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One of the skin’s major functions is sensation of pressure, temperature and
pain. The sensory receptors which detect pressure or movement are called
mechanoreceptors. There are several types of mechanoreceptors, and each
one is specialized for a particular type of pressure or vibration. Pacinian
corpuscles, for example, respond to fast vibrations and deep pressure while
Meissner’s corpuscles respond to slow vibration and light pressure.
Thermoreceptors detect heat and cold. There are also several types of
thermoreceptors, each having a different range of temperatures to which it
responds. Nociceptors detect tissue damage and stimulate the perception of
pain. Unlike most sensory receptors, nociceptors do not show sensory
adaptation over time – they continue to signal pain as long as tissue damage
is present.
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These are some examples of cutaneous sensory receptors found in the skin.
Note that they are all located in the dermis and that some types are more
superficial than others, depending on their functions.
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Skin is at the interface between our internal and external environments. It
makes sense, then, that it offers protection against a lot of things in the outside
world. Since the skin is strong (collagen fibers in the dermis) and tough
(keratin in the epidermis), underlying tissues are protected against mechanical
damage like bumps and scrapes. Since it’s waterproof, the skin also helps
prevent water loss from the body.
Tissues are protected against damage from corrosive chemicals (acids, bases,
etc.) both because the skin is a physical barrier and because sensory
receptors in the skin let us know when we’re begin damaged by chemicals.
The skin is the body’s first line of defense against bacterial infection.
Unbroken, it’s a physical barrier that keeps bacteria out. In addition, the skin’s
secretions contain an enzyme called lysozyme which can kill bacteria. Also,
the skin is kept slightly acidic, which creates a hostile environment for bacteria.
Epidermal cells can contain melanin, which is a dark pigment that absorbs
ultraviolet radiation before it can damage the DNA of skin cells. This is so
effective that very little UV radiation makes it through the epidermis.
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While many systems of the body help regulate temperature, the integument
contributes in many ways.
As we’ve already seen previously, eccrine glands secrete sweat onto the skin,
which absorbs heat as it evaporates. This helps cool the body when it is too
warm.
The body can regulate the amount of blood that reaches the surface of the
skin by changing the diameters of certain blood vessels. When the body is too
hot, vasodilation occurs – the arteriole lumens increase in size. This routes
more blood to the surface, so it can radiate more heat to the outside world
(sweating helps get rid of this heat, too). When the body is too cold,
vasoconstriction happens – the arteriole lumens decrease in size. This
keeps the warmer blood deeper in the body so it doesn’t lose heat to the
outside world.
In addition, remember that the hypodermis beneath the skin is mostly adipose
(fat) tissue, which acts as a pretty good insulator. So we have a layer of
insulation that covers the entire body.
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Vitamin D is critical nutrient that is involved in (among other things) using
calcium to build bone tissue. A deficiency of vitamin D leads to weakened
bones and can affect the muscular and nervous system as well. It may also
contribute to skin cancer. Unfortunately, not very many foods contain much
vitamin D. Fortunately, though, the skin can make it for us! When certain skin
cells in the stratum basale and stratum spinosum are exposed to ultraviolet B
light, they convert a cholesterol molecule into vitamin D. Note that sunlight
exposure is required for this to happen. (Since adequate sunlight exposure
isn’t guaranteed, many foods in industrialized countries are fortified with
vitamin D, by the way.)
The skin plays only a minor role in excretion, but sweat does have a small
amount of nitrogenous waste products in it.
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There are there main determinants of skin color.
•Melanin is a pigment that determines the darkness of your skin. It can be yellow, dark yellow,
brown, or even very dark brown. Melanin is produced by cells called melanocytes that are
interspersed throughout the stratum basale with the skin cells. The melanocytes have lots of
melanin-filled projections that stick out of them like little fingers. As new cells are formed, the
cells absorb the ends of these projections and incorporate the melanin (in vesicles called
melanosomes). The melanin is arranged above the nucleus like a little umbrella, where it can
absorb ultraviolet radiation before it can damage the DNA.
Melanin is the main pigment that accounts for the difference in skin color between ethnic
groups, but everyone has about the same number of melanocytes. The amount of melanin
produced by these melanocytes is genetically-determined and varies from group-to-group and
person-to-person, though.
Sunlight exposure can cause an increase in melanin production as well.
•Carotene is a yellow or orange pigment that imparts an orangish color to the skin. It is a
nonpolar pigment that dissolves in our body fat and in the oils of the stratum corneum. Carrots
are high in beta carotene, so eating a lot of carrots over a long period of time can cause the
skin to change color. Carotene is most-easily visible in the palmar and plantar regions, where
there is little melanin to mask the orange color.
•Blood is red, so it just makes sense that where it’s close to the surface, the skin appears pink
or red. When we blush, blood flow to the surface of the skin in our cheeks increases,
accounting for the red color. Our lips are redder than other skin areas because they have a
superficial blood flow, too.
Sometimes, though, dark colors under the skin can look a different color than they really are. In
areas where the dermis is thin, collagen fibers scatter the light reflected off of blood, blood
vessels, tattoo dyes, etc. Only blue light gets to your eyes, so these images may look blue. The
superficial veins in your wrists probably look blue for this very reason. In reality, the blood in
those vessels is red, but the light is scattered by the collagen fibers, so you only see blue. This
is the same reason the sky looks blue. (Sunlight is white, but particles in the atmosphere
scatter the light in all directions so that only the blue light reaches your eyes.)
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When tissues (not just skin) are damaged, a series of events begins to repair the damage.
There are two types of tissue repair. In regeneration, the damaged tissue is replaced by the
same type of tissue. In fibrosis, the damaged tissue is replaced by dense connective tissue
instead. Which type of repair happens depends on both the tissue type (skin and bone
regenerate well, for example, while muscle and some nervous tissues don’t regenerate) and
the extent of the damage. In the skin, you can tell when fibrosis has occurred because the
dense connective tissue left behind is visible as a scar.
Three events happen when skin is damaged: the capillaries in the damaged area become
more permeable (leaky). This allows the blood to deliver more fluid, repair proteins, and white
blood cells. Second, granulation tissue forms. Granulation tissue is a delicate mass of
connective tissue and capillaries that joins the broken ends of the skin while new cells are
being produced. Finally, regeneration creates new skin cells that, usually, make the skin as
good as new.
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