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Healing (literally meaning to make whole) is the process of the restoration of health to an unbalanced, diseased or damaged organism. Healing may be physical or psychological and not without the mutual reception of these two dimensions of human health. With respect to 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.
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 and increasingly traditional spiritual approaches.
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.
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. cardiac muscle or 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.
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.
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 removed by apoptosis, and type III collagen is largely replaced by type I. Collagen which was originally disorganized is cross-linked and aligned along tension lines. This phase can last a year or longer. Ultimately a scar made of collagen, containing a small number of fibroblasts is left.
Healing of tissue damaged by inflammation 
Recall phase 
Resolution phase 
In the Resolution phase, pathogens and damaged tissue are removed by macrophages (white blood cells). Red blood cells are also removed from the damaged tissue by macrophages. Failue to remove all of the damaged cells and pathogens may retrigger inflammation.
Regeneration phase 
In the Regeneration phase, blood vessels are repaired and new cells form in the damaged site similar to the cells that were damaged and removed. Some cells such as neurons and muscle cells (especially in the heart) are slow to recover.
Repair phase 
In the Repair phase, new tissue is generated which requires a balance of anti-inflammatory and pro-inflammatory eicosanoids. Anti-inflammatory eicosanoids include lipoxins, epi-lipoxins, and resolvins, which cause release of growth hormones.
- Oxford English Dictionary
- The Anti-Inflammation Zone, Barry Sears, pages 230-233, 2005.
- How wounds heal and tumors form With this simple Flash demonstration, Harvard professor Donald Ingber explains how wounds heal, why scars form, and how tumors develop. Presented by Children's Hospital Boston.
- Wound Healing and Repair
- Lorenz H.P. and Longaker M.T. Wounds: Biology, Pathology, and Management. Stanford University Medical Center.
- Romo T. and McLaughlin L.A. 2003. Wound Healing, Skin. Emedicine.com.
- Rosenberg L. and de la Torre J. 2003. Wound Healing, Growth Factors. Emedicine.com.
- After the Injury- Children's Hospital Of Philadelphia