Pathophysiology of the Skin
Skin and Skin Functions
The skin is the largest organ of the human body. It is also our first line of defense against many types of infections. Its health and well being are critical to us as lymphedema patients.
The skin is composed of many layers of cells and each is named for its unique function, texture or position. Because of the immunocompromised condition of a lymphovenous limb, I feel it is important for us to understand the skin, its functions, skin care and health, possible skin growths and infections.
Major functions of skin
We will discuss each of these in turn. Please note, however, that sensation was already covered in enough detail in the Unit 4 main page.
Skin protects everything that lies beneath it. It acts as a cushion against insult to the body. It alerts the body of danger (through sensation). Its continuous, tightly connected surface, lightly coated with oily sebum maintains a distinct fluid environment within our bodies from that environment encountered outside of our bodies. Also, the skin, by being such an effective barrier, is our bodies' first line of defense against invading parasites/diseases.
Our bodies are supposed to maintain a specific temperature, 37 ºC (or 98.6 ºF) at all times. We need to maintain that temperature in order for our cells to perform properly. So, our bodies use energy to produce heat just to keep our temperature up.
Meanwhile, heat continually escapes from our bodies. It escapes in four ways: 1) radiation; 2) conduction; 3) convection; and 4) evaporation. These four ways require some further explanation. After the following explanation, we will return to the notion of insulation and how that helps to prevent heat loss.
You know how if you light a match, the heat spreads out in all directions from the flame? We talk about the heat as radiating out from the heat source, the flame. Well, our bodies radiate heat, too. They contain heat, much like a flame contains heat (but just less of it). Heat radiates out from our bodies in all directions. Heat can be described as traveling in rays that are called infrared heat rays.
Some organisms can actually see infrared rays. For example, killer snakes have organs that detect these rays and use them to find warm-blooded animals-- even in the dark. For these snakes, it is as if they see the heat rays radiating out of the warm-blooded animal. Humans do not have any ability to see infrared heat rays, that's why the notion of their existence seems weird to us. But humans have manufactured machines that can sense these infrared heat rays-- so that through this machine people seem colored from red (very hot) to blue (very cold).
All that this means is that if your body is up against something that is colder than it, heat gets transferred from your body to the colder item. For example, after you have been sitting in a chair for a while, you may notice that you have warmed it up. Another more important example is that the air that is around your body, assuming it is cooler than 37 ºC also warms up just by being next to your body.
You know that heat rises and cold air falls. We all know that. Well, the warmer air that is next to your body, as mentioned above in the "conduction" section, also rises. When it does so, cooler air that has fallen replaces it. This means that we continually lose the insulating, warmer air layer from our bodies, and so we end up losing more heat.
In order for water to evaporate, it needs more kinetic energy (energy of molecular motion)... this is proportional to temperature. So, another way to say it is that water needs higher temperatures or more heat to evaporate. Where does it get this heat from? The water comes from within our bodies and is released to the surface of our bodies by the sweat glands. If it is going to evaporate, it needs more heat-- so it gets this heat from our bodies and takes it with it when it evaporates. That leaves less heat behind, and we feel cooler.
The underlying layer of hypodermis with its adipose tissue (as well as the dense nature of the dermis) creates an insulating layer against heat loss. For example, infrared rays may not make it through the adipose, but instead may be absorbed by the adipose tissue.
Also, when we begin to get cold, we excite our arrector pili muscles and then our skin hairs stand up on end. This increases the thickness of the insulating layer of air around the outside of our bodies, and helps to slow down conduction and convection heat loss.
Finally, we can slow down sweat gland secretion, to decrease the loss of heat through evaporation. But since we can also speed it up, this fits even better in the "temperature regulation" section.
This is a bit different from mere insulation. In order to regulate temperature you have to be able to both increase it and decrease it. Imagine walking outside on a hot summer's day... it is hotter outside than your body temperature. How come you don't overheat right away? How come you can stay outside for hours (if it is not really, really hot) without passing out? Then you go inside. You enter an air conditioned building, which is set really low. It seems cold at first, but you "get used to it." How come? What did your body do?
Your body handles these situations by having your skin respond to them. Your skin can help you to lose or retain heat, depending on the situation. Kind of like having a thermostat in your home that always keeps it at 72 ºF, whether it has to run the heat or the air-conditioning to do it. Such a fancy thermostat in our homes would be rather expensive. Our skin does it automatically, using only cellular energy.
The main way that our body handles temperature changes is by altering blood vessel diameter in your skin. If you are too hot, then your blood is also too hot. You can cool down the entire body by cooling down your blood. How? Dilate your dermal blood vessels. Dilation increases their diameter, and, since we have a constant volume of blood that is running through our blood vessels under pressure, if this blood has a lower pressure way to go, like through large vessels, it will (*click here for another explanation of this pressure notion). So blood will flow into your dermal blood vessels. This causes it to run very near the surface of your body, and allows for heat to escape from your blood quite easily into the air.
Note that in the above situation, more blood enters the skin-- this causes your skin to look flushed. That is a common symptom of being hot.
If you are too cold, you can warm up your body by constricting your dermal blood vessels. This prevents too much blood from entering your skin, keeping it well inside your body and away from any possible heat loss to the environment. Your book describes this mechanism in Figure 6.12.
The other means by which we can regulate our temperature are: regulating sweat gland secretion, contracting our arrector pili muscles, and shivering. Shivering, however, occurs in muscles deeper than the skin... it's just that the heat that makes is trapped inside our bodies by our insulation. Shivering works because active muscles release heat, so it forces muscles to become active.
Keep in mind that this type of temperature regulation shows very fine tuning characteristic of all homeostatic mechanisms.
*note: Think about it this
way, if you want to water your huge garden with a hose, and you have
different hoses available (one with a wide diameter and one with a
diameter), which hose would you use? You would probably use
diameter hose, because with the same amount of water pressure from your
you could water your garden much faster. Water runs through a
faster than it does through a narrow hose. Same with
will go faster and more easily into larger diameter blood
blood is constantly moving through our bodies' blood vessels, more will
through the dilated vessels over the course of minutes or hours than
through constricted vessels
Link no longer available
Different Parts of the Skin
This layer is seen on the surface of the skin. It is made up of cells called keratinocytes, which are stacked on top of each other, forming different sub-layers. The keratinocytes develop at the bottom and rise to the top, where they are shed from the surface as dead cells. So this layer is constantly renewing itself, the live cells changing into dead, hard, flattened cells. Melanocytes and Langerhans cells are other important cells found in the epidermis which have special functions
The dermis consists mostly of connective tissue and is much thicker than the epidermis. It is responsible for the skin's pliability and mechanical resistance and is also involved in the regulation of the body temperature. The dermis supplies the avascular epidermis with nutrients by means of its vascular network. It contains sense organs for touch, pressure, pain and temperature (Meissner´s corpuscles, Pacinian corpuscles, free nerve endings), as well as blood vessels, nerve fibres, sebaceous and sweat glands and hair follicles.
subcutaneous layer below the dermis consists of
connective tissue and much fat. It acts as a protective
cushion and helps
to insulate the body by monitoring heat gain and heat loss. Not all
consider this layer a part of the skin, but it definitely has a strong
the way the skin looks
Layers of the Skin
By Heather Brannon, MD, About.com
Updated: April 9, 2007
About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD
The skin is an ever-changing organ that contains many specialized cells and structures. The skin functions as a protective barrier that interfaces with a sometimes-hostile environment. It is also very involved in maintaining the proper temperature for the body to function well. It gathers sensory information from the environment, and plays an active role in the immune system protecting us from disease. Understanding how the skin can function in these many ways starts with understanding the structure of the 3 layers of skin - the epidermis, dermis, and subcutaneous tissue.
The epidermis is the outer layer of skin. The thickness of the epidermis varies in different types of skin. It is the thinnest on the eyelids at .05 mm and the thickest on the palms and soles at 1.5 mm.
The epidermis contains 5 layers. From bottom to top the layers are named:
The bottom layer, the stratum basale, has cells that are shaped like columns. In this layer the cells divide and push already formed cells into higher layers. As the cells move into the higher layers, they flatten and eventually die.
The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks.
There are three types of specialized cells in the epidermis.
The dermis also varies in thickness depending on the location of the skin. It is .3 mm on the eyelid and 3.0 mm on the back. The dermis is composed of three types of tissue that are present throughout - not in layers. The types of tissue are:
Layers of the Dermis
The two layers of the dermis are the papillary and reticular layers.
Specialized Dermal Cells
The dermis contains many specialized cells and structures.
The subcutaneous tissue is a layer of fat and connective tissue that houses larger blood vessels and nerves. This layer is important is the regulation of temperature of the skin itself and the body. The size of this layer varies throughout the body and from person to person.
The skin is a complicated structure with many functions. If any of the structures in the skin are not working properly, a rash or abnormal sensation is the result. The whole specialty of dermatology is devoted to understanding the skin, what can go wrong, and what to do if something does go wrong.
Functions of the Skin
The skin has very important vital functions for keeping the physiological and biochemical conditions of the body in its optimum state. The most important functions of the skin are:
Regulates body temperature.
Prevents loss of essential body fluids, and penetration of toxic substances.
Protection of the body from harmful effects of the sun and radiation.
Excretes toxic substances with sweat.
Immunological function mediated by Langerhans cells.
Sensory organ for touch, heat, cold, socio-sexual and emotional sensations.
Vitamin D synthesis from its precursors under the effect of sunlight and introversion of steroids.
The epidermis is the outer most layer of skin that acts as a barrier preventing toxic chemical and other materials from penetrating deeper into the skin. This is relatively porous and undergoes changes in thickness in response to different factors such as trauma or pressure.
The layers of the epidermis differentiate and gradually develop to a more rigid structure, which provides a barrier to excessive loss of body fluids and the penetration of noxious substances. The basal layer is the precursor of the different cells of the epidermis which divide, pushed further upwards, loosing much of their metabolic function and enzymatic activity. The spinous layer is characterized by growth of keratin fibrils where these are present also in the cells of the basal layer.
Epidermal cells as they are pushed up away from the basal layer, begin to dehydrate and become filled with cross-linked keratin, which gives the cells a granular appearance. Lamellar bodies containing structured lipids play an important role in skin protection. The intercellular lipids, the corneocytes, amino acids, and other salts from sweat, sebaceous secretions, degradation products from corneal proteins besides lipids and others all have an important barrier effect preventing loss of water and keep the skin pH in its optimum condition (5.5).
The stratum corneum provides most of the barrier function.
The skin acts as a two-way barrier to prevent the inward or outward passage of water and electrolytes. The epidermis largely represents the barrier; whereas once the epidermis is removed the residual dermis is almost completely permeable.
There are two possible routes for the passage of drugs through the epidermis, through the transcellular, which is probably the major pathway for polar substances, and through the intercellular.
The penetration of substances through the skin surface depends upon different factors:
Age - penetration is more in newborn and children than in adults.
Skin condition - penetration is more on injured or abraded skin surfaces. Chemicals may cause injury and increase penetration.
Hydration of the skin - penetration is more in hydrated skin than dry skin. Hydration increases the permeability of the stratum corneum. Water is an effective penetration enhancer.
Fat content of the epidermis has no much effect on penetration.
Type of vehicles: vehicles may increase penetration and absorption of the drug from the skin surface. This depends on the type of vehicle and the condition of the skin. Certain vehicles that may cause injury to the skin even minimal injury predispose to more penetration of the drugs or other materials applied topically to the skin surface.
Hyperemia - vasodilatation of the blood vessels in response to different stimuli either local or generalized increases the penetration.
Physiological and pharmacological factors
The penetration in vivo of topically applied substances can be assessed by physiological or pharmacological signs or analyzed by chemical or histological techniques:
Vasoconstriction has been utilized for corticosteroids.
Vasodilatation for nicotinates.
Whealing for histamines.
Sweating for pilocarpine.
Anesthesia for local anesthetics.
Lipoid soluble substances facilitate penetration of substances applied to the skin surface. Steroid hormones and vitamin D, salts such as chloride and sulfate can penetrate the skin surface. Gases and volatile substances can pass through the skin.
Anatomy of the Skin
The skin is a vital organ that covers the entire outside of the body, forming a protective barrier against pathogens and injuries from the environment. The skin is the body's largest organ; covering the entire outside of the body, it is about 2 mm thick and weighs approximately six pounds. It shields the body against heat, light, injury, and infection. The skin also helps regulate body temperature, gathers sensory information from the environment, stores water, fat, and vitamin D, and plays a role in the immune system protecting us from disease.
The color, thickness and texture of skin vary over the body. There are two general types of skin; thin and hairy, which is more prevalent on the body, and thick and hairless, which is found on parts of the body that are used heavily and endure a large amount of friction, like the palms of the hands or the soles of the feet.
Basically, the skin is comprised of two layers that cover a third fatty layer. These three layers differ in function, thickness, and strength. The outer layer is called the epidermis; it is a tough protective layer that contains the melanin-producing melanocytes. The second layer (located under the epidermis) is called the dermis; it contains nerve endings, sweat glands, oil glands, and hair follicles. Under these two skin layers is a fatty layer of subcutaneous tissue, known as the subcutis or hypodermis. The skin contains many specialized cells and structures:
Basket Cells – Basket cells surround the base of hair follicles and can sense pressure. They are evaluated when assessing overall nerve health and condition.
Blood Vessels – Blood vessels carry nutrients and oxygen-rich blood to the cells that make up the layers of skin and carry away waste products.
Hair Erector Muscle (Arrector Pili Muscle) – The arrector pili muscle is a tiny muscle connected to each hair follicle and the skin. When it contracts it causes the hair to stand erect, and a "goosebump" forms on the skin.
Hair Follicle – The hair follicle is a tube-shaped sheath that surrounds the part of the hair that is under the skin and nourishes the hair. It is located in the epidermis and the dermis.
Hair Shaft - The hair shaft is the part of the hair that is above the skin.
Langerhans Cells - These cells attach themselves to antigens that invade damaged skin and alert the immune system to their presence.
Melanocyte – A melanocyte is a cell that produces melanin, and is located in the basal layer of the epidermis.
Merkel Cells – Merkel cells are tactile cells of neuroectodermal origin located in the basal layer of the epidermis.
Pacinian Corpuscle – A pacinian corpuscle is a nerve receptor located in the subcutaneous fatty tissue that responds to pressure and vibration.
Sebaceous Gland - Sebaceous glands are small, sack-shaped glands which release an oily substance onto the hair follicle that coats and protects the hair shaft from becoming brittle. These glands are located in the dermis.
Sensory Nerves – The epidermis is innervated with sensory nerves. These nerves sense and transmit heat, pain, and other noxious sensations. When they are not functioning properly sensations such as numbness, pins-and-needles, pain, tingling, or burning may be felt. When evaluating a skin biopsy, total number, contiguity, diameter, branching, swelling, and overall health of the sensory nerves are assessed.
Stratum Corneum – The stratum corneum is outermost layer of the epidermis, and is comprised of dead skin cells. It protects the living cells beneath it by providing a tough barrier between the environment and the lower layers of the skin. The stratum corneum is useful for diagnosis because in some conditions it will become thinner than normal.
Sweat Gland (Sudoriferous Gland) - These glands are located in the epidermis and produce moisture (sweat) that is secreted through tiny ducts onto the surface of the skin (stratum corneum). When sweat evaporates, skin temperature is lowered.
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Layers of the Skin
The epidermis is the outermost layer of the skin, and protects the body from the environment. The thickness of the epidermis varies in different types of skin; it is only .05 mm thick on the eyelids, and is 1.5 mm thick on the palms and the soles of the feet. The epidermis contains the melanocytes (the cells in which melanoma develops), the Langerhans' cells (involved in the immune system in the skin), Merkel cells and sensory nerves. The epidermis layer itself is made up of five sublayers that work together to continually rebuild the surface of the skin:
The basal layer is the innermost layer of the epidermis, and contains small round cells called basal cells. The basal cells continually divide, and new cells constantly push older ones up toward the surface of the skin, where they are eventually shed. The basal cell layer is also known as the stratum germinativum due to the fact that it is constantly germinating (producing) new cells.
The basal cell layer contains cells called melanocytes. Melanocytes produce the skin coloring or pigment known as melanin, which gives skin its tan or brown color and helps protect the deeper layers of the skin from the harmful effects of the sun. Sun exposure causes melanocytes to increase production of melanin in order to protect the skin from damaging ultraviolet rays, producing a suntan. Patches of melanin in the skin cause birthmarks, freckles and age spots. Melanoma develops when melanocytes undergo malignant transformation.
Merkel cells, which are tactile cells of neuroectodermal origin, are also located in the basal layer of the epidermis.
The squamous cell layer is located above the basal layer, and is also known as the stratum spinosum or “spiny layer” due to the fact that the cells are held together with spiny projections. Within this layer are the basal cells that have been pushed upward, however these maturing cells are now called squamous cells, or keratinocytes. Keratinocytes produce keratin, a tough, protective protein that makes up the majority of the structure of the skin, hair, and nails.
The squamous cell layer is the thickest layer of the epidermis, and is involved in the transfer of certain substances in and out of the body. The squamous cell layer also contains cells called Langerhans cells. These cells attach themselves to antigens that invade damaged skin and alert the immune system to their presence.
The keratinocytes from the squamous layer are then pushed up through two thin epidermal layers called the stratum granulosum and the stratum lucidum. As these cells move further towards the surface of the skin, they get bigger and flatter and adhere together, and then eventually become dehydrated and die. This process results in the cells fusing together into layers of tough, durable material, which continue to migrate up to the surface of the skin.
The stratum corneum is the outermost layer of the epidermis, and is made up of 10 to 30 thin layers of continually shedding, dead keratinocytes. The stratum corneum is also known as the “horny layer,” because its cells are toughened like an animal’s horn. As the outermost cells age and wear down, they are replaced by new layers of strong, long-wearing cells. The stratum corneum is sloughed off continually as new cells take its place, but this shedding process slows down with age. Complete cell turnover occurs every 28 to 30 days in young adults, while the same process takes 45 to 50 days in elderly adults.
The dermis is located beneath the epidermis and is the thickest of the three layers of the skin (1.5 to 4 mm thick), making up approximately 90 percent of the thickness of the skin. The main functions of the dermis are to regulate temperature and to supply the epidermis with nutrient-saturated blood. Much of the body’s water supply is stored within the dermis. This layer contains most of the skins’ specialized cells and structures, including:
Blood Vessels - The blood vessels supply nutrients and oxygen to the skin and take away cell waste and cell products. The blood vessels also transport the vitamin D produced in the skin back to the rest of the body.
Lymph Vessels - The lymph vessels bathe the tissues of the skin with lymph, a milky substance that contains the infection-fighting cells of the immune system. These cells work to destroy any infection or invading organisms as the lymph circulates to the lymph nodes.
Hair Follicles - The hair follicle is a tube-shaped sheath that surrounds the part of the hair that is under the skin and nourishes the hair.
Sweat Glands – The average person has about 3 million sweat glands. Sweat glands are classified according to two types:
Apocrine glands are specialized sweat glands that can be found only in the armpits and pubic region. These glands secrete a milky sweat that encourages the growth of the bacteria responsible for body odor.
Eccrine glands are the true sweat glands. Found over the entire body, these glands regulate body temperature by bringing water via the pores to the surface of the skin, where it evaporates and reduces skin temperature. These glands can produce up to two liters of sweat an hour, however, they secrete mostly water, which doesn’t encourage the growth of odor-producing bacteria.
Sebaceous glands - Sebaceous, or oil, glands, are attached to hair follicles and can be found everywhere on the body except for the palms of the hands and the soles of the feet. These glands secrete oil that helps keep the skin smooth and supple. The oil also helps keep skin waterproof and protects against an overgrowth of bacteria and fungi on the skin.
Nerve Endings – The dermis layer also contains pain and touch receptors that transmit sensations of pain, itch, pressure and information regarding temperature to the brain for interpretation. If necessary, shivering (involuntary contraction and relaxation of muscles) is triggered, generating body heat.
Collagen and Elastin - The dermis is held together by a protein called collagen, made by fibroblasts. Fibroblasts are skin cells that give the skin its strength and resilience. Collagen is a tough, insoluble protein found throughout the body in the connective tissues that hold muscles and organs in place. In the skin, collagen supports the epidermis, lending it its durability. Elastin, a similar protein, is the substance that allows the skin to spring back into place when stretched and keeps the skin flexible.
The dermis layer is made up of two sublayers:
The upper, papillary layer, contains a thin arrangement of collagen fibers. The papillary layer supplies nutrients to select layers of the epidermis and regulates temperature. Both of these functions are accomplished with a thin, extensive vascular system that operates similarly to other vascular systems in the body. Constriction and expansion control the amount of blood that flows through the skin and dictate whether body heat is dispelled when the skin is hot or conserved when it is cold.
The lower, reticular layer, is thicker and made of thick collagen fibers that are arranged in parallel to the surface of the skin. The reticular layer is denser than the papillary dermis, and it strengthens the skin, providing structure and elasticity. It also supports other components of the skin, such as hair follicles, sweat glands, and sebaceous glands.
The subcutis is the innermost layer of the skin, and consists of a network of fat and collagen cells. The subcutis is also known as the hypodermis or subcutaneous layer, and functions as both an insulator, conserving the body's heat, and as a shock-absorber, protecting the inner organs. It also stores fat as an energy reserve for the body. The blood vessels, nerves, lymph vessels, and hair follicles also cross through this layer. The thickness of the subcutis layer varies throughout the body and from person to person.llink no longer available
What are the Main Functions of the Skin?
Skin Diseases, Condition, Symptoms and Procedures
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