an initrd is actually a REALLY tiny linux distro
An initrd may have a tiny linux system in it, but it doesn't have it's own kernel. The initrd functions as the temporary root filesystem for the kernel, then only the filesystem is switched. If you look at the bootloader configuration file, you'll note that the initrd is specified with the kernel:
kernel (hd0,0)/boot/vmlinuz root=/dev/hda1
The kernel boots with the initrd as it's root fs, executes linuxrc if needed, then switches to the original root fs (hda1) unless told otherwise by linuxrc.