macbook pro fans and heat

I have a question on the fans/heat. I mainly use the MBP in conjunction with my 21" display. I have it below my desk, closed for the most part and have an external keyboard and trackball. Is there any issue in terms of heat or the fans if the MBP is closed? I do hear the fans kicking in and wonder if the vents still work if the display is closed and not being used. I have it leaning vertically with the back of the unit facing up, I figured that was a least better as it seems that the fans blow out towards the back of the unit. I don't want it to over heat or run hot if possible, especially now since I don't have AppleCare.