Regeneration in humans is the regrowth of lost tissues or organs in response to injury. This is in contrast to wound healing, which involves closing up the injury site with a scar. Some tissues such as skin and large organs including the liver regrow quite readily, while others have been thought to have little or no capacity for regeneration. However ongoing research, particularly in the heart and lungs, suggests that there is hope for a variety of tissues and organs to eventually become regeneration-capable. With non-injured tissues, the tissue is naturally regenerated over time; by default these tissues have new cells available to replace expended cells. With injured tissue, the body usually has a different response – this emergency response usually involves building a degree of scar tissue over a time period longer than a regenerative response, as has been proven clinically.