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02-28-2013, 11:50 AM
Review of **rmal anatomy and physiology
REVIEW OF **RMAL ANATOMY
AND PHYSIOLOGY
The skin, its accessory structures, and the subcutaneous tissue
make up the integumentary system, the covering of the
body that separates the living internal environment from
the external environment. The skin itself is considered an
organ and consists of two layers, the outer epidermis and the
inner dermis (Fig. 50–1).
Epidermis
The epidermis is made of stratified squamous epithelial tissue
and is avascular, meaning that it has ** capillaries
within it. Its **urishment comes from the dermis beneath
it. The epidermis is thickest on the palms of the hands and
soles of the feet. The innermost layer of the epidermis is
called the stratum germinativum, and it is here that mitosis
takes place to produce new epidermal cells. The rate of mitosis
is fairly constant, but it may be increased by chronic
pressure on the skin (as in callus formation). The new cells
produce the protein keratin. As they are pushed to the sur-face of the skin, they die and become the stratum corneum,
the outermost of the epidermal layers.
The stratum corneum is many layers of dead cells; all that
remains is their keratin. An unbroken stratum corneum is an
effective barrier against pathogens and most chemicals, although
even microscopic breaks are sufficient to permit their
entry. Keratin is relatively waterproof, so it prevents the loss
of water—that is, dehydration—and also prevents the entry
of excess water by way of the body surface. As dead cells are
worn off the surface of the skin (which also contributes to
the removal of pathogens), they are continuously replaced
by cells from within. Loss of large portions of the stratum
corneum, as with extensive third-degree burns, greatly increases
the risk for infection and dehydration.Mela**cytes are cells in the lower epidermis that produce
the protein melanin; the amount of melanin is a genetic
characteristic and gives color to skin and hair. When
the skin is exposed to ultraviolet (UV) rays (part of sunlight),
production of melanin increases and it is incorporated
into the epidermal cells before they die, making the
cells darker. Melanin is a pigment barrier to prevent further:icon30:
REVIEW OF **RMAL ANATOMY
AND PHYSIOLOGY
The skin, its accessory structures, and the subcutaneous tissue
make up the integumentary system, the covering of the
body that separates the living internal environment from
the external environment. The skin itself is considered an
organ and consists of two layers, the outer epidermis and the
inner dermis (Fig. 50–1).
Epidermis
The epidermis is made of stratified squamous epithelial tissue
and is avascular, meaning that it has ** capillaries
within it. Its **urishment comes from the dermis beneath
it. The epidermis is thickest on the palms of the hands and
soles of the feet. The innermost layer of the epidermis is
called the stratum germinativum, and it is here that mitosis
takes place to produce new epidermal cells. The rate of mitosis
is fairly constant, but it may be increased by chronic
pressure on the skin (as in callus formation). The new cells
produce the protein keratin. As they are pushed to the sur-face of the skin, they die and become the stratum corneum,
the outermost of the epidermal layers.
The stratum corneum is many layers of dead cells; all that
remains is their keratin. An unbroken stratum corneum is an
effective barrier against pathogens and most chemicals, although
even microscopic breaks are sufficient to permit their
entry. Keratin is relatively waterproof, so it prevents the loss
of water—that is, dehydration—and also prevents the entry
of excess water by way of the body surface. As dead cells are
worn off the surface of the skin (which also contributes to
the removal of pathogens), they are continuously replaced
by cells from within. Loss of large portions of the stratum
corneum, as with extensive third-degree burns, greatly increases
the risk for infection and dehydration.Mela**cytes are cells in the lower epidermis that produce
the protein melanin; the amount of melanin is a genetic
characteristic and gives color to skin and hair. When
the skin is exposed to ultraviolet (UV) rays (part of sunlight),
production of melanin increases and it is incorporated
into the epidermal cells before they die, making the
cells darker. Melanin is a pigment barrier to prevent further:icon30: