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Human Body^133

HOW DOES SKIN CONTROL BODY

TEMPERATURE?

When the body is hot, sweat glands

secrete a watery fluid onto the skin.

The water draws heat from the skin

as it evaporates. At the same time,

blood vessels in the skin widen to

release excess heat. When the body

is cold, the skin’s blood vessels

narrow to reduce heat loss, and tiny

muscles pull hairs erect, trapping

warm air over the skin.

WHY DOES SKIN COLOUR VARY?

Skin colour comes from the pigment melanin,

made by cells called melanocytes in the

epidermis. Everyone has the same number

of melanocytes, but they are much more

active in people with dark skin. Melanocytes

also become more active after exposure to

the sun, producing pigment to protect the

skin from damage from the sun.

An outer covering of skin protects the body from

injury, germs, water loss, and sunlight. Skin is also an

important sensory organ, full of receptor cells sensitive

to touch, heat, cold, and pain.

WHAT MAKES SKIN TOUGH?

Skin gets its strength from a supple outer layer called

the epidermis, made largely of dead cells packed with

a hard protein called keratin. The epidermis is

continually wearing away and renewing itself. Its

bottom layer of living cells keeps dividing, producing

new cells that slowly move to the surface. As they

travel up, they flatten, harden, and die.

WHAT’S UNDER THE EPIDERMIS?

Beneath the epidermis is the dermis, a layer of tissue

containing blood vessels, nerves, sensory receptors,

sweat glands, and hair roots. Under the dermis

is a layer of fat cells that cushions the skin

and traps heat deep within the body.

4 FINGERNAILS

Nails are made of overlapping plates

of dead cells filled with the protein

keratin. Keratin is tough and

waterproof and is also found

in hair and skin. Nails protect

the sensitive tips of the fingers

and toes.

BEARD STUBBLE 3

These shafts of hair from a man’s beard have been shaved

short. Hair is found all over the body to protect the skin and help

us feel things that come near to the skin’s surface. It is made of dead

cells and keratin, and can grow up to 1 cm (½ in) a month.

FIND OUT MORE. Body 128–129 • Circulation 134 • Growth 149

Skin

1 SWEAT

Droplets of sweat cover the skin on

the back of a hand. The fluid is

produced in sweat glands in the

dermis, and rises up through pores

onto the surface of the skin. Sweat

lowers the body’s temperature by

using its heat to evaporate.

1 A CROSS-SECTION THROUGH THE SKIN

This highly magnified image shows the layers in healthy skin.

The top of the epidermis (red) is made up of flattened dead

cells full of keratin. Surface cells continually flake away.

Melanocyte cells in

this area at the base of

the epidermis produce

melanin – the pigment

that gives skin its

colour and filters out

ultraviolet light

Epidermis

cells continually

divide to produce

new skin cells

Dermis

is a layer of living

tissue below the

epidermis full of

blood vessels and

sensory receptors

Skin surface

is made up of

dead, flattened

cells from the

epidermis

Hair follicles

are rooted deep

in the dermis

Hair shaft

skin