When I plug my MacBookPro into a wall socket does it run power through the wall into my battery? Or is it a separate function?

I have the MacBookPro that was installed with 2.7ghz intel i7. So does it run the electricity from the outlet into my battery and then the computer? Or is it directly from the wall to computer, passing up the battery? The reason I ask is because I like to plug it in the wall, but I feel like it might drain the battery faster in that manner? What do you know of the situation? Thanks!