With physical damage or disease suffered by an organism, healing involves the repair of living tissue(s), organs and the biological system as a whole and resumption of (normal) functioning. Medicine includes the process by which the cell(s) in the body regenerate and repair to reduce the size of a damaged or necrotic area and replace it with new living tissue. The replacement can happen in two ways: by "regeneration" in which the necrotic cells are replaced by new cells that form "like" tissue as was originally there; or by "repair" in which injured tissue is replaced with scar tissue. Most organs will heal using a mixture of both mechanisms.