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Skin has many functions, and is far more than a mere decoration for the
body. Some of these functions are so important that unless most of the
skin is working efficiently, we will die.
This is the reason why second- or third-degree
burns are so serious. When the skin is destroyed over a large area, there
is no way of controlling the rate at which water is lost to the outside
environment, or of regulating the temperature of the body or of controlling
infection. Someone who has lost over half the skin this way is unlikely
to survive.
Although we think of the skin as a single organ,
the epidermis and dermis have to some extent separate functions. The function
of water conservation is however dependent on both; the role of the stratum
corneum in this field is absolutely vital, as it acts as a semipermeable
barrier and allows us to survive in a hostile environment.
Functions of the
epidermis
The epidermis has three principal functions:
- protecting the body from the environment, particularly the sun
- preventing excessive water loss from the body
- protecting the body from infection.
Protection
from the environment
The sun produces enormous amounts of heat and light, some of which reaches
the earth. Without this heat and light no life could ever have evolved.
Unfortunately the sun also produces less beneficial
rays, which are completely invisible to us, called ultraviolet radiation.
(Sun beds also expose their users to these rays.) Part of this radiation
is reflected by the stratum corneum at the skin surface, part is absorbed
by the melanin
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in the epidermal cells, and some is scattered within the skin. All three
processes contribute to the vital function of protecting the nuclei of
the cells in the epidermis and the collagen of the dermis.
This scattered radiation creates a lot of high-energy
particles, which are called free radicals. Free radicals are very
reactive, and attack the constituents of the skin: this is why over a
long time ultraviolet radiation produces so much damage. This will be
discussed in Chapter 4 ('Skin and aging').
Sunlight reflected
from snow - a damaging combination for our skin, since it contains a
substantial proportion of ultraviolet radiation.
Prevention
of water loss from the body
Throughout our lives our bodies naturally lose water by constant gentle
evaporation through our skins (transepidermal water loss, TEWL), although
we are unaware of the process. Preventing excessive water loss is exceptionally
important in itself - both to the skin itself and to the body as a whole.
In the normal epidermis the water content gets less the closer we get
to the surface. Water makes up to 70-75% of the weight of the basal layer,
but only 10-15% of the stratum corneum.
The stratum corneum is a particularly important
barrier to the control of moisture loss.
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