system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What happens to the empty space in your body when you have a kidney/lung removed?
gpt
It depends which organ. If a lung is removed, the other lung expands a bit and takes up some of the space. The remaining space fills up with fluid. The kidneys and the organs surrounding it don't have well-defined positions, so if a kidney is removed the other organs shift around and the space fills in with fluid. Eventually fascia (connective tissue) will grow in as well.

It's rare for a procedure to involve putting a foreign object in to compensate. It's usually done in cases where the organ was providing structure itself, like when breast reconstruction is performed after a mastectomy.