The skin, along with hair and nails, provides the body with
a protective outer covering that shields it, for example, from harmful solar
rays. It also provides our first line of defence against infection, helps
control water loss from the body, plays an important role in TEMPERATURE CONTROL, and contains the receptors that provide the sense of touch.
The skin has two main layers, called the epidermis and dermis. The
epidermis consists of an upper layer of dead cells and a lower living layer,
which replaces cells as they are lost from the upper layer. Beneath the
epidermis is the thicker dermis, which overlies an insulating layer of fatty
tissue.
Hair grows from follicles, pockets of epidermal tissue that extend
down into the dermis. Hair has a cycle of growth, rest, and then loss,
when the new hair pushes the old hair out of the follicle. About 100 hairs are
lost and replaced in a person’s scalp every day.
The ends of the fingers and toes are covered by nails. These plates
of tough protective tissue are made mainly of keratin, a protein also found in
hair and skin. Nails grow from a region of living cells called the germinal
matrix, which lies underneath a fold of skin called the cuticle.
The blood vessels, hairs, and sweat glands of the skin
work together to help control body temperature. If we get too hot, our sweat
production increases and blood vessels widen to allow more blood to reach the
skin’s surface, where it cools. If we get too cold, these processes go
into reverse. In addition, tiny muscles attached to the hair follicles pull the
hairs erect, trapping an insulating layer of air next to the skin.
Sweat, a salty liquid, reaches the surface of the skin through
pores. The pore is surrounded by dead epidermal cells. Sweat
evaporates from the surface of the skin and so helps to lower the body
temperature. Sweating also rids the body of excess water and some waste
products.
