162
UNIT 2
Covering, Support, and Movement of the Body
5
tissues from damaging external agents. Even this short list hints
that the skin and its derivatives perform a variety of functions, in-
cluding: protection, body temperature regulation, cutaneous sen-
sation, metabolic functions, blood reservoir, and excretion.
Protection
Given its superficial location, the skin is our most vulnerable or-
gan system, exposed to microorganisms, abrasion, temperature
extremes, and harmful chemicals. Te skin constitutes at least
three types of barriers: chemical, physical, and biological.
Chemical Barriers
Chemical barriers include skin secretions and melanin. Al-
though the skin’s surface teems with bacteria, the low pH of
skin secretions—the so-called
acid mantle
—retards their mul-
tiplication. In addition, dermcidin in sweat and bactericidal
substances in sebum kill many bacteria outright. Skin cells also
secrete natural antibiotics called
defensins
that literally punch
holes in bacteria, making them look like sieves. Wounded skin
releases large quantities of protective peptides called
cathelici-
dins
that are particularly effective in preventing infection by
group A streptococcus bacteria.
As discussed earlier, melanin provides a chemical pigment
shield to prevent UV damage to skin cells.
Physical Barriers
Te continuity of skin and the hardness of its keratinized cells pro-
vide physical barriers. As a physical barrier, the skin is a remarkable
compromise. A thicker epidermis would be more impenetrable,
but we would pay the price in loss of suppleness and agility.
Te outstanding barrier capacity of the skin arises from the
structure of its stratum corneum, which has been compared
to bricks and mortar. Multiple layers of dead flat cells are the
bricks and the glycolipids surrounding them are the mortar.
Epidermal continuity works hand in hand with the acid mantle
and certain chemicals in skin secretions to ward off bacterial
invasion. Te water-resistant glycolipids of the epidermis block
most diffusion of water and water-soluble substances between
cells, preventing both their loss from and entry into the body
through the skin. However, there is a continual small loss of wa-
ter through the epidermis, and if immersed in water (other than
salt water), the skin will take in some water and swell slightly.
Substances that
do
penetrate the skin in limited amounts
include
Lipid-soluble substances
, such as oxygen, carbon dioxide, fat-
soluble vitamins (A, D, E, and K), and steroids (estrogens)
Oleoresins
(o
0
le-o-rez
9
inz) of certain plants, such as poison
ivy and poison oak
Organic solvents
, such as acetone, dry-cleaning fluid, and
paint thinner, which dissolve the cell lipids
Salts of heavy metals
, such as lead and mercury
Selected
drugs
(nitroglycerine,
nicotine,
seasickness
medications)
found all over the body except in the thick skin of the palms
and soles. Tey are small on the body trunk and limbs, but
quite large on the face, neck, and upper chest. Tese glands
secrete an oily substance called
sebum
(se
9
bum). Te central
cells of the alveoli accumulate oily lipids until they become
so engorged that they burst, so functionally these glands are
holocrine glands
(see p. 125). Te accumulated lipids and cell
fragments constitute sebum.
Most, but not all, sebaceous glands develop as outgrowths
of hair follicles and secrete sebum into a hair follicle, or occa-
sionally to a pore on the skin surface. Arrector pili contractions
force sebum out of the hair follicles to the skin surface. Sebum
soFens and lubricates the hair and skin, prevents hair from be-
coming brittle, and slows water loss from the skin when external
humidity is low. Perhaps even more important is its
bactericidal
(bacterium-killing) action.
Hormones, especially androgens, stimulate the secretion of
sebum. Sebaceous glands are relatively inactive during child-
hood but are activated in both sexes during puberty, when an-
drogen production begins to rise.
Homeostatic Imbalance
5.6
If accumulated sebum blocks a sebaceous gland duct, a
white-
head
appears on the skin surface. If the material oxidizes and
dries, it darkens to form a
blackhead
.
Acne
is an active inflam-
mation of the sebaceous glands accompanied by “pimples”
(pustules or cysts) on the skin. It is usually caused by bacterial
infection, particularly by staphylococcus, and can range from
mild to severe, leading to permanent scarring.
Overactive sebaceous glands can cause
seborrhea
(seb
0
o-re
9
ah;
“fast-flowing sebum”), known as “cradle cap” in infants. Sebor-
rhea begins on the scalp as pink, raised lesions that gradually be-
come yellow to brown and begin to slough off oily scales.
Check Your Understanding
17.
Which cutaneous glands are associated with hair follicles?
18.
When Anthony returned home from a run in 85°F weather,
his face was dripping with sweat. Why?
19.
What is the difference between heat-induced sweating and a
“cold sweat,” and which variety of sweat gland is involved?
20.
Sebaceous glands are not found in thick skin. Why is their
absence in those body regions desirable?
For answers, see Appendix H.
Functions of the Integumentary
System
Describe how the skin accomplishes at least five different
functions.
±irst and foremost, our skin is a barrier. Like the skin of a grape, it
keeps its contents juicy and whole. Te skin is also a master at self
(wound) repair, and interacts immediately with other body sys-
tems by making potent molecules, all the while protecting deeper
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