What is the physical representation of the heap and the stack?
Note: I refer mostly to x86 as I'm most familiar with it and it's widely documented:

On x86 (and most other architectures) the stack is just resident in memory like everything else. The processor manipulates the stack through a pointer, which is usually stored in a register (the stack pointer). Push and pop instructions will change the value of the stack pointer, changing where the top of the stack is.

The heap is a software abstraction. Like the name implies, it's just a lump of RAM which an allocator slices up and hands out pointers to. In physical terms, the heap doesn't look any different to any other part of RAM.

Paging and segmentation are tangentially related to the heap, however. They're mostly to do with multi-tasking, where processes can't be allowed to modify RAM they don't explicitly own.