What is healing?

The Healing Process

The human body is not able to reproduce injured parts during the healing process. Most injured tissue in the body is replaced with collagen, a white protein known as scar tissue. Body areas capable of reproducing, or regenerating, include the outer layer of skin and the inner layers of the intestines.

The human body is involved in a continuous process of self-healing. The outer layer of the skin is constantly rubbed off, yet the body is able to replace (regenerate) new skin to take its place. Another body area capable of regeneration is the innermost layer of the intestine. All other types of tissue, however, such as muscle, fat, blood vessels, or even bones, must rely on other ways to heal when injured. How quickly the body heals depends on many factors, but the process is a predictable one. The healing process includes three phases: the acute inflammatory phase, the repair or regeneration phase, and the remodeling phase.

The first phase of healing, the immediate inflammatory phase, includes the first three or four days after the injury. This process, carried out by vascular, chemical, and cellular events, leads to the repair of tissue, to regeneration, or to scar tissue formation. If the hand is sliced open by a piece of broken glass, the first healing response would be a temporary decrease in blood flow, known as vasoconstriction, or narrowing of the blood vessels at the injury site to prevent the person from bleeding to death. With extensive vessel damage, however, the body is unable to close off enough vessels, and life-threatening hemorrhaging may occur.

The blood begins to seal the broken vessels by coagulation, also known as blood clot formation. The next step is activation of the chemicals needed in the healing process, which is possible only after the blood vessel diameter increases in a process called vasodilation. During vasodilation, the blood flow is slowed and the blood becomes thicker, resulting in swelling. At this point, a buildup or accumulation of fluid results from the seeping of plasma, the fluid portion of the blood, through the vessel walls. This seeping or leakage results from the difference in pressure within the vessel and outside its walls, as well as in the increased permeability of blood vessel walls during the inflammatory process. The amount of swelling at the injury site depends on the amount of seeping, which in turn depends on how much tissue damage has occurred.

Because the blood flow is slower, the concentration of red blood cells and white blood cells is increased. The white blood cells line up and adhere to the inside walls of small blood vessels, known as venules. These white blood cells then pass through the venule walls and are chemically attracted to the injury site over the next several hours. A specialized connective tissue cell, known as a mast cell, is also sent to the injury site. Mast cells contain heparin and histamine. Heparin prolongs the clotting time of blood by temporarily preventing coagulation, while histamine causes dilation of the capillaries. During this earliest phase, both heparin and histamine are important factors, since their actions allow other specialized cells to move into the injured area. The amount of bleeding and fluid buildup at the injury site depends on the extent of damage and how easily materials can cross the walls of intact vessels.
Both of these conditions influence the healing process.

The second phase of healing can be called the repair or regeneration phase. For tissue capable of regeneration, this phase involves the restoration of destroyed or missing tissue. For other types of tissue, this second phase would entail the repair process. The healing of a deep cut in the hand would not be considered regeneration, since the body is not able to remake all the different layers of skin and muscle injured. This healing phase would extend forward from the previously described inflammatory phase. During this phase, the cut is naturally cleaned through the body’s ability to remove cellular waste, the help of the red blood cells, and the formation of a blood clot.

Two types of healing can occur. Primary healing, or healing by primary intention, could take place in the hand laceration example, since the edges are even and close together. If this injury resulted in a large piece of tissue being removed, then the body would fill the gap with scar tissue. The replacement of tissue with scar tissue is an example of secondary healing, or healing by secondary intention. A torn muscle would be an example of secondary healing if it is allowed to heal on its own by the formation of scar tissue within the muscle.

No matter which type of healing occurs, several factors regulate how quickly and how completely this process takes place. Because blood vessels and cells are deprived of oxygen and die from the injury, this new cellular waste or debris must be cleaned from the area before repair or regeneration can take place. This tissue death promotes the formation of new capillary buds on the walls of the intact vessels. As these mature, the injury site is newly supplied with oxygenated blood and the healing process continues into the third phase.

The third phase of healing, known as the remodeling phase, includes the laying down of young scar tissue that increases in strength over the next year. Although the healing process has no distinct time frame, it is believed that three to six weeks are needed for the production of scar tissue. There must be a balance between the toughness and the elasticity of the scar. The amount of stress placed on a newly formed scar will determine the tensile strength of the collagen content. If stress or strain is placed on this forming scar tissue too early, the healing process will take longer. A desirable outcome would be a scar of adequate collagen content through the development of sufficient mature collagen fibers of proper tensile strength. Adequate tensile strength is also affected by how long inflammation is present.

If an injury site has inflammation that lasts up to one month, it is considered a subacute inflammation. When it lasts for months or years, it is then called chronic inflammation. Chronic inflammation is a condition in which small traumas happen repeatedly; it is often seen in overuse injuries. Because this type of injury lasts longer, different types of chemicals try to initiate complete healing. The role of some of these special chemicals is not completely understood.

The healing of a broken bone, similar in many ways to the healing of the skin, is somewhat easier to understand. The first phase shows the same acute inflammation that lasts about four days, involving clotting blood, dead bone

cells, and soft tissue damage around the injury site. The second phase, the repair and regeneration phase, differs slightly when a bone is broken, since the blood clot (hematoma) becomes granulated and builds between the two bone ends. The bone produces a specialized cell that turns into a soft or hard fibrous callus, matures into cartilage, and finally becomes bone with a firmly woven network of cells.

The beginning soft callus is a network of unorganized bone that forms at the two broken edges and is later absorbed and replaced by a hard callus. With appropriate care, a broken bone will develop a new network in the center and eventually become primary bone. The amount of oxygen available in the area determines this development. It is important to keep in mind that when the injury is severe enough to break a bone, then the blood supply is interrupted, lowering the amount of oxygen that is available. Low oxygen could result in the formation of only fibrous tissue or cartilage. Strong, healthy bone results when oxygen and the correct amount of compression are available. The third phase, the remodeling phase, describes the time when the callus has been reabsorbed and special intersecting bone fibers cover the broken area. It may take many years for this entire process to be completed, until the bone has regained its normal shape and ability to withstand stresses.

Disorders and Treatment

Any of the three stages of healing can be delayed or prevented. The three main causes for failed healing are poor blood supply, poor immobilization, or infection.

The healing process within the body can be seriously hampered if the blood supply is poor, since the delivery of nutrients, chemicals, hormones, and specialized building materials to the injury site is hampered. It is extremely important that oxygen levels are adequate for proper healing. If the blood supply is not sufficient, then the tissue may die, especially in broken bone fragments. Fortunately, most tissues of the body have a good blood supply, as demonstrated by the amount of bleeding that takes place when the skin and underlying tissues are injured.

The second condition that interferes with healing is excessive movement because the body part was not immobilized. For the scar tissue or even new bone to become well organized, the two edges of the injured tissue must be kept close together.

The third reason for poor healing is infection. Although the body has many defenses against infection, foreign material can slow healing. If this infectious material invades the space between the two bone ends of a fracture, the necessary building materials may not reach the site. Infection invading the hand tissue cut by the glass could prevent the edges from healing together because of pus, scab formation, or the interference of germs.

There are many different types of injuries, and several steps must be taken in caring for each type. Soft tissues, the first line of defense against injuries, can be used to describe all tissues other than bone. Soft tissue injuries are classified as either closed or open. In a closed wound, the damage lies below the surface of the skin and the skin remains intact. A sprained ankle or a bruised knee are classified as closed wounds. In an open wound, the skin or mucous membranes, such as the lining of the mouth, are broken or torn.

There are four types of open soft tissue injuries; each has specific characteristics and heals differently. The first type is an abrasion, in which part of the outer layer of the skin and some underlying tissue is rubbed or scraped off. A common injury of this type is a scraped knee resulting from a fall on the sidewalk. The second type, a laceration, results from a sharp object cutting the skin, such as the previous example of a piece of glass cutting the skin either superficially or very deeply. The third type, an avulsion, results when a piece of skin or even an entire fingertip is torn off or left loosely hanging by a small flap of skin. It is important that this flap not be removed since a physician can sometimes reattach the part. The last type of soft tissue injury is the puncture wound, which results when a sharp object penetrates the skin and into a body part. Such an injury could be a stab from a knife or an ice pick, a splinter stuck in the foot, or even a bullet shot into the leg. The initial management is the same for all four types of injury.

Management of open wounds must include the control of bleeding, infection prevention, and immobilization. Two of the above injuries, an avulsion and a puncture wound, require additional special care. In the case of an avulsed body part, the amputated part should be saved; wrapped in a dry, sterile piece of gauze; and placed in a plastic bag. If this bag is kept in something cool, such as a bucket of ice, the possibility of reattachment is increased. An impaled object remaining in a puncture wound should never be removed but held in place, all movement restricted until medical care can be given.

Several medical treatments can aid in promoting the healing process, as can commonsense first aid measures taken immediately after an injury occurs. For example, with a glass cut to the hand one should immediately stop the bleeding by placing a sterile piece of gauze, or a very clean cloth, directly over the laceration. By adding direct pressure over the gauze, the circulation is reduced. If the cut is deep, if the bleeding cannot be controlled, or if a piece of glass remains in the wound, then it is advisable to seek medical attention. A physician would then thoroughly clean the injury site and stitch the two edges together. Immobilizing the two flaps of skin together by sewing them will allow the first two phases of healing to progress. By having the wound inspected and cleaned by medical personnel, the risk of infection is reduced. A small injury can be cared for at home, but infection must be prevented through proper cleansing. Even soap and water, along with a bandage or dressing, will help ward off infections.

Perspective and Prospects

Many strategies to improve the healing of human tissue have evolved over time—from ancient times, when healers packed mud on the top of sores to draw out the infection, to modern alternative medicines. Every person, at one time or another, receives a cut, scrape, bump, or bruise. Therefore, there is much interest in speeding up the healing process.

Renewed interests in nontraditional approaches to medicine explore the healing powers locked within the human body. The use of homeopathy, acupuncture, and acupressure are examples of alternatives to antibiotics and standard first aid measures to help an injury heal. Holistic health care, hypnosis, and osteopathic medicine offer other areas of exploration. The practice of Chinese medicine includes the use of herbs, crystals, massage, and meditation to allow healing to proceed quickly but through natural means. Even the use of aromatherapy—treatment through the inhalation of specific smells—has gained a foothold in the medical world. The manipulations done by chiropractic doctors offer other possibilities. Some seek cures in nature, from sources below the sea or deep in the forest. Yet, many untapped resources remain. The continuing research in genetics offers vast possibilities, and the link between mental attitude and the immune system presents a rich area for further exploration. Even innovations as simple as a special glue, used to replace sutures or staples for closing wounds, would have an important influence on the future of the healing process.

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