Body organs aren't all internal like the brain or the heart. There's
one we wear on the outside. Skin is our largest organ—adults carry some
8 pounds (3.6 kilograms) and 22 square feet (2 square meters) of it.
This fleshy covering does a lot more than make us look presentable. In
fact, without it, we'd literally evaporate.
Skin acts as a waterproof, insulating shield, guarding the body
against extremes of temperature, damaging sunlight, and harmful
chemicals. It also exudes antibacterial substances that prevent
infection and manufactures vitamin D for converting calcium into
healthy bones. Skin additionally is a huge sensor packed with nerves
for keeping the brain in touch with the outside world. At the same
time, skin allows us free movement, proving itself an amazingly
versatile organ.
Skin is made up of three layers. The outermost is the epidermis. This consists mainly of cells called keratinocytes, made from the tough protein keratin
(also the material in hair and nails). Keratinocytes form several
layers that constantly grow outwards as the exterior cells die and
flake off. It takes roughly five weeks for newly created cells to work
their way to the surface. This covering of dead skin is known as the stratum corneum,
or horny layer, and its thickness varies considerably, being more than
ten times thicker on the soles of the feet than around the eyes. The
epidermis harbors defensive Langerhans cells, which alert the body's
immune system to viruses and other infectious agents.
The epidermis is bonded to a deeper skin layer below known as the dermis,
which gives the organ its strength and elasticity thanks to fibers of
collagen and elastin. Blood vessels here help regulate body temperature
by increasing blood flow to the skin to allow heat to escape, or by
restricting the flow when it's cold. A network of nerve fibers and
receptors pick up feelings such as touch, temperature, and pain,
relaying them to the brain.
The dermis houses hair follicles and
glands with ducts that pass up through the skin. Sweat glands bring
down internal temperature through perspiration while ridding the body
of the waste fluids urea and lactate. Apocrine glands, which develop
during puberty, produce a scented sweat linked to sexual attraction
that can also cause body odor, especially around the armpits. Sebaceous
glands secrete oil-like sebum for lubricating the hair and skin.
The skin's base layer is the subcutis,
which includes a seam of fat laid down as a fuel reserve in case of
food shortage. It also works as insulation and cushions us from knocks
and falls.
Skin Color
Skin color is due to melanin,
a pigment produced in the epidermis to protect us from the sun's
potentially cancer-causing ultraviolet (UV) rays. Dark-skinned people
produce more numerous and deeper-colored melanin particles. People with
the darkest complexions are native to tropical regions, particularly
those with few densely forested areas.
Fair skin is an adaptation
found in people from northern latitudes where solar rays are relatively
weak. Here the benefits of dark skin are outweighed by the need for
bone-strengthening vitamin D, produced through exposure to UV rays. But
hotter, sunnier environments bring the risk of serious skin damage.
Australia, where the majority of the population is of northern European
descent, has the world's highest rates of skin cancer, accounting for more than 80 percent of all cancers diagnosed there each year.
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