Our skin is the largest organ in our bodies, with a surface area of about 20 square
feet in adults. When we are cut or wounded, our skin begins to repair itself through
a complex, well-coordinated process.
The largest organ in your body isn't your liver or your brain. It's your skin,
with a surface area of about 20 square feet in adults. Though different areas
of the skin have different characteristics, much of this surface performs similar
functions, such as sweating, feeling heat and cold, and growing hair. But after
a deep cut or wound, the newly healed skin will look different from the
surrounding area, and may not fully regain all its abilities for a while, or at all.
To understand why this happens, we need to look at the structure of the
human skin. The top layer, called the epidermis, consists mostly of hardened
cells, called keratinocytes,and provides protection. Since its outer layer is
constantly being shed and renewed,it's pretty easy to repair. But sometimes
a wound penetrates into the dermis, which contains blood vessels and the
various glands and nerve endings that enable the skin's many functions.
And when that happens, it triggers the four overlapping
stages of the regenerative process. The first stage, hemostasis, is the skin's
response to two immediate threats: that you're now losing blood and that the
physical barrier of the epidermis has been compromised. As the blood vessels
tighten to minimize the bleeding, in a process known as vasoconstriction,
both threats are averted by forming a blood clot. A special protein known as fibrin
forms cross-links on the top of the skin, preventing blood from flowing out and
bacteria or pathogens from getting in. After about three hours of this, the skin
begins to turn red, signaling the next stage, inflammation. With bleeding under
control and the barrier secured, the body sends special cells to fight any
pathogens that may have gotten through. Among the most important of these
are white blood cells, known as macrophages, which devour bacteria and
damage tissue through a process known as phagocytosis, in addition to producing
growth factors to spur healing. And because these tiny soldiers need to travel
through the blood to get to the wound site, the previously constricted blood vessels
now expand in a process called vasodilation. About two to three days after the
wound, the proliferative stage occurs, when fibroblast cells begin to enter the
wound. In the process of collagen deposition, they produce a fibrous protein
called collagen in the wound site, forming connective skin tissue to replace the
fibrin from before. As epidermal cells divide to reform the outer layer of skin,
the dermis contracts to close the wound. Finally, in the fourth stage of remodeling,
the wound matures as the newly deposited collagen is rearranged and converted
into specific types. Through this process, which can take over a year, the tensile
strength of the new skin is improved, and blood vessels and other connections
are strengthened. With time, the new tissue can reach from 50-80% of some
of its original healthy function, depending on the severity of the initial wound and
on the function itself. But because the skin does not fully recover, scarring
continues to be a major clinical issue for doctors around the world. And even
though researchers have made significant strides in understanding the healing
process, many fundamental mysteries remain unresolved. For instance,
do fibroblast cells arrive from the blood vessels or from skin tissue adjacent to
the wound? And why do some other mammals, such as deer, heal their
wounds much more efficiently and completely than humans? By finding the
answers to these questions and others, we may one day be able to heal
ourselves so well that scars will be just a memory.
1. What is the largest organ in a human body?
2. Enumerate the parts of the skin.
3. Summarize the process on how a wound heals itself.