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Healing (literally meaning to make whole[1]) is the process of the restoration of health from an unbalanced, diseased or damaged organism. The result of healing can be a cure to a health challenge, but one can heal without being cured.[2]

The profession of nursing has been traditionally concerned with matters of healing, whereas historically the profession of medicine has been concerned with curing.[3]

With physical damage or disease suffered by an organism, healing involves the repair of living tissue, organs and the biological system as a whole and resumption of normal functioning. It is the process by which the cells 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 similar 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.

It is also referred to in the context of the grieving process.

In psychiatry and psychology, healing is the process by which neuroses and psychoses are resolved to the degree that the client is able to lead a normal or fulfilling existence without being overwhelmed by psychopathological phenomena. This process may involve psychotherapy, pharmaceutical treatment or alternative approaches such as traditional spiritual healing.

In order for an injury to be healed by regeneration, the cell type that was destroyed must be able to replicate. Cells also need a collagen framework along which to grow. Alongside most cells there is either a basement membrane or a collagenous network made by fibroblasts that will guide the cells’ growth. Since ischaemia and most toxins do not destroy collagen, it will continue to exist even when the cells around it are dead.

Example
Acute tubular necrosis (ATN) in the kidney is a case in which cells heal completely by regeneration. ATN occurs when the epithelial cells that line the kidney are destroyed by either a lack of oxygen (such as in hypovolemic shock, when blood supply to the kidneys is dramatically reduced), or by toxins (such as some antibiotics, heavy metals or carbon tetrachloride).

Although many of these epithelial cells are dead, there is typically patchy necrosis, meaning that there are patches of epithelial cells still alive. In addition, the collagen framework of the tubules remains completely intact.

The existing epithelial cells can replicate, and, using the basement membrane as a guide, eventually bring the kidney back to normal. After regeneration is complete, the damage is undetectable, even microscopically.

Healing must happen by repair in the case of injury to cells that are unable to regenerate (e.g. neurons). Also, damage to the collagen network (e.g. by enzymes or physical destruction), or its total collapse (as can happen in an infarct) cause healing to take place by repair.

Genetics
Many genes play a role in healing.[4] For instance, in wound healing, P21 has been found to allow mammals to heal spontaneously. It even allows some mammals (like mice) to heal wounds without scars.[5][6] The LIN28 gene also plays a role in wound healing. It is dormant in most mammals.[7] Also, the proteins MG53 and TGF beta 1 play important roles in wound healing.[8]

Wound healing
Main article: Wound healing
In response to an incision or wound, a wound healing cascade is unleashed. This cascade takes place in four phases: clot formation, inflammation, proliferation, and maturation.

Clotting phase
Healing of a wound begins with clot formation to stop bleeding and to reduce infection by bacteria, viruses and fungi. Clotting is followed by neutrophil invasion three to 24 hours after the wound has been incurred, with mitoses beginning in epithelial cells after 24 to 48 hours .[citation needed]

Inflammation phase
In the inflammatory phase, macrophages and other phagocytic cells kill bacteria, debride damaged tissue and release chemical factors such as growth hormones that encourage fibroblasts, epithelial cells and endothelial cells which make new capillaries to migrate to the area and divide.

Proliferative phase
In the proliferative phase, immature granulation tissue containing plump, active fibroblasts forms. Fibroblasts quickly produce abundant type III collagen, which fills the defect left by an open wound. Granulation tissue moves, as a wave, from the border of the injury towards the center.

As granulation tissue matures, the fibroblasts produce less collagen and become more spindly in appearance. They begin to produce the much stronger type I collagen. Some of the fibroblasts mature into myofibroblasts which contain the same type of actin found in smooth muscle, which enables them to contract and reduce the size of the wound.

Maturation phase
During the maturation phase of wound healing, unnecessary vessels formed in granulation tissue are remov