Trauma (or any injury) results in tissue damage. Immediately after traumatic injury, a pattern of local reactions and systemic changes is launched. This reparative process involves almost all organ systems.
The local response to trauma serves three goals: stop blood loss, clear tissue debris, and restore normal biological function in the affected area, with the use of scar tissue.
Limitation of blood loss. This begins with a brief constriction of the blood vessels to reduce blood flow to the affected area. Meanwhile, platelets are activated to form a clot or mesh of fibrin to block the bleeding blood vessels. The platelets then release substances such as histamine, serotonin, and cytokines, which activate the next stage of healing, inflammation.
Clearing of tissue debris. Once the bleeding is under control, the body begins to remove damaged and dysfunctional tissue through the inflammatory response. Only after the debris is completely reabsorbed can the body lay down a new tissue framework. Inflammation requires the activation of certain enzyme systems and pro-inflammatory cells that dissolve the damaged tissue.
During this inflammatory period, blood flow to the wound is increased. This vasodilation, which follows the intense vasoconstriction seen immediately after the wound is created, is mediated by chemicals such as histamine, prostaglandins, and those found in the complement cascade, which is part of the immune system response to injury. Under their influence the blood vessel walls in the area of the wound become leaky, allowing repair cells and protein-rich plasma to gather in the damaged tissues. This process results in swelling.
The plasma spilling into the wound serves multiple functions. It dilutes any irritants in the injured area and brings protein molecules called fibrinogen, which link with each other, forming a fibrin mesh big enough to occupy the entire wound. This clot helps trap foreign particles, enhances immune cell effectiveness, and forms the scaffolding over which new tissues are laid down.
Neutrophils are among the first white blood cells to arrive at the site of injury. They remove the dead and dying cells, blood clots, and the fibrin mesh to clean up the wound. Other immune cells, including monocytes, lymphocytes, eosinophils, and basophils, join the neutrophils later. Together these immune cells engulf and digest any bacteria.
Scar Tissue: Long-Term Healing
Once the bleeding has stopped and the body's immediate inflammatory response has been activated, long-term healing and tissue regeneration can begin. The growth of new tissue consists of three different processes:
Angiogenesis, or creation of new blood vessels
Formation of granulation tissue
Remodeling of the scar to suit changing functional requirements
Angiogenesis. The cells lining the damaged capillaries start multiplying to form fresh blood vessels, a process known as angiogenesis. The new capillaries not only help clear the dead tissue but also support the growing cells by supplying oxygen and nutrients.
Granulation and Remodeling