By replacing damaged or destroyed cells with healthy new cells, the processes of repair and regeneration work to restore an individuals health after injury. Unlike the salamander, which is capable of regenerating a limb if it is lost, human beings cannot regenerate whole organs or limbs. If one kidney is destroyed by disease, it is permanently lost. However, the remaining contralateral kidney, if normal, is capable of limited regeneration to compensate for the decrease in kidney mass. The manycelltypes of the body have varying capacities for regeneration. Regeneration is the production of new cells exactly like those destroyed. Of the three categories of human cells(1) thelabile cells, which multiply throughout life, (2) thestable cells, which do not multiply continuously but can do so when necessary, and (3) thepermanent cells, incapable of multiplication in the adultonly the permanent cells are incapable of regeneration. These are the brain cells and the cells of the skeletal and heart muscles.Labile cells are those of the bone marrow, the lymphoid tissues, the skin, and the linings of most ducts and hollow organs of the body.Stable cells are found in the liver, in many of the glands of the body, such as the pancreas and salivary glands, in the lining of the kidney tubules, and in the connective tissues. Normally these cells do not divide unless some are destroyed by disease or injury and must be replaced.If only a small area of theliver(made up of stable cells) is damaged or destroyed, unaffected cells around the area of injury can replace those that were lost. When large areas of the liver are destroyed, however, cellular regeneration cannot occur, and the area of cell loss is replaced by new healthy connective-tissue cells, which produce scars. If aheart attackoccurs, a certain number of heart muscle cells (permanent cells) are killed because of loss of blood supply. Because heart muscle cells cannot regenerate, the area of injury is replaced by a scar (if the patient survives). Such repair is by no means perfect, but it nonetheless permits restoration of reasonable heart function with perhaps only a slightly reduced level of health, depending on the number of heart muscle cells that have been lost.Cellular regeneration in humans is limited by many other factors, such as the availability of blood supply and a supporting connective tissue. When the blood vessels and supporting cells (connective tissue) are destroyed in the liver along with the liver cells, perfect reconstitution of the liver is not possible. There may be some regrowth of liver cells, but they do not form the normal liver architecture, and the newly regenerated cells cannot function because they do not have an appropriate orientation to the blood vessels and bile ducts.A review of the events that occur after a simple cut in the skin provides a good example of the processes ofregeneration. At first, the area becomes red, swollen, and painful because of the inflammatory reaction. A scab forms. Beneath the scab, while the inflammatory process is going on, the cells from the adjacent healthy skin begin to regenerate by dividing and growing over the damaged area. If the damage is minor, perfect reconstruction of the skin and its appendages is likely to result. If the damage has extended below the skin surface, deeper connective-tissue cells, notably thefibroblasts, proliferate and fill the area. These cells lay down collagen (connective-tissue protein) composed of tough, durable fibrils (minute fibres), and, eventually,scarformation ensues. Once scarring has occurred, it cannot be reversed, although considerable shrinking of the scar may occur. If scar formation is limited, total function will return. On the other hand, if scar tissue formation is excessive, it often leads to a loss of function of the part.HemostasisAnother mechanism of defense is hemostasis, the prevention of loss of blood from damaged blood vessels by formation of a clot. (This process is covered more at length in the articleblood: Bleeding and blood clotting.) Simply stated, a break in a blood vessel leads to activation of a complex sequence of events that results in the formation of a solid plug of platelets, red blood cells, and fibrin (a fibrous protein formed from fibrinogen). This plug, or clot, seals the damaged vessel and prevents further loss ofblood(hemorrhage). The numerous components of the blood called clotting factors contribute in sequential fashion to the formation of the clot. (The clotting factors are commonly referred to by a roman numeral rather than by name. Fibrinogen, for example, is clotting factor I.) A defect in one of these factors can undermine hemostasis; for example, the absence of clotting factor VIII leads to hemophilia A, a disorder of uncontrolled bleeding.