I.
The skin is composed of two layers, the
outer EPITHELIUM and the thicker
inner DERMIS. A fatty
SUBCUTANEOUS LAYER lies beneath
the skin.
A.
The epidermis
1.
It is composed of stratified squamous epithelium
and lacks blood vessels.
2.
Cells in the stratum basale divide quickly (close to
blood in dermis), and push older
cells toward the surface where
they die.
3.
The older cells are called
keratinocytes because they
produce keratin (tough, waterproof protein). A thick
layer of keratinocytes forms the stratum corneum,
the uppermost layer of the skin.
4. In most areas, there are only four skin layers: the
stratum basale, the stratum spinosum, the stratum
granulosum, and the stratum corneum. In the palms
of the hands and the soles of the feet there is an
additional layer, called the stratum lucidum, between
the stratum granulosum and the stratum corneum.
Cells change
shape as they
move toward
the surface.
5. Melanocytes are specialized epidermal cells that
make the dark pigment melanin (provides skin color).
B. The dermis
1. The dermis binds the epidermis to the underlying
tissues.
2. It is mainly composed of collagenous and elastic
fibers that gives the skin toughness and elasticity.
3. Muscle and nerve fibers can be found in the dermis,
along with blood vessels, hair follicles, sebaceous
glands, and sweat glands.
C. Subcutaneous layer
1. It is made of loose
connective and adipose
tissues.
2. It regulates heat by
insulating the body.
3. It contains the major blood vessels in the skin.
II. Accessory skin organs (located in the dermis)
A.
HAIR FOLLICLES
1.
Hair develops from hair follicles (a group of
epidermal cells).
2.
As hair grows, hair cells get pushed out, become
keratinized, and die.
3. Goosebumps are produced
when the ARRECTOR PILI
MUSCLE that attaches to
each hair follicle contracts.
B. NAILS
1. They are protective covering at the ends of the
fingers and toes.
2. Nails grow from the whitish, thickened, half moonshaped nail bed. Nail cells are full of keratin.
C. SKIN GLANDS
1. SEBACEOUS GLANDS make sebum, a fatty material
that helps keep the hair and skin soft and flexible.
They are usually connected to hair follicles.
2. There are two kinds of SWEAT GLANDS, ECCRINE
and APOCRINE GLANDS.
a. Eccrine glands are found
on the forehead, neck and
back and produce lots of
sweat during exercise or
when its hot. Sweat is
mostly water, but contains some waste.
b. Appocrine glands are found in the groin and around
the nipples. They become active at puberty and
produce odorous sweat. They are active when a
person is upset, frightened, in pain, or sexually
aroused.
III. Body temperature and homeostasis
A. The body has a set point of 98.6˚ F and when
temperature rises above it, structures in the skin
release heat.
1. VASODILATION (dilation of the blood vessels)
occurs causing more blood to enter the vessels and
heat to escape.
Heat
2. RADIATION (heat rays move from warmer to
cooler surroundings in all areas) occurs.
3. CONDUCTION (heat moves from from the body to a
cooler object that the body is touching) occurs.
4. CONVECTION (air becomes heated as it comes into
contact with the body and moves away from it)
occurs.
5. EVAPORATION occurs when the body sweats. As
the water is carried away, heat leaves with it.
B. When the body drops below the set point the skin
responds to warm it up.
1. VASOCONSTRICTION happens to decrease the
heat lost from radiation, conduction, and convection.
2. Skeletal muscles may contract slowly (or more quickly
as in shivering), releasing energy and heat as a
byproduct.