i've tried putting it on low graphic settings etc, but it doesn't make a difference anyway this is the problem with most games that i have tried to play apart from some (onverse works pretty well) so I'm just wondering if the sims 3 will come with the same problem i just don't want to waste money and since there isn't a trial for the game I'm hesitant about buying the game
I've looked at the minimum system requirements for the sims 3 but quite frankly i dont really know what half the stuff means....
some info about my comp: macbook pro os x
Version 10.8
processor:2.4 GHz Intel Core i5
graphics: intel HD graphics 3000 384 MB
tell me if i need to put anymore info thanks

And the fans running - like they're suppose to - is a problem because???