The integumentary system consists of the skin--the body's largest organ--and its accessory features and glands. Like other body organs, the skin is essential to maintenance of life and health, and has functions including temperature regulation and protection from infection. While skin is exposed to the environment and is susceptible to damage, it is self-repairing.
Immediate Response
When skin becomes damaged, the body's initial response is to stop bleeding. The skin is rich with blood vessels, meaning that cuts and scrapes can bleed profusely. Clotting keeps the body from losing too much blood, and initiates the integumentary repair process. Damaged blood vessels send distress signals, explains Dr. Lauralee Sherwood in her book "Human Physiology," that activate proteins in the blood. These proteins form a fibrous net that traps blood cells, producing a clot, which eventually dries into a scab.
Disinfection
One of the skin's primary purposes is to keep infectious agents such as bacteria out of the body. Cuts give pathogens an easy road into the bloodstream, so shortly after the skin becomes damaged by a cut or scrape, the body works to preempt infection. Iimmune cells called neutrophils flock to the site of a skin wound and release antibacterial and antiviral chemicals. They also release chemicals that break down tissue close to the surface of the wound, with the idea that most surface cells have been badly damaged by the injury, and would die and rot regardless.This provides a clean slate for the repair process.
Blood Vessel Proliferation
Within a few days of receiving a skin injury, it becomes less swollen, but tissue around the scab appears bright pink or red. This is because the process of building new skin requires lots of blood, bringing plenty of oxygen and nutrition to hard-working cells. The body therefore builds extra capillaries, or small blood vessels, in the area of an injury. In his book "Anatomy and Physiology," Dr. Gary Thibodeau explains that this is called angiogenesis.
New Matrix
Below the outer layers of skin lies a dermal layer of the stretchy protein collagen, explains Thibodeau, that forms part of the extracellular matrix that is the basis for skin. Cuts and scrapes deep enough to bleed indicate that there has been damage not only the outer layers, but also to the matrix of skin. Before the body grows new outer layers, therefore, it must build new extracellular matrix. The initial matrix is called "granular tissue," and it's slightly different from regular matrix. Still, it provides a base for new epithelium, or outer layers of skin.
New Epithelium
The epithelium of skin consists of flattened cells filled with the tough protein keratin. These cells come from keratinocytes, which migrate into the area of the wound and form a base layer. As they proliferate and reproduce, they migrate upward and become less metabolically active as they fill with keratin, Thibodeau, says. Within weeks of an injury, new keratinocytes have formed a complete, new epithelial layer over the area of injury. Unnecessary blood vessels formed during the healing process then begin to die, returning skin to its original color.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
