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Chapter 6: e Integumenta System
Chapter Introduction
Our skin ties us to the world. rough senso reception, our skin lets us know about dangers. Our skin is our most fundamental protection against pathogens and environmental conditions that could harm us. Our skin is a mechanism of self-expression, it is the prima thing that other humans see about us. Unfo unately, in many countries and regions where racism is prevalent and built into our institutions, our skin may even determine much about our experience in the world. Our skin, along with its accesso structures (such as hair), comprises one of the body’s most essential and dynamic systems: the integumenta system. e integumenta system refers to the skin, hair, nails, and exocrine glands, and it is responsible for much more than simply lending to your outward appearance. e skin makes up about 16 percent of your body weight, making the skin and accesso structures the human body’s largest organ system. e skin protects the inside of your body, including all your other organs; it is of vital impo ance to your health. is chapter will introduce the structure and functions of the integumenta system, as well as some of the diseases, disorders, and injuries that can a ect this system.
6.1Layers of the Skin
Although you may not typically think of the skin as an organ, it meets the de nition of one, in that it is made of tissues that work together as a single structure to pe orm unique and critical functions. e skin and its accesso structures make up the integumenta system, which provides the body with overall protection. e skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue (Figure 6.1). e deeper layer of skin is well vascularized (has numerous blood vessels). It is also highly inne ated; that is to say, it has numerous ne e bers ensuring communication to and from the brain.
Figure 6.1Layers of Skin
e skin is composed of many layers. e epidermis, which is the major outermost layer of the skin, is composed of epithelial tissue. e dermis, which lies deep to the epidermis, is composed of connective tissue. Beneath the dermis lies the hypodermis, which has a di erent composition of connective tissues than the dermis. Fascia is the term for the connective tissue that anchors the skin to the muscle beneath.
Student Study Tip
Remember that epi - means “on” and hypo - means “under.” erefore, the three skin layers are literally the dermis, the layer on the dermis, and the layer under the dermis.
You are ve familiar with a macro view of the skin. You see skin eve day; you may even be looking at skin right now. As we dive into the micro view of the skin, in our broadest view we can divide the skin into three layers: the epidermis, dermis, and hypodermis. Below these layers, a sheet of connective tissue, called fascia , ties the skin to the underlying muscles.
6.1a e Epidermis
e epidermis is composed of strati ed squamous epithelium. It is actually a specialized strati ed squamous epithelium called keratinized strati ed squamous. e lining of structures that are open to the outside is also made of strati ed squamous epithelium, but the skin, because it is a d organ, requires special protection and so is keratinized. Like all epithelia, it is avascular (does not contain blood vessels). is epithelium is made of four, or sometimes ve, layers of epithelial cells, depending on body location. From deep to supe icial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum (Figure 6.2).
Figure 6.2Layers of the Epidermis
e epidermis has four layers in most areas of the body: stratum basale, stratum spinosum, stratum granulosum, and stratum corneum.
Student Study Tip
ick skin and strati ed squamous epithelium operate under the same principle: More friction means more layers needed for protection!
e skin on the palms of the hands and the soles of the feet has a fth layer, called the stratum lucidum , located between the stratum corneum and the stratum granulosum (Figure 6.4). Skin in these two locations, with its fth layer of cells, is referred to as “thick” skin, and all other skin is referred to as “thin” skin. at being said, the relative thickness of the skin (for example, the thick skin of your back versus the ve thin skin of your eyelids) is contributed to much more by the dermis than the epithelium. in skin and thick skin also di er in their glands, as we will discuss later.
Figure 6.4 in Skin Versus ick Skin
(A and B) in skin exists all over the body and has four distinguishable cell layers in the epidermis. (C and D) ick skin, found only on the palms of the hands and soles of the feet, has an additional layer, the stratum lucidum.
Stratum Basale
e stratum basale is the deepest epidermal layer and attaches the epidermis to its basement membrane, which ties the epidermis to the dermis below. e stratum basale is a single layer of cells, most of which are stem cells that are a precursor of the keratinocytes of the epidermis. All the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce replacement cells. As new cells are produced, the existing cells of the epidermis are pushed supe icially away from the stratum basale. Just like all stem cells, the cells of the stratum basale exhibit decreased function as we age, leading to a thinner epidermis and slower wound healing.
Two other cell types are found dispersed among the stem cells in the stratum basale. e rst is a Merkel cell, which functions as a senso receptor and is connected to senso ne es that send signals about touch to the brain. ese cells are especially abundant on the su aces of the hands and feet. e second is a melanocyte, a cell that produces the pigment melanin. Melanin, a protein that can be made in two forms, functions to protect cells from ultraviolet (UV) radiation damage.
Melanin occurs in two prima forms. Whereas eumelanin exists as black and brown, pheomelanin provides a more reddish hue. While all humans have a relatively similar concentration of melanocytes, some individuals make less
Stratum Lucidum
e stratum lucidum, present in the skin of the palms of the hands and soles of the feet, is a smooth, seemingly translucent layer of the epidermis located just supe icial to the stratum granulosum and deep to the stratum corneum. e keratinocytes that compose the stratum lucidum are dead and attened (see Figure 6.4). In addition to the melanin, keratohyalin, and keratin of the deeper keratinocytes, these cells are densely packed with eleidin, a clear protein derived from keratohyalin, which gives these cells their transparent appearance. e high lipid content of this protein provides a barrier to water to these regions.
Stratum Corneum
e stratum corneum is the most supe icial layer of eve epidermis and is the layer of the skin that is exposed to the outside environment. e stratum corneum is usually composed of about 15 to 30 layers of dead skin cells. is d , dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the underlying strata. e entire layer is replaced during a period of about four weeks.
Considering Friction
Wring your hands, scratch an itch, walk across a sandy beach. ink about how much friction your skin is exposed to daily. e skin has many structural adaptations to meet this functional need. We have discussed already that the keratinocytes of the epidermis are connected by desmosomes to keep them from separating. e stratum corneum, a layer of dead cells, functions by shedding cells in response to contact and friction, preventing the underlying living cells from being damaged. Another structural adaptation to better endure friction is the dermal papillae. ese ngerlike projections of the dermis interdigitate with downward projections of the epidermis (Figure 6.6A). To imagine how this structure assists with the function of resisting friction, look at your hands. Imagine one hand is the epidermis, the other is the dermis. Put your hands together as if they are high- ving each other. Now, slide one up or down. See how easy it is for two smooth su aces to slide past each other? Now, interlace your ngers like the photo in Figure 6.6B. T sliding your hands now. e interdigitation, or interlocking, of the dermal papillae with the ridges of the stratum basale fo i es the connection between the epidermis and the dermis, so that these two layers do not detach when exposed to friction. ese bumps and ridges form during the fetal period and are noticeable externally; you may recognize them as ngerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes.
Figure 6.6Dermal Papillae
(A) e bumps and ridges of the dermis and epidermis interlock to form a combined structure that rarely pulls apa when exposed to friction or shear force. (B) e interdigitation can be likened to the interlocking of ngers.
6.1b e Dermis
As with all epithelia, the epidermis is avascular and therefore gets its nutrients from an underlying connective tissue layer. is layer, the dermis, contains blood and lymph vessels, ne es, and other structures, such as the deeper po ions of hair follicles and sweat glands. e dermis itself has two layers, but both are connective tissue proper and composed of an interconnected mesh of elastin and collagenous bers, produced by broblasts (Figure 6.7). e thickness of the dermis varies depending on location, with the back of the torso having the most substantial dermal layer and the eyelids having the thinnest. roughout the dermis, collagen bers provide structure and tensile strength, with strands of collagen extending throughout the two layers of the dermis and even into the hypodermis. Elastin bers provide some elasticity to the skin, enabling the skin to stretch and recoil during movement, such as the deformations in the skin when a person’s face breaks into a smile. In addition, collagen binds water to keep the skin hydrated. As with all connective tissue, scattered broblasts are responsible for the production of these bers. As we age, the broblasts decrease in number and production and the dermis decreases in its density of both collagen and elastin. e loss of collagen makes the skin thinner and slower to heal, and the decline in elastin leads to a decreased capacity to recoil after stretching, which results in the formation of wrinkles.
Figure 6.7 e Dermis e two layers of the dermis—papilla and reticular—are di erentiated by the thickness and weave of the collagen bers.
e body mass index (BMI) is often used as a measure of fat, although this measure is in fact derived from a mathematical formula that compares body weight (mass) to height. BMI numbers are broadly divided into groups of “underweight,” “normal,” “overweight,” and “obese.” is measure, as with a lot of medical reference values, was developed and validated on studies of primarily white individuals who identify as men and therefore may not be accurate when applied to most humans. In fact, only about 25 percent of Americans fall into the BMI range of “normal,” not only due to the obesity epidemic in the United States but also because many body types, including individuals who are extremely muscular, will fall outside this range. Epidemiologists do note that BMI can be a broad indicator of mo ality risk, with more signi cant risk of disease among those in the underweight and obese categories, but a person’s BMI value tells us little about their individual health.
6.2Accesso Structures of the Skin
Accesso structures of the skin include hair, nails, sweat glands, and sebaceous glands. ese structures span the depth of the epidermis and can extend down through the dermis into the hypodermis. Glands are made of living epithelial cells that are actively producing secretions; in contrast, hair and nails are composed of dead, keratin- lled cells similar to the stratum corneum.
6.2aHair
You are already ve familiar with the pa of hair that you can see external to your skin. is externally visible po ion, a long lament of keratin, is the hair shaft. is shaft of hair is grown within an epidermal structure called the hair follicle. e hair follicle is a structure that develops from the epidermis, but as it grows larger it pushes down into the dermis (see the “Anatomy of Hair” feature). It is still an almost entirely epidermal structure, even though a considerable po ion of it is found in the dermis. As the hair follicle develops (see “Anatomy of Hair” pa (A)), the epidermis invaginates, pushing down into the dermis. It takes with it stem cells of the stratum basale and melanocytes. As these developing epidermal hair cells push in on the basement membrane, a tiny cluster of dermal cells begins to di erentiate beneath the developing follicle. is cluster of dermal cells becomes the dermal papilla (see “Anatomy of Hair”). Being connective tissue, the papilla contains blood vessels and ne es, and nutrients that di use from the vascularized papilla feed the entire follicle. e deepest po ion of the hair follicle is the hair bulb. Epithelial cells within the hair bulb form the hair matrix and function ve similarly to cells of the stratum basale; they divide rather continuously, producing keratinocytes that ll themselves with keratin until they die, leaving membranous sacs of keratin protein behind. ese cells become the hair root—the po ion of the follicle between the bulb and the su ace of the skin—and the hair shaft (the hair exposed at the skin’s su ace).
If we examine the hair itself, the central core of the hair—the medulla—is a fragile inner core made up of some living cells and the spaces between them. icker hair strands have a medulla, while thinner hair strands do not. Whether (and where) you grow hair with a medulla or not is determined by your genetics. If present, the medulla is surrounded by the co ex, a layer of compressed, keratinized cells that is covered by an outer layer of ve hard keratinized cells known as the cuticle. ese layers are depicted in longitudinal and cross sections of the hair (see the “Anatomy of Hair” feature, pa (C)). Whether your hair is curly or straight is determined by the structure of the co ex and, to the extent that it is present, the medulla. e shape and structure of these layers are, in turn, determined by the shape of the hair follicle. As new cells are deposited within the hair bulb, the hair shaft is pushed through the follicle toward the su ace. Keratinization is completed as the cells are pushed to the skin su ace to form the externally visible shaft of hair. e external hair is completely dead and composed entirely of keratin. For this reason, our hair itself does not have sensation (you know this because it doesn’t hu you to cut or shave your hair). ere are ne es within the hair papilla, however, so pulling on hair yields sensation.
Anatomy of…
Hair
Each hair follicle is associated with an oil gland known as a sebaceous gland. We will explore the structure and function of these glands in more detail in the next section.
Now that we have learned all about hair’s structure, you may be left with some questions about hair function. Why do we have it at all? Why do some people have more of it than others? Why does the hair on our heads grow so long while the hair on our bodies stops at a ce ain length? Depending on your age, you might be facing a ve impo ant question: why does hair go gray? Hair se es a variety of functions, including physical protection, senso input, thermoregulation, and UV protection. For example, hair around the eyes (eyelashes) and the hair of the eyebrows prevents sweat and other pa icles from dripping into and bothering the eyes. Hair within the nose defends the body by trapping and excluding dust pa icles that may contain allergens and microbes. Hair also has a senso function due to senso inne ation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin su ace. is feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin su ace. e distribution of hair on the human body changed dramatically during evolution as early humans evolved into a bipedal, walking species, di erentiating themselves from their tree-dwelling ancestors. In the trees, primates live in the shade. eir bodies are cooler and sun exposure happens all over the body. Early humans, walking across the open savannah, were exposed to the sun primarily on their heads, as the sun was directly above their upright bodies. ey also were likely much hotter than their tree-dwelling ancestors. erefore, our modern body hair distribution evolved, with longer, denser hair on top of our heads to protect us from the sun and sho er, sparser hair over our bodies. Hair on the human body provides both UV protection and thermoregulation.
Body hair plays a more substantial role in thermoregulation in animals such as dogs and cats, most of which have a much heavier coat than most humans. Each hair root is connected to a small muscle called the arrector pili that
e structure of each follicle and hair determines its unique prope ies. Hair grows from the bulb within the follicle and emerges out of the skin. e hair that we can see has an outer layer of cuticle, which may be 2 – 10 layers thick. If we use heat or products, these things a ect the cuticle of the hair. Deep to the cuticle, the co ex is built of accumulated keratin and pigment. e relative amount and type of pigment determines your hair color. ese pigments are produced within the bulb. Two melanin pigments, eumelanin (brown/black) and pheomelanin (orange/red) are the prima determinants of hair color. erefore, if you choose to dye your hair you need the dye to access the co ex, below the cuticle. is is why bleaching and dyeing can be damaging to hair, because these processes a ect both the cuticle and the co ex.
Some hairs have a medulla, and some hairs do not. e medulla may continue all the way through the length of the hair, or it may be interrupted. e presence or absence of the medulla determines a lot about your experience with your hair. Some ethnicities, Native Americans and Asians in pa icular, consistently have medulla in all their hairs. Coarse hair, such as beard hair, has a medulla that is doubly thick. Some hair, such as ve ne, straight hair, lacks a medulla altogether.
e last biological and anatomical characteristic of hair is its curliness. Two factors that contribute to hair curl have been identi ed. e rst is anatomical. Within the hair follicle, the growing hair travels through a tunnel to reach the skin su ace. e shape of this tunnel may be straight or curly, and this plays a large role in determining the curl of the emerging hair. Another factor is that the keratin proteins themselves are di erent in curly hair. Within these hairs the keratin molecules form covalent bonds, holding them in a pattern that curls the hair. Chemical straightening products break these bonds, permanently straightening the hair. Many of these anatomical and biological traits are rooted in the genome and, to some degree, ethnicity.
As we age, all stem cells slow their mitotic capability, including those in the hair bulb. Over time, more and more hair will be lost. In addition, eventually the melanocytes in the hair bulb will die, and they will stop lending melanin (and color) to the hair shaft; if the hair continues to grow, it will be gray.
6.2bNails
e nail bed is the living component of nails, a specialized structure of the epidermis, the cells of which produce the nail body. e nail body, the hard, bladelike structure you might paint, functions to protect the tips of our ngers and toes, as they are the fa hest extremities and the pa s of the body that experience the most frequent mechanical stress (Figure 6.9). e nail body also may assist in picking up small objects and, of course, scratching an itch. e nail body is composed of densely packed dead keratinocytes. e nail body forms at the nail root (a protected region at the proximal side of the nail bed), which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously. All around the perimeter of the nails are aps of skin called nail folds that help to anchor the nail body. At the meeting point between the proximal nail fold and the nail body is a narrow and thin strip of just epidermis, called the nail cuticle (also called the eponychium ). e majority of the nail bed is rich in blood vessels, except at the base, where a thick layer of epithelium overlies the nail bed in a crescent- shaped region called the lunula.
Figure 6.9Nails
Nails are accesso structures of the epithelium made of dead cells packed with keratin.
6.2cSweat Glands
We have talked a lot about human evolutiona adaptations to moving around on two feet. Humans are capable not only of walking bipedally, but also of running. In fact, humans are one of only two mammal species well adapted to long-distance endurance running (horses are the other). Most mammal species do not sweat, but humans, horses, and apes use sweat as a means of thermoregulation both when the body temperature rises due to external temperatures and when it rises due to exercise such as running. Humans sweat all over their bodies, and the vast majority of sweat glands exist for this function of thermoregulation. ere is a di erent type of sweat gland present in the axilla and inguinal regions (armpits and groin) that functions a bit di erently. ese glands are di erent in structure, as explored in the “Anatomy of Sweat” feature, but also produce a di erent kind of sweat that may pe orm another function altogether. e theo is still under debate, but some scientists hypothesize that the sweat from the glands of the axilla and inguinal region contain chemicals called pheromones, a type of chemical signal that organisms can use to communicate with each other. Eccrine sweat glands are the type of sweat gland found all over the body; they produce sweat for thermoregulation. While these glands are found all over the skin’s su ace, they are especially abundant on the palms of the hands, the soles of the feet, and the forehead. ey are coiled glands lying deep in the dermis, with the duct rising to a pore on the skin su ace, where the sweat is released (see the “Anatomy of Sweat” feature). is type of sweat, released by exocytosis, making it merocrine-type secretion, is hypoosmotic compared to interstitial uid. It is composed mostly of water, with some salt, antibodies, traces of metabolic waste, and dermcidin, an antimicrobial chemical. We tend to think of sweat as salty, but it is actually much lower in minerals than our other body uids. When we release sweat onto the su ace of the skin, the temperature of the body causes most of the liquid of sweat to evaporate, which causes a cooling e ect. e water turns to vapor, leaving behind the small amounts of salts and other compounds on the su ace. e skin su ace therefore becomes saltier during the process of sweating. e dermcidin and antibodies left behind help control the growth of bacteria on the skin su ace.
Student Study Tip Apocrine glands develop with Age, After pube y! Eccrine glands are Ever-present at Eve stage of life!
Apocrine sweat glands are usually associated with hair follicles and are found in densely hai areas, such as armpits and genital regions. Apocrine sweat glands are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the hypodermis, with the duct normally emptying into the hair follicle (see the
When hormone levels are changing or pa icularly high, especially in pube y, an overproduction and accumulation of sebum, along with keratin or dead cells, can block hair follicles. e bacteria ( Propionibacterium and Staphylococcus species) that live around the follicle, feeding o sebum and apocrine sweat, will multiply and may trigger an immune reaction and in ammation (Figure 6.11). If neutrophils are involved, a white pus may be present (neutrophil reactions are discussed in more detail in Chapter 21 ), and the pus may turn black over time and with exposure to air.
Figure 6.11Acne Acne is the name for the disorder produced by an immune reaction that can occur when the hair follicle becomes in amed. e bacteria that feed o sebum can overgrow, triggering in ammation.
As we age, the skin’s accesso structures also have lowered activity, generating thinner hair and nails and reduced amounts of sebum and sweat. Reduced sweating ability can cause some older adults to be intolerant to extreme heat. Other cells in the skin, such as melanocytes and dendritic cells, also become less active, leading to a paler skin tone and lowered immunity.
6.3Functions of the Integumenta System
e skin—the largest organ in the body—and its accesso structures are vital to the maintenance of homeostasis. e skin not only se es as a barrier, allowing the world within to maintain homeostatic conditions even when environmental conditions change, but the skin also pe orms a vital role in connecting other body systems to the outside world so that they can pe orm their own homeostatic functions.
6.3aProtection
e skin’s structure allows it to withstand a variety of environmental conditions, so it can protect the more vulnerable pa s of the body from wind, water, and UV sunlight. Due to the presence of layers of keratin and glycolipids in the stratum corneum, the skin also acts as a protective barrier against water loss. It also is the rst line of defense against abrasive activity (friction, shear forces) and protects us from contact with grit, microbes, or
harmful chemicals. Even sweat has protective functions by deterring microbes with its antibiotic ingredient, dermcidin.
6.3bSenso Function
An average mosquito weighs 5 mg (about 1/1000 the weight of a piece of paper) and yet, you can feel a mosquito land on your skin, allowing you to ick it o before it bites.
is is because the skin, and especially the hairs projecting from hair follicles in the skin, can sense changes in the environment. Ne e bers surrounding the hair bulb sense a disturbance and transmit the information to the brain and spinal cord, which can then respond by activating the skeletal muscles of your eyes to see the mosquito and the skeletal muscles of the body to act in response.
e skin acts as a sense organ because the epidermis, dermis, and hypodermis contain specialized senso ne e structures that detect touch, pressure, su ace temperature, and pain. ese receptors are more concentrated on the tips of the ngers, which are most sensitive to touch, especially the tactile corpuscles (Figure 6.12), which respond to light touch, and the lamellated corpuscles (also known as Pacinian corpuscles ), which respond to pressure and vibration. Merkel cells, seen scattered in the stratum basale, are also touch receptors. In addition to these specialized receptors, there are nociceptors, which are ne e bers speci c to communicating pain to the brain and spinal cord, and thermoreceptors, which are senso ne es adapted to detecting changes in temperature. As we learned earlier, there are also ne e bers connected to each hair follicle. is rich inne ation helps us sense our environment and react accordingly.
Figure 6.12Senso Inne ation of the Skin
e skin is one of the main structures through which the central ne ous system gains information about the environment. ermoreceptors detect changes in temperature, tactile corpuscles sense touch, nociceptors relay pain information, and deep lamellated corpuscles sense pressure and vibration.
6.3c ermoregulation
e integumenta system helps regulate body temperature through its collaboration with the brain and spinal cord. Body temperature is speci cally regulated by a division of the ne ous system called the sympathetic ne ous system ; this pa of the ne ous system is best known for its involvement in our fear or ght-or- ight responses.
In addition to its essential role in bone health, vitamin D is essential for general immunity against bacterial, viral, and fungal infections. Vitamin D has been found to be e ective in immune responses against tuberculosis and COVID-19, the disease caused by the novel coronavirus. Recent studies are also nding a link between insu cient vitamin D and cancer.
6.4Healing the Integument
6.4aInjuries
Because the skin is the pa of our bodies that meets the world most directly, it is especially vulnerable to inju . Injuries include burns and wounds, as well as scars, blisters, and calluses. ey can be caused by sharp objects, heat, or excessive pressure or friction to the skin. Skin injuries set o a healing process that occurs in several overlapping stages. e rst step to repairing damaged skin is the formation of a blood clot, which helps stop the ow of blood and scabs over with time (Figure 6.15A). Many di erent types of cells are involved in wound repair, especially if the su ace area that needs repair is extensive. Before the basal stem cells of the stratum basale can recreate the epidermis, broblasts must mobilize and divide the epidermis, broblasts must mobilize, divide, and rapidly deposit collagen to form a bridge across which the epidermis can grow (Figure 6.15B). is quickly-made collagen bridge is called granulation tissue. Blood vessels in ltrate the area, bringing nutrients and an oxygen supply to the healing tissue. Immune cells, such as macrophages, roam the area and engulf any debris from the damaged tissue, or pathogens that invaded the wound, to reduce the chance of infection. Because the granulation tissue and epithelial covering are created so quickly, this healed area may have a di erent feel or texture to the surrounding skin. e dermis may be thicker, raised, or more rigid, and the epidermis may be thinner. e intense brous nature of the new tissue does not allow for the regeneration of accesso structures, such as hair follicles, sweat glands, or sebaceous glands. e combination of these characteristics is what we know of as a scar.
Figure 6.15Wound Healing (A) e rst step in wound healing is clotting of any broken blood vessels. A local in ammato response includes chemicals that cause vasodilation and the attraction of white blood cells and broblasts. (B) Fibroblasts are recruited to the wound, and quickly create a collagen matrix to plug the wound. is collagen-rich tissue is called granulation tissue. Once granulation tissue is in place, epithelial cells can begin to
Sometimes the production of scar tissue continues after the wound is healed. e scar becomes raised compared to the tissue around it and is called a keloid. Scars can also have a sunken appearance (atrophic scars) if insu cient collagen is laid down in the healing process. Atrophic scars are more common following acne and chickenpox.
Burns
A burn results when the skin is damaged by intense heat, radiation, electricity, or chemicals. e damage results in the death of skin cells. While the skin can typically heal, the danger of burns is that a massive loss of uid and resulting dehydration, electrolyte imbalance, and infection can occur while the inside tissues of the body are exposed. Renal and circulato failure can follow, which can be fatal. Burn patients are treated with intravenous uids to o set dehydration and loss of nutrients.
Burns are sometimes measured in terms of the size of the total su ace area a ected. is is referred to as the rule of nines , which associates speci c anatomical areas with a percentage that is a factor of nine (Figure 6.16). Burns are also classi ed by the depth of damage sustained. A rst-degree burn, such as a mild sunburn, is a supe icial burn that a ects only the epidermis. Although the skin may be painful and swollen, these burns typically heal on their own within a few days. A second-degree burn goes deeper and a ects both the epidermis and a po ion of the dermis. ese burns result in swelling and a painful blistering of the skin. It is impo ant to keep the burn site sterile to prevent infection. If this is done, the burn will heal within several weeks. A third-degree burn fully extends into the epidermis and dermis, destroying the tissue and a ecting the ne e endings and senso function. ese are serious burns that may appear white, red, or black; they require medical attention and will heal slowly without it (Figure 6.17). Counterintuitively, third-degree burns are usually not as painful because the ne e endings themselves are damaged and therefore can no longer communicate pain signals to the brain.
Figure 6.16Estimating the Size of a Burn e amount of body area a ected by a burn is a critical piece of data in the calculation of treatment options.
migrate over the wound. (C) Epithelial cells fully close the wound; the epithelium and underlying dermis often have a di erent consistency than the surrounding tissue.
