What are the phases of wound healing?
An otherwise healthy young man is stabbed in the stomach by a two-inch pocket knife, which he will not pull out. After receiving adequate treatment for the stab wound, how long will recovery be?
Wound healing is the process by which the skin, or any injured organ, repairs itself after injury. Wound healing aims to prevent or limit further damage to clean and seal the wound against infection, to restore tissue strength, and, if possible, tissue function. Skin is composed of two basic layers: the epidermis and the dermis. The epidermis is a superficial thin layer composed of epithelial tissue. The dermis is a deeper, thick layer composed primarily of connective tissue. When the skin is damaged, the type of healing process depends on the depth of the injury. An epidermal abrasion from a minor abrasion or burn does not penetrate the dermis. Here healing is a simple process in which surrounding basal epidermal cells multiply and migrate across the wound until the wound is covered.
A dermal wound, however, penetrates the dermis and involves multiple tissue layers. The healing process is more complex than epidermal healing and occurs in three phases. The first phase involves hemostasis and inflammation. Here fibrin and blood platelets form a loose blood clot to prevent further blood loss. The damaged tissue causes the release of histamine which triggers vasodilation and increases the permeability of the blood vessels. This increases the delivery of white blood cells, which help remove microbes and foreign particles via phagocytosis.
Two or three days later the proliferation and migratory, the second phase, begin. In the initial phase of the stage, the clot exterior dries forming a scab. Fibroblasts infiltrate the wound and secrete collagen to strengthen the clot. Fibroblast also triggers the endothelial cells surrounding the wound to proliferate and injured blood vessels to start regrowing. These cells form a delicate mesh known as granulation tissue.
The third and final phase is the maturation and remodeling phase, which may last between three weeks and six months. The scab sloughs off. Collagen fibers become more organized and fewer fibroblasts are present. The blood vessels are restored to normal. Finally, scar tissue forms through a process called fibrosis. The scar tissue differs from normal skin in that it has a denser arrangement of collagen fibers and reduced elasticity.
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