Chapter 5
The Integumentary System
163
5
Cutaneous Sensation
Te skin is richly supplied with
cutaneous sensory receptors
,
which are actually part of the nervous system. Te cutaneous re-
ceptors are classified as
exteroceptors
(ek
0
ster-o-sep
9
torz) because
they respond to stimuli arising outside the body. For example, tac-
tile (Meissner’s) corpuscles (in the dermal papillae) and tactile discs
allow us to become aware of a caress or the feel of our clothing
against our skin, whereas lamellar (also called Pacinian) corpuscles
(in the deeper dermis or hypodermis) alert us to bumps or con-
tacts involving deep pressure. Hair follicle receptors report on wind
blowing through our hair and a playful tug on a pigtail. Free nerve
endings that meander throughout the skin sense painful stimuli
(irritating chemicals, extreme heat or cold, and others). We defer
detailed discussion of these cutaneous receptors to Chapter 13.
Figure 5.1 illustrates all the cutaneous receptors mentioned
above except for tactile corpuscles, which are found only in skin
that lacks hairs, and tactile cells, shown in Figure 5.2b.
Metabolic Functions
Te skin is a chemical factory, fueled in part by the sun’s rays.
When sunlight bombards the skin, modified cholesterol mol-
ecules are converted to a vitamin D precursor, which is trans-
ported via the blood to other body areas to be ultimately
converted to vitamin D, which plays various roles in calcium
metabolism. For example, calcium cannot be absorbed from the
digestive tract without vitamin D.
Among its other metabolic functions, the epidermis makes
chemical conversions that supplement those of the liver. For
example, keratinocyte enzymes can
“Disarm” many cancer-causing chemicals that penetrate the
epidermis
Convert some harmless chemicals into carcinogens
Activate some steroid hormones—for instance, they can
transform cortisone applied to irritated skin into hydrocorti-
sone, a potent anti-inflammatory drug
Skin cells also make several biologically important proteins,
including collagenase, an enzyme that aids the natural turnover
of collagen (and deters wrinkles).
Blood Reservoir
Te dermal vascular supply is extensive and can hold about 5%
of the body’s entire blood volume. When other body organs,
such as vigorously working muscles, need a greater blood sup-
ply, the nervous system constricts the dermal blood vessels. Tis
constriction shunts more blood into the general circulation,
making it available to the muscles and other body organs.
Excretion
Te body eliminates limited amounts of nitrogen-containing
wastes (ammonia, urea, and uric acid) in sweat, although most
such wastes are excreted in urine. Profuse sweating is an impor-
tant avenue for water and salt (sodium chloride) loss.
Drug agents called
penetration enhancers
that help ferry
other drugs into the body
Alcoholic drinks dramatically enhance skin permeability for at
least 24 hours a±er their ingestion.
Homeostatic Imbalance
5.7
Organic solvents and heavy metals are devastating to the body and
can be lethal. Passage of organic solvents through the skin into
the blood can shut down the kidneys and also cause brain dam-
age. Absorption of lead results in anemia and neurological defects.
Tese substances should never be handled with bare hands.
Biological Barriers
Biological barriers include the dendritic cells of the epidermis,
macrophages in the dermis, and DNA itself.
Dendritic cells are active elements of the immune system. ²o
activate the immune response, the foreign substances, or
anti-
gens
, must be presented to specialized white blood cells called
lymphocytes. In the epidermis, the dendritic cells play this role.
Dermal macrophages constitute a second line of defense to
dispose of viruses and bacteria that manage to penetrate the
epidermis. Tey, too, act as antigen “presenters.”
Although melanin provides a fairly good chemical sun-
screen, DNA itself is a remarkably effective biologically based
sunscreen. Electrons in DNA molecules absorb UV radiation
and transfer it to the atomic nuclei, which heat up and vibrate
vigorously. Since the heat dissipates to surrounding water mole-
cules instantaneously, the DNA converts potentially destructive
radiation into harmless heat.
Body Temperature Regulation
Te body works best when its temperature remains within
homeostatic limits. Like car engines, we need to get rid of the
heat generated by our internal reactions. As long as the external
temperature is lower than body temperature, the skin surface
loses heat to the air and to cooler objects in its environment,
just as a car radiator loses heat to the air and other nearby parts.
Under normal resting conditions, and as long as the environ-
mental temperature is below 31–32°C (88–90°F), sweat glands
secrete about 500 ml (0.5 L) of sweat per day. Tis routine and un-
noticeable sweating is called
insensible perspiration
. When body
temperature rises, the nervous system stimulates dermal blood
vessels to dilate and the sweat glands into vigorous secretory ac-
tivity. Indeed, on a hot day, sweat becomes noticeable and can ac-
count for the loss of up to 12 L (about 3 gallons) of body water in
one day. Tis visible output of sweat is called
sensible perspiration
.
Evaporation of sweat from the skin surface dissipates body heat
and efficiently cools the body, preventing overheating.
When the external environment is cold, dermal blood vessels
constrict. Teir constriction causes the warm blood to bypass the
skin temporarily and allows skin temperature to drop to that of
the external environment. Tis slows passive heat loss from the
body, conserving body heat. Chapter 24 discusses body tempera-
ture regulation.
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