Human Anatomy and Physiology PDF

HUMAN ANATOMY AND PHYSIOLOGY: A Reading Material IV. INTEGUMENTARY SYSTEM Body membranes cover surfaces, line body cavities, and form protective (and often lubricating) sheets around organs. They fall into two major groups: (1) epithelial membranes, which include the cutaneous, mucou...

HUMAN ANATOMY AND PHYSIOLOGY: A Reading Material IV. INTEGUMENTARY SYSTEM Body membranes cover surfaces, line body cavities, and form protective (and often lubricating) sheets around organs. They fall into two major groups: (1) epithelial membranes, which include the cutaneous, mucous, and serous membranes; and (2) connective tissue membranes, represented by synovial membranes. The cutaneous membrane, generally called the skin or integumentary system, is the outer covering that we all rely on for protection. A. Classification of Body Membranes 1. Epithelial Membranes The epithelial membranes, also called covering and lining membranes, include the cutaneous membrane (skin), the mucous membranes, and the serous membranes. However, calling these membranes “epithelial” is misleading because it is not the whole story. Although they all do contain an epithelial layer, it is always combined with an underlying layer of connective tissue. Hence these membranes are actually simple organs. Because we will discuss the skin in detail shortly, we will list it here solely as a subcategory of the epithelial membranes. Cutaneous Membrane The cutaneous membrane is composed of two layers, the superficial epidermis and the underlying dermis. The epidermis is composed of stratified squamous epithelium, whereas the dermis is mostly dense (fibrous) connective tissue. Unlike other epithelial membranes, the cutaneous membrane is exposed to air and is a dry membrane. Mucous Membranes A mucous membrane (mucosa) is composed of epithelium (the type varies with the site) resting on a loose connective tissue membrane called a lamina propria. This membrane type lines all body cavities that open to the exterior, such as those of the hollow organs of the respiratory, digestive, urinary, and reproductive tracts. Notice that the term mucosa refers only to the location of the epithelial membranes, not their cellular makeup, which varies. However, most mucosae contain either stratified squamous epithelium (as in the mouth and esophagus) or simple columnar epithelium (as in the rest of the digestive tract). In all cases, they are moist membranes that are almost continuously bathed in secretions or, in the case of the urinary mucosae, urine. The epithelium of mucosae is often adapted for absorption or secretion. Although many mucosae secrete mucus, not all do. The mucosae of the respiratory and digestive tracts secrete large amounts of protective, lubricating mucus; that of the urinary tract does not. Serous Membranes A serous membrane, or serosa, is composed of a layer of simple squamous epithelium resting on a thin layer of areolar connective tissue. In contrast to mucous membranes, which line open body cavities, serous membranes line body cavities that are closed to the exterior (except for the dorsal body cavity and joint cavities). Serous membranes occur in pairs. The parietal layer lines a specific portion of the wall of the ventral body cavity. It folds in on itself to form the visceral layer, which covers the outside of the organ(s) in that cavity. You can visualize the relationship between the serosal layers by pushing your fist into a limp balloon only partially filled with air. The part of the balloon that clings to your fist can be compared to the visceral serosa clinging to the organ’s external surface. The outer wall of the balloon represents the parietal serosa that lines the walls of the cavity and that, unlike the balloon, is never exposed but is always fused to the cavity wall. In the body, the serous layers are separated not by air but by a scanty amount of thin, clear fluid, called serous fluid, which is secreted by both membranes. Although there is a potential space between the two membranes, they tend to lie very close to each other. The lubricating serous fluid allows the organs to slide easily across the cavity walls and one another without friction as they carry out their routine functions. This is extremely important when mobile organs such as the pumping heart and expanding lungs are involved. The specific names of the serous membranes depend on their locations. The serosa lining the abdominal cavity and covering its organs is the peritoneum. In the thorax, serous membranes isolate the lungs and heart from one another. The membranes surrounding the lungs are the pleurae; those around the heart are the pericardia. 2. Connective Tissue Membranes Synovial membranes are composed of loose areolar connective tissue and contain no epithelial cells at all. These membranes line the fibrous capsules surrounding joints, where they provide a smooth surface and secrete a lubricating fluid. They also line small sacs of connective tissue called bursae and the tube-like tendon sheaths. Both of these structures cushion organs moving against each other during muscle activity—such as the movement of a tendon across a bone’s surface. B. The Integumentary System (SKIN) Skin is absolutely essential because it keeps water and other precious molecules in the body. It also keeps excess water (and other things) out. This is why you can swim for hours without becoming waterlogged. Structurally, the skin is a marvel. It is pliable yet tough, which allows it to take constant punishment from external agents. Without our skin, we would quickly fall prey to bacteria and perish from water and heat loss. The skin and its appendages (sweat and oil glands, hair, and nails) are collectively called the integumentary system. 1. Functions of the Integumentary System The integumentary system, also called the integument, which simply means “covering,” performs a variety of functions; most, but not all, of which are protective. It insulates and cushions the deeper body organs and protects the entire body from mechanical damage (bumps and cuts), chemical damage (such as from acids and bases), thermal damage (heat and cold), ultraviolet (UV) radiation (in sunlight), and microbes. The uppermost layer of the skin is hardened, to help prevent water loss from the body surface. The skin’s rich capillary network and sweat glands (both controlled by the nervous system) play an important role in regulating heat loss from the body surface. The skin acts as a mini-excretory system; urea, salts, and water are lost when we sweat. The skin is also a chemical plant; it manufactures several proteins important to immunity and synthesizes vitamin D. (Modified cholesterol molecules in the skin are converted to vitamin D by sunlight.) The skin also produces acidic secretions, called the acid mantle, that protect against bacterial invasion.  2. Structure of the Skin The skin is composed of two kinds of tissue. The outer epidermis is made up of stratified squamous epithelium that is capable of becoming hard and tough. The underlying dermis is made up mostly of dense connective tissue. The epidermis and dermis are firmly connected and the dermis is fairly tear resistant. However, a burn or friction (such as the rubbing of a poorly fitting shoe) may cause them to separate, allowing interstitial fluid to accumulate in the cavity between the layers, which results in a blister. Deep to the dermis is the subcutaneous tissue, or hypodermis, which essentially is adipose (fat) tissue. It is not considered part of the skin, but it does anchor the skin to underlying organs and provides a site for nutrient storage. Subcutaneous tissue serves as a shock absorber and insulates the deeper tissues from extreme temperature changes occurring outside the body. It is also responsible for the curves that are more a part of a woman’s anatomy than a man’s. Epidermis Most cells of the epidermis are keratinocytes (keratin cells), which produce keratin, the fibrous protein that makes the epidermis a tough protective layer in a process called keratinization. These keratinocytes are connected by desmosomes throughout the epidermis. Like all other epithelial tissues, the epidermis is avascular; that is, it has no blood supply of its own. This explains why a man can shave daily and not bleed even though he cuts off many cell layers each time he shaves. The epidermis is composed of up to five layers, or strata. From the inside out these are the stratum basale, spinosum, granulosum, lucidum, and corneum. The deepest cell layer of the epidermis, the stratum basale, lies closest to the dermis and is connected to it along a wavy border that resembles corrugated cardboard. This basal layer contains the most adequately nourished of the epidermal cells because nutrients diffusing from the dermis reach them first. Stem cells in this layer are constantly dividing, and millions of new cells are produced daily; hence its alternate name, stratum germinativum (“germinating layer”). Of the new cells produced, some become epidermal cells, and others maintain the population of stem cells by continuing to divide. The daughter cells destined to become epidermal cells are pushed upward, away from the source of nutrition, to become part of the epidermal layers closer to the skin surface. As they move away from the dermis and become part of the more superficial layers, the stratum spinosum and then the stratum granulosum, they become flatter and increasingly keratinized. As these cells leave the stratum granulosum, they die, forming the clear stratum lucidum. This latter epidermal layer is not present in all skin regions. It occurs only where the skin is hairless and extra thick, that is, on the palms of the hands and soles of the feet. The combination of accumulating keratin inside them, secreting a water-repellent glycolipid into the extracellular space, and their increasing distance from the blood supply (in the dermis) effectively dooms the stratum lucidum cells and the more superficial epidermal cells because they are unable to get adequate nutrients and oxygen. The outermost layer, the stratum corneum, is 20 to 30 cell layers thick, but it accounts for about three-quarters of the epidermal thickness. The shingle-like dead cell remnants, completely filled with keratin, are referred to as cornified, or horny, cells. The common saying “Beauty is only skin deep” is especially interesting in light of the fact that nearly everything we see when we look at someone is dead! The abundance of the tough keratin protein in the stratum corneum allows that layer to provide a durable “overcoat” for the body, which protects deeper cells from the hostile external environment and from water loss, and helps the body resist biological, chemical, and physical assaults. The stratum corneum rubs and flakes off slowly and steadily as the dandruff familiar to everyone. The average person sheds about 18 kg (40 lb) of these flakes in a lifetime, providing a food source for the dust mites that inhabit our homes and bed linens. This layer is replaced by cells produced by the division of the deeper stratum basale cells. Indeed, we have a totally “new” epidermis every 25 to 45 days. Melanin, a pigment that ranges in color from yellow to brown to black, is produced by special spider-shaped cells called melanocytes, found chiefly in the stratum basale. Freckles and moles are seen where melanin is concentrated in one spot. Scattered in the epidermis are epidermal dendritic cells, which are important “sentries” that alert and activate immune system cells to a threat such as bacterial or viral invasion. Seen here and there at the epidermal-dermal junction are Merkel cells, which are associated with sensory nerve endings and serve as touch receptors called Merkel discs. Homeostatic Imbalance 1: Despite melanin’s protective effects, excessive exposure to UV light (via sunlight or tanning beds) eventually damages the skin, leading to a leathery appearance. It also depresses the immune system. This may help to explain why people infected with the human herpesvirus 1, which causes cold sores, are more likely to have an eruption after sunbathing. Overexposure to the sun can also alter the DNA of skin cells, leading to skin cancer. People with very dark skin seldom have skin cancer, attesting to melanin’s amazing effectiveness as a natural sunscreen. Dermis The dermis is your “hide.” It is a strong, stretchy envelope that helps to bind the body together. When you purchase leather goods (bags, belts, shoes, and the like), you are buying the treated dermis of animals. The connective tissue making up the dermis consists of two major regions—the papillary and the reticular areas, which are composed of areolar and dense irregular connective tissue, respectively. Like the epidermis, the dermis varies in thickness. For example, it is particularly thick on the palms of the hands and soles of the feet but is quite thin on the eyelids. The papillary layer is the superficial dermal region. It is uneven and has peg-like projections from its superior surface, called dermal papillae, which indent the epidermis above. Many of the dermal papillae contain capillary loops, which furnish nutrients to the epidermis. Others house pain receptors (free nerve endings) and touch receptors. On the palms of the hands and soles of the feet, the papillae are arranged in definite patterns that form looped and whorled ridges on the epidermal surface that increase friction and enhance the gripping ability of the fingers and feet. Papillary patterns are genetically determined. The ridges of the fingertips are well provided with sweat pores and leave unique, identifying films of sweat called fingerprints on almost anything they touch. The reticular layer is the deepest skin layer. It contains dense irregular connective tissue, as well as blood vessels, sweat and oil glands, and deep pressure receptors called lamellar corpuscles. Other cutaneous sensory receptors, which are actually part of the nervous system, are also located in the skin. These tiny sensors, which include touch, pressure, temperature, and pain receptors, provide us with a great deal of information about our external environment. They alert us to sources of heat or cold, or the tickle of a bug exploring our skin. In addition to detecting stimuli from our environment, phagocytes found here (and, in fact, throughout the dermis) act to prevent microbes that have managed to get through the epidermis from penetrating any deeper into the body. Both collagen and elastic fibers are found throughout the dermis. Collagen fibers are responsible for the toughness of the dermis; they also attract and bind water and thus help to keep the skin hydrated. Elastic fibers give the skin its elasticity when we are young. As we age, the number of collagen and elastic fibers decreases, and the subcutaneous tissue loses fat. As a result, the skin loses its elasticity and begins to sag and wrinkle. The dermis is abundantly supplied with blood vessels that play a role in maintaining body temperature homeostasis. When body temperature is high, the capillaries of the dermis become engorged, or swollen, with heated blood, and the skin becomes reddened and warm. This allows body heat to radiate from the skin surface. If the environment is cool and body heat must be conserved, blood bypasses the dermis capillaries temporarily, allowing internal body temperature to remain high. Homeostatic Imbalance 2: Any restriction of the normal blood supply to the skin results in cell death and, if severe or prolonged enough, skin ulcers. Decubitus ulcers (bedsores) occur in bedridden patients who are not turned regularly or who are dragged or pulled across the bed repeatedly. The weight of the body puts pressure on the skin, especially over bony projections. Because this pressure restricts the blood supply, the skin becomes pale or blanched at pressure points. At first, the skin reddens when pressure is released, but if the situation is not corrected, the cells begin to die, and small cracks or breaks in the skin appear at compressed sites. Permanent damage to the superficial blood vessels and tissue eventually results in degeneration and ulceration of the skin. 3. Skin Color Three pigments contribute to skin color: melanin, carotene, and hemoglobin. The amount and kind (yellow, reddish brown, or black) of melanin in the epidermis. Skin exposure to sunlight stimulates melanocytes to produce more melanin pigment, resulting in tanning of the skin. As the melanocytes produce melanin, it accumulates in their cytoplasm in membrane-bound granules called melanosomes. These granules then move to the ends of the melanocytes’ spidery arms, where they are taken up by nearby keratinocytes. Inside the keratinocytes, the melanin forms a pigment umbrella over the superficial, or “sunny,” side of their nuclei and shields their genetic material (DNA) from the damaging effects of UV radiation in sunlight. People who produce a lot of melanin have brown-toned skin, whereas people with less melanin are light skinned. The amount of carotene deposited in the stratum corneum and subcutaneous tissue. Carotene is an orange-yellow pigment plentiful in carrots and other orange, deep yellow, or leafy green vegetables. In people who eat large amounts of carotene- rich foods, the skin tends to take on a yellow-orange cast. The amount of oxygen-rich hemoglobin (pigment in red blood cells) in the dermal blood vessels. In light-skinned people, the crimson color of oxygen-rich hemoglobin in the dermal blood supply flushes through the transparent cell layers above and gives the skin a rosy glow. Emotions also influence skin color, and many alterations in skin color signal certain disease states: Redness, or erythema. Reddened skin may indicate embarrassment (blushing), fever, hypertension, inflammation, or allergy. Pallor, or blanching. Under certain types of emotional stress (fear, anger, and others), some people become pale. Pale skin may also signify anemia, low blood pressure, or impaired blood flow into the area. Jaundice or a yellow cast. An abnormal yellow skin tone usually signifies a liver disorder in which excess bile pigments accumulate in the blood, circulate throughout the body, and become deposited in body tissues. Bruises. The black-and-blue marks of bruising reveal sites where blood has escaped from the circulation and has clotted in the tissue spaces. Such clotted blood masses are called hematomas. An unusual tendency to bruise may signify a deficiency of vitamin C in the diet or hemophilia (bleeder’s disease). Homeostatic Imbalance 3: When hemoglobin is poorly oxygenated, both the blood and the skin of light-skinned people appear blue, a condition called cyanosis. Cyanosis is common during heart failure and severe breathing disorders. In dark-skinned people, the skin does not appear cyanotic in the same situations because of the masking effects of melanin, but cyanosis is apparent in their mucous membranes and nail beds. 4. Appendages of the Skin The skin appendages include cutaneous glands, hair and hair follicles, and nails. Each of these appendages arises from the epidermis and plays a unique role in maintaining body homeostasis. Cutaneous Glands The cutaneous glands are all exocrine glands that release their secretions to the skin surface via ducts. They fall into two groups: sebaceous glands and sweat glands. As these glands are formed by the cells of the stratum basale, they push into the deeper skin regions and ultimately reside almost entirely in the dermis. Sebaceous (Oil) Glands The sebaceous glands, or oil glands, are found all over the skin, except on the palms of the hands and the soles of the feet. Their ducts usually empty into a hair follicle, but some open directly onto the skin surface. The product of the sebaceous glands, sebum (grease), is a mixture of oily substances and fragmented cells. Sebum is a lubricant that keeps the skin soft and moist and prevents the hair from becoming brittle. Sebum also contains chemicals that kill bacteria, so it is important in preventing bacterial infection of the skin. The sebaceous glands become very active when androgens (male sex hormones) are produced in increased amounts (in both sexes) during adolescence. Thus, the skin tends to become oilier during this period of life. Homeostatic Imbalance 4: When sebaceous gland ducts are blocked by sebum, acne appears on the skin surface. Acne is an active infection of the sebaceous glands. If the accumulated material oxidizes and dries, it darkens, forming a blackhead. If the material does not dry or darken, a whitehead forms. Acne can be mild or extremely severe, leading to permanent scarring. Seborrhea (“fast-flowing sebum”), known as “cradle cap” in infants, is caused by over-activity of the sebaceous glands. It begins on the scalp as pink, raised lesions that gradually form a yellow-to-brown crust that sloughs off oily scales and dandruff. Careful washing to remove the excessive oil often helps cradle cap in a newborn baby. Sweat Glands Sweat glands, also called sudoriferous glands, are widely distributed in the skin. Their number is staggering—more than 2.5 million per person. There are two types of sweat glands, eccrine and apocrine. The eccrine glands are far more numerous and are found all over the body. They produce sweat, a clear secretion that is primarily water plus some salts (sodium chloride), vitamin C, traces of metabolic wastes (ammonia, urea, uric acid), and lactic acid (a chemical that accumulates during vigorous muscle activity). Sweat is acidic (pH from 4 to 6), a characteristic that inhibits the growth of certain bacteria, which are always present on the skin surface. Typically, sweat reaches the skin surface via a duct that opens externally as a funnel-shaped sweat pore. Notice, however, that the facial “pores” commonly referred to when we talk about our complexion are the external outlets of hair follicles where sebaceous ducts empty, not these sweat pores. The eccrine sweat glands are an important and highly efficient part of the body’s heat- regulating equipment. They are supplied with nerve endings that cause them to secrete sweat when the external temperature or body temperature is too high. When sweat evaporates off the skin surface, it carries large amounts of body heat with it. On a hot day, it is possible to lose up to 7 liters of body water in this way. The heat-regulating functions of the body are important—if internal temperature changes more than a few degrees from the normal 37°C (98.6°F), life-threatening changes occur in the body Apocrine glands are largely confined to the axillary (armpit) and genital areas of the body. They are usually larger than eccrine glands, and their ducts empty into hair follicles. Their secretion contains fatty acids and proteins, as well as all the substances present in eccrine sweat; consequently, it may have a milky or yellowish color. The secretion is odorless, but when bacteria that live on the skin use its proteins and fats as a source of nutrients for their growth, it can take on a musky, sometimes unpleasant odor. Apocrine glands begin to function during puberty under the influence of androgens. Although they produce their secretions almost continuously, apocrine glands play a minimal role in thermoregulation. Their precise function is not yet known, but they are activated by nerve fibers during pain and stress and during sexual arousal. Hair and Hair Follicles Hair is an important part of our body image. Consider, for example, the spiky hair style of punk rockers and the flowing locks of high- fashion models. Millions of hairs, produced by hair follicles, are found all over the body surface except on the palms of the hands, soles of the feet, nipples, and lips. Humans are born with as many hair follicles as they will ever have, and hairs are among the fastest growing tissues in the body. Hair serves a few minor protective functions, such as guarding the head against bumps, shielding the eyes (via eyelashes), and helping to keep foreign particles out of the respiratory tract (via nose hairs). Hair may also help to attract sexual partners, as evidenced by its place in our body image. Hormones account for the development of hairy regions—the scalp and, in the adult, the pubic and axillary areas. Despite these functions, however, our body hair has lost much of its usefulness. Hair served early humans (and still serves hairy animals) by providing insulation in cold weather, but now we have other means of keeping warm. Hairs A hair is a flexible epithelial structure. The part of the hair enclosed in the hair follicle is called the root, and the part projecting from the surface of the scalp or skin is called the shaft. A hair forms by division of the well-nourished stratum basale epithelial cells in the matrix (growth zone) of the hair bulb at the deep end of the follicle. As the daughter cells are pushed farther away from the growing region, they become keratinized and die. Thus the bulk of the hair shaft, like the bulk of the epidermis, is dead material and almost entirely protein. Each hair is made up of a central core called the medulla, consisting of large cells and air spaces, surrounded by a bulky cortex layer composed of several layers of flattened cells. The cortex is, in turn, enclosed by an outermost cuticle formed by a single layer of cells that overlap one another like shingles on a roof. This arrangement of the cuticle cells helps to keep the hairs apart and keeps them from matting. The cuticle is the most heavily keratinized region; it provides strength and helps keep the inner hair layers tightly compacted. Because it is most subject to abrasion, the cuticle tends to wear away at the tip of the shaft, allowing the keratin fibrils in the inner hair regions to frizz out, a phenomenon called “split ends.” Hair pigment is made by melanocytes in the hair bulb, and varying amounts of different types of melanin (yellow, rust, brown, and black) combine to produce all varieties of hair color from pale blond to red to pitch black. Hairs come in a variety of sizes and shapes. They are short and stiff in the eyebrows, long and flexible on the head, and usually nearly invisible almost everywhere else. When the hair shaft is oval, hair is smooth, silky, and wavy. When the shaft is flat and ribbon-like, the hair is curly or kinky. If it is perfectly round, the hair is straight and tends to be coarse. Hair Follicles Hair follicles are actually compound structures. The inner epithelial root sheath is composed of epithelial tissue and forms the hair. The outer fibrous sheath is actually dermal connective tissue. This dermal region supplies blood vessels to the epidermal portion and reinforces it. Its nipple-like hair papilla provides the blood supply to the matrix in the hair bulb (the deepest part of the follicle). If you look carefully at the structure of the hair follicle, you will notice that it is slightly slanted. Small bands of smooth muscle cells— arrector pili — connect each side of the hair follicle to the dermal tissue. When these muscles contract (as when we are cold or frightened), the hair is pulled upright, dimpling the skin surface with “goose bumps.” Nails A nail is a scale-like modification of the epidermis that corresponds to the hoof or claw of other animals. Each nail has a free edge, a body (visible attached portion), and a root (embedded in the skin). The borders of the nail are overlapped by folds of skin called nail folds. The edge of the thick proximal nail fold is commonly called the cuticle. The stratum basale of the epidermis extends beneath the nail as the nail bed. Its thickened proximal area, called the nail matrix, is responsible for nail growth. As the matrix produces nail cells, they become heavily keratinized and die. Thus, nails, like hairs, are mostly nonliving material. Nails are transparent and nearly colorless, but they look pink because of the rich blood supply in the underlying dermis. The exception to this is the region over the thickened nail matrix that appears as a white crescent and is called the lunula/lunule. As noted earlier, when the supply of oxygen in the blood is low, the nail beds take on a cyanotic (blue) cast. C. Homeostatic Imbalances of the Skin Loss of homeostasis in body cells and organs reveals itself on the skin, sometimes in startling ways. The skin can develop more than 1,000 different ailments. The most common skin disorders are infections with pathogens such as bacteria, viruses, or fungi. Allergies, which are caused by abnormally strong immune responses, are also commonly seen in the skin. Less common, but far more damaging to body well-being, are burns and skin cancers. Infections and Allergies Infections and allergies cause the following commonly occurring skin disorders: Athlete’s foot. An itchy, red, peeling condition of the skin between the toes, resulting from an infection with the fungus Tinea pedis. Boils (furuncles) and carbuncles. Boils are caused by inflammation of hair follicles and surrounding tissues, commonly on the dorsal neck. Carbuncles are clusters of boils often caused by the bacterium Staphylococcus aureus. Cold sores (fever blisters). Small fluid-filled blisters that itch and sting, caused by human herpesvirus 1 infection. The virus localizes in a cutaneous nerve, where it remains dormant until activated by emotional upset, fever, or UV radiation. Cold sores usually occur around the lips and in the oral mucosa of the mouth and nose. Contact dermatitis. Itching, redness, and swelling of the skin, progressing to blistering. It is caused by exposure of the skin to chemicals (such as those in poison ivy) that provoke allergic responses in sensitive individuals. Impetigo. Pink, fluid-filled, raised lesions (commonly around the mouth and nose) that develop a yellow crust and eventually rupture. Caused by highly contagious staphylococcus or streptococcus infections, impetigo is common in elementary school–aged children. Psoriasis. Characterized by reddened epidermal lesions covered with dry, silvery scales that itch, burn, crack, and sometimes bleed. A chronic condition, psoriasis is believed to be an autoimmune disorder in which the immune system attacks a person’s own tissues, leading to the rapid overproduction of skin cells. Attacks are often triggered by trauma, infection, hormonal changes, or stress. When severe, psoriasis may be disfiguring. Burns There are few threats to life more serious than burns. A burn is tissue damage and cell death caused by intense heat, electricity, UV radiation (sunburn), or certain chemicals (such as acids), which denature proteins and cause cell death in the affected areas. When the skin is burned and its cells are destroyed, two life-threatening problems result. First, without an intact boundary, the body loses its precious supply of fluids containing proteins and electrolytes as these seep from the burned surfaces. Dehydration and electrolyte imbalance follow and can lead to a shutdown of the kidneys and circulatory shock (inadequate circulation of blood caused by low blood volume). To save the patient, lost fluids must be replaced immediately. The volume of fluid lost can be estimated indirectly by determining how much of the body surface is burned (extent of burns), using the rule of nines. This method divides the body into 11 areas, each accounting for 9 percent of the total body surface area, plus an additional area surrounding the genitals (the perineum) representing 1 percent of body surface area. Later, infection becomes the most important threat and is the leading cause of death in burn victims. Burned skin is sterile for about 24 hours. But after that, pathogens easily invade areas where the skin has been destroyed and multiply rapidly in the nutrient-rich environment of dead tissues. To make matters worse, the patient’s immune system becomes depressed within one to two days after severe burn injury. Burns are classified according to their severity (depth) as first-degree (superficial), second- degree (superficial partial-thickness burns), third degree (full-thickness burns), or fourth- degree (full-thickness burns with deep-tissue involvement). In first-degree burns, only the superficial epidermis is damaged. The area becomes red and swollen. Except for temporary discomfort, first-degree burns are not usually serious and generally heal in two to three days. Sunburn without blistering is a first-degree burn. Second- degree burns involve injury to the epidermis and the superficial part of the dermis. The skin is red, painful, and blistered. Because sufficient numbers of epithelial cells are still present, regrowth (regeneration) of the epithelium can occur. Ordinarily, no permanent scars result if care is taken to prevent infection. Third-degree burns destroy both the epidermis and the dermis and often extend into the subcutaneous tissue, reflecting their categorization as full thickness burns. Blisters are usually present, and the burned area appears blanched (gray-white) or blackened. Because the nerve endings in the area are destroyed, the burned area is not painful. In third degree burns, regeneration is not possible, and skin grafting must be done to cover the underlying exposed tissues. Fourth-degree burns are also full-thickness burns, but they extend into deeper tissues such as bone, muscle, or tendons. These burns appear dry and leathery, and they require surgery and grafting to cover exposed tissue. In severe cases, amputation may be required to save the patient’s life. In general, burns are considered critical if any of the following conditions exists: Over 30 percent of the body has second-degree burns. Over 10 percent of the body has third- or fourth-degree burns. There are third- or fourth-degree burns of the face, hands, feet, or genitals. Burns affect the airway. Circumferential (around the body or limb) burns have occurred. Facial burns are particularly dangerous because of the possibility of burns in respiratory passageways, which can swell and cause suffocation. Joint injuries are troublesome because the scar tissue that eventually forms can severely limit joint mobility. Circumferential burns can restrict movement, and depending on location, can interfere with normal breathing. Skin Cancer Numerous types of neoplasms (tumors) arise in the skin. Most skin neoplasms are benign and do not spread (metastasize) to other body areas. For example, warts are caused by human papillomaviruses but are benign and do not spread. However, some skin neoplasms are malignant, or cancerous, and they tend to invade other body areas. Skin cancer is the single most common type of cancer in humans. The most important risk factor is overexposure to UV radiation in sunlight and tanning beds. Frequent irritation of the skin by infections, chemicals, or physical trauma also seems to be a predisposing factor. Let’s look in a bit more detail at the three most common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and malignant melanoma. Basal Cell Carcinoma Basal cell carcinoma is the least malignant and most common skin cancer. Cells of the stratum basale, altered so that they cannot form keratin, no longer honor the boundary between epidermis and dermis. They proliferate, invading the dermis and subcutaneous tissue. The cancerous lesions occur most often on sun-exposed areas of the face and often appear as shiny, dome-shaped nodules that later develop a central ulcer with a “pearly” beaded edge. Basal cell carcinoma is relatively slow-growing, and metastasis seldom occurs before the lesion is noticed. When the lesion is removed surgically, 99 percent of cases are completely cured. Squamous Cell Carcinoma Squamous cell carcinoma arises from the cells of the stratum spinosum. The lesions appear as scaly, reddened papules (small, rounded swellings) that gradually form shallow ulcers with firm, raised borders. This variety of skin cancer appears most often on the scalp, ears, back of the hands, and lower lip, but can appear anywhere on the skin. It grows rapidly and metastasizes to adjacent lymph nodes if not removed. This epidermal cancer is also believed to be induced by UV exposure. If it is caught early and removed surgically or by radiation therapy, the chance of complete cure is good. Malignant Melanoma Malignant melanoma is a cancer of melanocytes. It accounts for only about 5 percent of skin cancers, but it is often deadly. Melanoma can begin wherever there is pigment; most such cancers appear spontaneously, but some develop from pigmented moles. It arises from accumulated DNA damage in a skin cell and usually appears as a spreading brown to black patch that metastasizes rapidly to surrounding lymph and blood vessels. The chance for survival is about 50 percent, and early detection helps. The American Cancer Society suggests that people who sunbathe frequently or attend tanning parlors examine their skin periodically for new moles or pigmented spots and apply the ABCDE rule for recognizing melanoma: (A) Asymmetry. Any two sides of the pigmented spot or mole do not match. (B) Border irregularity. The borders of the lesion are not smooth but exhibit indentations. (C) Color. The pigmented spot contains areas of different colors (black, brown, tan, and sometimes blue or red). (D) Diameter. The lesion is larger than 6 millimeters (mm) in diameter (the size of a pencil eraser). (E) Evolution. One or more of these characteristics (ABCD) is evolving, or changing. The usual therapy for malignant melanoma is wide surgical excision along with immunotherapy, a treatment that involves the patient’s immune system. Large lesions may also require radiation or chemotherapy after surgical removal. D. Developmental Aspects of Skin and Body Membranes During the fifth and sixth months of development, a fetus is covered with a downy type of hair called lanugo. By the time the infant is born, it has usually shed this hairy cloak, and instead its skin is covered with an oily coating called the vernix caseosa. This white, cheesy-looking substance, produced by the sebaceous glands, protects the baby’s skin while it is floating in its water-filled sac inside the mother. The newborn’s skin is very thin, and blood vessels are easily seen through it. Commonly, there are accumulations in the sebaceous glands, which appear as small white spots called milia on the baby’s nose and forehead. These normally disappear by the third week after birth. As the baby grows, its skin becomes thicker, and more subcutaneous fat is deposited. During adolescence, the skin and hair become oilier as sebaceous glands are activated, and acne may appear. Acne usually subsides in early adulthood, and the skin reaches its optimal appearance when we are in our twenties and thirties. Then visible changes in the skin begin to appear, as it is continually assaulted by abrasion, chemicals, wind, sun, and other irritants and as its pores become clogged with air pollutants and bacteria. As a result, pimples, scales, and various kinds of dermatitis, or skin inflammation, become more common. As we get even older, the amount of subcutaneous tissue decreases, leading to sensitivity to cold. The skin also becomes drier (because of decreased oil production), and as a result, it may become itchy and bothersome. Thinning of the skin, another result of the aging process, makes it more susceptible to bruising and other types of injuries. The decreasing elasticity of the skin, along with the loss of subcutaneous fat, allows bags to form under our eyes, and our jowls begin to sag. Smoking and sunlight speed up this loss of elasticity, so three of the best things you can do for your skin are to avoid smoking, shield your skin from the sun, and do not go to tanning beds. In doing so, you will also be decreasing the chance of skin cancer. There is no way to avoid aging skin, but good nutrition, plenty of fluids, and cleanliness help delay the process. Hair loses its luster as we age, and by age 50 the number of hair follicles has dropped by one-third and continues to decline, resulting in hair thinning and some degree of baldness, or alopecia, in most people. Many men become bald as they age, a phenomenon called male pattern baldness. A bald man is not really hairless—he does have hairs in the bald area. But, because those hair follicles have begun to degenerate, the vellus hairs are colorless and very tiny (and may not even emerge from the follicle). Another phenomenon of aging is graying hair. Like balding, this is usually genetically controlled by a “delayed-action” gene. Once the gene takes effect, the amount of melanin deposited in the hair decreases or becomes entirely absent, which results in gray-to- white hair. Homeostatic Imbalance 5: Certain events can cause hair to gray or fall out prematurely. For example, many people have claimed that they turned gray nearly overnight because of some emotional crisis in their life. In addition, we know that anxiety, protein-deficient diets, therapy with certain chemicals (chemotherapy), radiation, excessive vitamin A, and certain fungal diseases (ringworm) can cause both graying and hair loss. However, when the cause of these conditions is not genetic, hair loss is usually not permanent. Reference: Marieb, Elaine Nicpon; Keller, Suzanne M. (2018). Essentials of Human Anatomy and Physiology 12th Edition Pearson Education Inc., 300 Hudson Street, NY NY 10013