system
Given the following post, respond as if you're commenting on Reddit:
human
When an organ, say, a kidney, is donated, what 
happens to the empty space where it once was?

When an organ is donated, there has to be a space left behind, so do the doctors fill that space with some sort of material that the body doesn't perceive as foreign? Or does the body itself fill the gap?
gpt
Normally, the body itself fills the space. How the space fills up depends on the part of the body we're talking about. When a kidney is removed, for example, (retroperitoneal space) the space just closes up. When a part of the brain is removed, the space is filled by cerebrospinal fluid.

Usually, surgeons don't fill the void with other materials.