Chapter 5
The Integumentary System
151
5
Te skin receives little respect from its inhabitants, but archi-
tecturally it is a marvel. It covers the entire body, has a surface
area of 1.2 to 2.2 square meters, weighs 4 to 5 kilograms (4–5 kg
5
9–11 lb), and accounts for about 7% of total body weight in
the average adult. Also called the integument (“covering”), the
skin multitasks. Its functions go well beyond serving as a bag for
body contents. Pliable yet tough, it takes constant punishment
from external agents. Without our skin, we would quickly fall
prey to bacteria and perish from water and heat loss.
Varying in thickness from 1.5 to 4.0 millimeters (mm) or
more in different parts of the body, the skin is composed of two
distinct regions
(Figure 5.1)
:
Te
epidermis
(ep
0
ĭ-der
9
mis), composed of epithelial cells, is
the outermost protective shield of the body (
epi
5
upon).
Te underlying
dermis,
making up the bulk of the skin, is a
tough, leathery layer composed mostly of fibrous connective
tissue.
Only the dermis is vascularized. Nutrients reach the epider-
mis by diffusing through the tissue fluid from blood vessels in
the dermis.
Te subcutaneous tissue just deep to the skin is known as
the
hypodermis
(Figure 5.1). Strictly speaking, the hypodermis
is not part of the skin, but it shares some of the skin’s protec-
tive functions. Te hypodermis, also called
superficial fascia
because it is superficial to the tough connective tissue wrapping
(fascia) of the skeletal muscles, consists mostly of adipose tissue.
Besides storing fat, the hypodermis anchors the skin to the un-
derlying structures (mostly to muscles), but loosely enough that
the skin can slide relatively freely over those structures. Sliding skin
protects us by ensuring that many blows just glance off our bodies.
Because of its fatty composition, the hypodermis also acts as a shock
absorber and an insulator that reduces heat loss. Te hypodermis
thickens markedly when a person gains weight. In females, this “ex-
tra” subcutaneous fat accumulates first in the thighs and breasts, but
in males it first collects in the anterior abdomen (as a “beer belly”).
Epidermis
Structurally, the
epidermis
is a keratinized stratified squamous
epithelium consisting of four distinct cell types and four or five
distinct layers.
Epidermis
Hair shaft
Dermis
Reticular
layer
Papillary
layer
Hypodermis
(subcutaneous
tissue; not part
of skin)
Dermal papillae
Sweat pore
Subpapillary
plexus
Appendages of skin
Eccrine sweat gland
Arrector pili muscle
Sebaceous (oil) gland
Hair follicle
Hair root
Nervous structures
Sensory nerve fiber
with free nerve endings
Lamellar corpuscle
Hair follicle receptor
(root hair plexus)
Cutaneous plexus
Adipose tissue
Figure 5.1
Skin structure.
Three-dimensional view of the skin and underlying subcutaneous
tissue. The epidermal and dermal layers have been pulled apart at the right corner to reveal the
dermal papillae.
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