How Does Cut Skin Heal?
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Skin and the Healing Process
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Skin is a complex organ that serves many vital functions. It protects internal organs, prevents intrusion of bacteria, controls body temperature, and receives sensory stimulation from the environment. A break in the skin threatens its ability to maintain these important roles. The body has a complex yet remarkable process to heal wounds that proceeds in four step-wise phases: hemostasis, inflammation, proliferative and remodeling.
Hemostasis
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Hemostasis literally means blood stagnation, which means to stop the bleeding. The blood vessels constrict and blood platelets accumulate to form a clot. A scab is formed as the clotted blood dehydrates.
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Inflammation
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The purpose of inflammation is to prevent infection. Following hemostasis, histamines cause the blood vessels to dilate, which allows white blood cells (WBCs) to flow into the wounded area. Leukocytes are the first WBCs on the scene. They begin to clean away debris and destroy bacteria. They are then replaced by white blood cells called macrophages. Macrophages continue cleaning the wound but they also secrete growth factors and proteins that facilitate the development of new cells to rebuild tissue. This phase begins as soon as the skin is wounded and lasts three to four days. It is marked by swelling as fluids infiltrate the wound, warmth from the histamine response and subsequent pain.
Proliferative
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The proliferative phase lasts from 2 days up to 3 weeks. During this time the wound begins to heal by building tissue, including skin and blood vessels. The proliferative phase is broken down into four sub-phases: granulation, contraction, angiogenesis, and re-epithelialization.
During granulation, cells called fibroblasts travel to the wound and begin to produce new connective tissue called collagen. The collagen fibers develop into a matrix that creates the foundation for further healing.
As the matrix forms and the collagen begins to strengthen, the edges of the wound start to contract towards one another. Wound contraction continues throughout the healing process.
Angiogenesis refers to the development of new blood vessels. This proceeds as parent blood vessels produce new offshoots.
The epithelial cells form the outer layer of skin. During re-epithelialization, new epithelial cells grow from the edges of the wound across the matrix of collagen. A thin epithelial layer bridges the wound in 48 to 72 hours.
The ultimate result of the proliferative phase is that the new collagen, epithelium and vessels build a scar to replace the original skin that was destroyed. Even though the wound is now healed, the process continues.
Remodeling
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The process of remodeling starts after 3 weeks and can last for years. This is a continuing alteration of the healed wound to improve and maintain its strength. Old collagen is broken down and then replaced with new collagen. The wound also continues to contract. Scar tissue is never as strong as the original skin, but both of these processes help the area to achieve optimal strength.
Factors Affecting Healing
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Several factors can affect the body's ability to properly heal. There must be adequate circulation of blood so that nutrients can be delivered to the healing wound and so that dead cells and debris can be removed. Age can also have an influence due to the slow down in metabolism. But perhaps the most important factor is nutrition. The healing process requires a lot of different nutrients: proteins to build cell structure, vitamin A for epithelial tissue, vitamin C is required for connective tissue such as collagen, vitamin K is required for clotting and vitamin E generally promotes healing.
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