Protective functions of Skin
by
Dr Aijaz
Body’s largest organ; covers area of 1.5 to 2.0 m2;
accounts for about 15% of the total body weight;
• Consist of 2 layers: epidermis (top) stratified,
dermis (deep) connective tissue layer;
• Below the skin is the hypodermis (another connective tissue
layer, it is not a technical part of the skin)
• Most of the skin is 1-2mm thick. although eyelids(0.5mm) and 6mm
between the shoulder blades. variation is due to the thickness of the
dermis;
• Classification of the skin is based on the epidermis alone especially
the surface layer of dead cells (stratum corneum): -thick: palm, soles,
fingers, and toes;has sweat glands but no hair follicles, nor oil glands
(sebaceous); -
thin: rest of the body; has hair follicles, sebaceous glands and sweat
glands;
• Hairless (Glabrous) skin has an epidermal layer of about 1.5 mm in
thickness and a dermis of about 3 mm
• Hairy skin has an epidermal layer of 0.07 mm in thickness and a
dermis of about 1-2 mm;
Epidermis
Consist of a surface comprised of dead cells packed with
keratin (a tough protein);
• Does not contain blood vessels. but it contains sparse nerve
endings for touch and pain (But most sensations are due to
the dermis);
• Cell types:
– Keratinocytes - produce keratin
– Melanocytes - produce melanin
Dermis
• Composed mainly of collagen, but also contains elastic
reticular fibers, blood vessels, sweat glands, sebaceous
glands, hair follicles, nail roots, sensory nerve endings and
muscular tissue (facial expressions are due to the skeletal
muscle connection to the dermal collagen fibers to produce
smile, frown, eyebrow movement…)
• The boundaries of dermis vs epidermis is not strict, it’s like
cardboard ridges that merge together. In sensitive areas, the
dermis is more extended and pushed to the surface allowing
blood vessels and nerve endings to reach closer to the
surface (look at hand, front vs back)
• Functions: Pressure detection; metabolism (duplication of
cells)…
Hypodermis
• A sub dermal layer of adipose tissue or otherwise called
subcutaneous fat; which is made up of loose, fibrous tissue, rich in
blood vessels, lymphatic vessels and nerves;
• The base of hair follicles and the coiled tubes of sweat glands
may also project down into the hypodermis;
• This is the layer that pads the body, serves as an energy
reservoir and provides thermal insulation, (it is
differently distributed in females vs males
White Reaction
When a pointed object is drawn lightly over the skin, the stroke
lines become pale (white reaction). The mechanical stimulus
apparently initiates contraction of the precapillary sphincters, and
blood drains out of the capillaries and small veins. The response
appears in about 15 seconds.
Triple Response
When the skin is stroked more firmly with a pointed instrument,
instead of the white reaction there is reddening at the site that
appears in about 10 seconds (red reaction).
This is followed in a few minutes by local swelling and diffuse,
mottled reddening around the injury. The initial redness is due to
capillary dilation, a direct response of the capillaries to pressure.
The swelling (wheal) is local edema due to increased permeability
of the capillaries and postcapillary venules, with consequent
extravasation of fluid.
The redness spreading out from the injury (flare) is due to
arteriolar dilation. This three-part response, the red reaction, wheal,
and flare, is called the triple response and is part of the normal
reaction to injury
The flare is absent in locally anesthetized skin and in denervated
skin after the sensory nerves have degenerated, but it is present
immediately after nerve block or section above the site of the injury.
This plus other evidence indicates that it is due to an axon reflex, a
response in which impulses initiated in sensory nerves by the injury
are relayed antidromically down other branches of the sensory
nerve fibers
Reactive Hyperemia
A response of the blood vessels that occurs in many organs but is
visible in the skin is reactive hyperemia, an increase in the
amount of blood in a region when its circulation is reestablished
after a period of occlusion. When the blood supply to a limb is
occluded, the cutaneous arterioles below the occlusion dilate.
When the circulation is reestablished, blood flowing into the dilated
vessels makes the skin become fiery red. O2 diffuses a short
distance through the skin, and reactive hyperemia is prevented if
the circulation of the limb is occluded in an atmosphere of 100%
O2. Therefore, the arteriolar dilation is apparently due to a local
effect of hypoxia.
Tissue Macrophages in the Skin and Subcutaneous Tissues
(Histiocytes).
Although the skin is mainly impregnable to infectious agents, this is
no longer true when the skin is broken. When infection begins in a
subcutaneous
tissue and local inflammation ensues, local tissue macrophages
can divide in situ and form still more macrophages. Then they
perform the usual functions of attacking and destroying the
infectious agents,
Wound Healing
When tissue is damaged, platelets adhere to exposed matrix via integrins that
bind to collagen and laminin.
Blood coagulation produces thrombin, which promotes platelet aggregation and
granule release. The platelet granules generate an inflammatory response.
White blood cells are attracted by selectins and bind to integrins on endothelial
cells, leading to their extravasation through the blood vessel walls.
Cytokines released by the white blood cells and platelets up-regulate integrins
on macrophages, which migrate to the area of injury, and on fibroblasts and
epithelial cells, which mediate wound healing and scar formation.
Plasmin aids healing by removing excess fibrin. This aids the migration of
keratinocytes into the wound to restore the epithelium under the scab. Collagen
proliferates, producing the scar. Wounds gain 20% of their ultimate strength in 3
weeks and later gain more strength, but they never reach more than about 70%
of the strength of normal skin.
Skin as Blood Reservoir
Shock is more profound in patients with elevated
temperatures because of the cutaneous vasodilation, and
patients in shock should not be warmed to the point that their
body temperature rises.
This is sometimes a problem because well-meaning laymen
have read in first-aid books that "injured patients should be
kept warm," and they pile blankets on accident victims who
are in shock.
Control of Skin Coloration and Proctection against Radiation.
Fish, reptiles, and amphibia change the color of their skin for
thermoregulation, camouflage, and behavioral displays. They do this in
part by moving black or brown granules into or out of the periphery of
pigment cells called melanophores. The granules are made up of
melanins, which are synthesized from dopa and dopaquinone. The
movement of these granules is controlled by a variety of hormones and
neurotransmitters
In mammals, there are no melanophores containing pigment granules
that disperse and aggregate, but there are melanocytes, which have
multiple processes containing melanin granules. The melanocytes
contain melanotropin-1 receptors, one of several types of melanotropin
receptors that have been cloned.
Pigment Abnormalities in Humans
Albinos have a congenital inability to synthesize melanin. Albinism occurs
in humans and many other mammalian species. It can be due to a variety
of different genetic defects in the pathways for melanin synthesis.
Piebaldism is characterized by patches of skin that lack melanin as a
result of congenital defects in the migration of pigment cell precursors
from the neural crest during embryonic development. Not only the
condition but also the precise pattern of the loss is passed from one
generation to the next.
Vitiligo is due to a similar patchy loss of melanin, but the loss develops
after birth and is progressive.
3-WATER PROOFING
• In dry environment , it is a mean of preventing fluid
loss .
• Impermeability of keratin to water is maintained by
the thickened lipid wall but some chemicals are lipid
soluble & can penetrate .
• The secretion of sebaceous glands over the skin &
the outer ear by the modified sweat glands or
ceruminous glands secrete water repellent wax .
• Daily Loss of Body Water Insensible Water Loss. Some of the water losses
cannot be precisely regulated.
• there is a continuous loss of water by evaporation from the respiratory
tract and diffusion through the skin, 700 ml/day of water loss under
normal conditions. This is termed insensible water loss because we are not
consciously aware of it.
• The insensible water loss through the skin occurs independently of
sweating and is present even in people who are born without sweat glands;
the average water loss by diffusion through the skin is about 300 to 400
ml/day. This loss is minimized by the cholesterol-filled cornified layer of
the skin, which provides a barrier against excessive loss by diffusion.
• When the cornified layer lost, as occurs with extensive burns, the rate of
evaporation can increase as much as 10-fold, to 3 to 5 L/day. For this
reason, burn victims must be given large amounts of fluid, usually
intravenously, to balance fluid loss.
Vitamin D synthesis
The active transport of Ca2+ and PO43- from the intestine is increased by a
metabolite of vitamin D.
The term "vitamin D" is used to refer to a group of closely related sterols
produced by the action of ultraviolet light on certain provitamins.
Vitamin D3, which is also called cholecalciferol, is produced in the skin of
mammals from 7-dehydrocholesterol by the action of sunlight.
Insulator System of the Body
The skin, the subcutaneous tissues, and especially the fat of the
subcutaneous tissues act together as a heat insulator for the body.
The fat is important because it conducts heat only one third as
readily as other tissues.
When no blood is flowing from the heated internal organs to the
skin, the insulating properties of the normal male body are about
equal to three quarters the insulating properties of a usual suit of
clothes.
In women, this insulation is even better. The insulation beneath the
skin is an effective means of maintaining normal internal core
temperature, even though it allows the temperature of the skin
to approach the temperature of the surroundings.
Blood Flow to the Skin from the Body Core Provides Heat Transfer
Blood vessels are distributed profusely beneath the skin.
Especially important is a continuous venous plexus that is supplied by
inflow of blood from the skin capillaries.
In the most exposed areas of the body—the hands, feet, and ears—blood
is also supplied to the plexus directly from the small arteries through
highly muscular arteriovenous anastomoses.
The rate of blood flow into the skin venous plexus can vary
tremendously—from barely above zero to as great as 30 per cent of the
total cardiac output.
A high rate of skin flow causes heat to be conducted from the core of the
body to the skin with great efficiency, whereas reduction in the rate of
skin flow can decrease the heat conduction from the core to very little.
Temperature-Decreasing Mechanisms When the Body Is Too
Hot
The temperature control system uses three important mechanisms to
reduce body heat when the body temperature becomes too great:
1. Vasodilation of skin blood vessels. In almost all areas of the body, the
skin blood vessels become intensely dilated. This is caused by inhibition
of the sympathetic centers in the posterior hypothalamus that cause
vasoconstriction. Full vasodilation can increase the rate of heat transfer
to the skin as much as eightfold.
2. Sweating. The effect of increased body temperature to cause sweating
when the body core temperature rises above the critical level of 37°C
(98.6°F). An additional 1°C increase in body temperature causes enough
sweating to remove 10 times the basal rate of body heat production.
3. Decrease in heat production.
The mechanisms thatcause excess heat production, such as shivering and
chemical thermogenesis, are strongly inhibited.
Temperature-Increasing Mechanisms When the Body Is Too Cold
When the body is too cold, the temperature control system institutes
exactly opposite procedures. They are:
1. Skin vasoconstriction throughout the body. This is caused by
stimulation of the posterior hypothalamic sympathetic centers.
2. Piloerection. Piloerection means hairs “standing on end.” Sympathetic
stimulation causes the arrector pili muscles attached to the hair follicles
to contract, which brings the hairs to an upright stance. This is not
important in human beings, but in lower animals, upright projection of
the hairs allows them to entrap a thick layer of “insulator air” next to the
skin, so that transfer of heat to the surroundings is greatly depressed.
3. Increase in thermogenesis:(heat production). Heat production by the metabolic systems is increased
by promoting shivering, sympathetic excitation of heat production, and
thyroxine secretion. These methods of increasing heat require additional
explanation, which follows.
Thanks