Tissue Response to Trauma
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 3 goals: stop blood loss, clear tissue debris, and restore normal biological function to affected area with use of scar tissue.
Limitation of blood loss. This begins with a brief constriction of 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. 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 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 damaged tissue.
During this inflammatory period, blood flow to the wound is increased. This vasodilation, which follows 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, blood vessel walls in the area of the wound become leaky, allowing repair cells and protein-rich plasma to gather in damaged tissues. This process results in swelling.
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 and form a fibrin mesh big enough to occupy the entire wound. This clot helps trap foreign particles, enhances immune cell effectiveness, and forms scaffolding over which new tissues are laid down.
Neutrophils are among the first white blood cells to arrive at the injury site. They remove dead and dying cells, blood clots, and the fibrin mesh to clean up the wound. Other immune cells including monocytes, lymphocytes, eosinophils, and basophils follow neutrophils. Together these immune cells engulf and digest any bacteria.
Scar tissue and long-term healing. Once 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 damaged capillaries start multiplying to form fresh blood vessels, a process known as angiogenesis. New capillaries not only help clear dead tissue, but also support growing cells by supplying oxygen and nutrients.
Granulation and remodeling. Once the wound is free of debris and new blood vessels have begun to form, repair cells called fibroblasts lay down a scaffolding of collagen. This collagen network matures into granulation tissue, which forms the foundation of scars. Granulation tissue matures as collagen fibers become increasingly interlinked. Externally, the scar can be seen to contract and close the wound; this process of collagen maturation and remodeling continues for a lifetime.