The best thing you could do in Particular 1.5 is to create a motion path with a light and then shoot the particles down that path. Use the spin settings to control how far from the center path the particles go. It wont be true "bouncing" off the walls, but may be close enough. Otherwise, you could do this easily in a 3d program like 3d Studio Max with physically correct particle interaction with the tube.
Its not a 3d tube. I actually have a cross-section of a stomach and I have to make the particles travel through the stomach. There is no 3d elements involved at all. I was just looking for a way to describe the problem without going into too much detail.
I need the particles to travel down the stomach and bounce off the walls of the stomach using only the picture of the stomach as a guide.