A person's skin covers, on average, an area of more than 20 square feet and accounts for approximately 15 percent of body weight. Our largest organ, the skin is far more than merely a thing of beauty. Complex and constantly changing, the skin plays an active role in our general health and well-being.
As you'll find when you read this book, there are many easy things you can do to take care of your skin and keep it at its best. Despite the fact that the skin is tough and resilient, problems do occur. According to the American Academy of Dermatology (AAD), some 7 million Americans each year visit a practitioner for advice on a new skin problem. The U.S. Department of Health and Human Services reports that skin diseases are to blame for 3 million lost work days annually. In addition, nearly 7 percent of the military population is affected by skin problemsfor example, rashes and infectionsmaking skin conditions the fourth largest cause of disability in the armed forces, claims the AAD.
Before we delve into what can go wrong with the skin, however, let's take a look at what's happening when everything is going right.
The Functions of the Skin
The immediate task of the skin is to act as a barrier. The visible layer of the skin, which consists of dead skin cells, serves as the immune system's first line of defense against bacteria, parasites, and other organisms trying to enter the body. In the event an invader does make it through, the skin calls in a cavalry of immune system cells to ward off the infection. The skin also works to protect internal muscles, tissues, and organs from injury, cushioning them from bumps and bruises with its shock-absorbing layer of fat.
The skin is waterproof (otherwise you'd soak up water like a sponge by just taking a shower), but it is not merely a barrier, it is also a gatekeeper. Some substances can be absorbed through the skinfor example, medications administered in ''patch” form. Similarly, some dangerous chemicals, such as arsenic, can also gain entry to the body through the skin.
Temperature regulation is another main job of the skin. When the brain signals that the body's temperature is rising, glands within the skin secrete sweat onto the skin's surface, where it evaporates, releasing body heat (and also, conveniently some of the body's waste products). In addition, blood vessels within the skin dilate, bringing the blood closer to the surface so that heat may be dissipated.
When body temperature drops, the same blood vessels contract and restrict blood flow. Because the blood is kept from the surface of the skin, less body heat is lost. The contraction of the vessels causes the contraction of nearby erector pili muscles attached to hair follicles within the skin: Thus we have, in medical terms, cutis anseriaor goosebumps.
Of course, the skin is also the organ through which we experience the sense of touch. In fact, all of our sensory tissuesthose of the ears, the eyes, the tongue, and the noseare created from the same type of cells (called epithelial cells) that make up the skin. The network of nerves in the skin allows us to gather information about our surroundings, alerts us to dangers (such as a hot stove), and allows us to feel pleasure as well as pain.
As if that isn't enough, the skin also contributes to good health by taking part in the production of vitamin D. Synthesized in a reaction between chemicals in the skin and sunlight, vitamin D is important for normal growth and development: Its presence is necessary in order for the body to absorb calcium, a mineral crucial to bone formation and strength.
The skin is also in many ways a barometer of health. It reflects the negative effects that smoking and alcohol and drug use have on the body. Stress, as well, can take its toll on the appearance of the skin.
Many physical conditions also have symptoms that manifest themselves in changes to the skin: A symptom of chicken pox, for example, is a skin rash; liver disease sometimes appears as jaundice; and illegal drug use may trigger sallow skin or blemishes (marijuana use often causes hives and acne flare-ups). For these reasons, the skin is important to any practitioner looking for a diagnosis because it provides valuable clues to what may be going on beneath the surface.
The Anatomy of the Skin
Although it measures only 6 millimeters thick on the soles of the feet and a mere 0.5 millimeter on the eyelids, the skin is well equipped to perform its many functions. The average square inch of skin contains some 600 sweat glands, 100 fat glands, 100 oil glands, 65 hairs, 20 blood vessels, and thousands of nerve endings. These structures are contained within the three layers of the skinthe epidermis, the dermis, and the subcutaneous tissue.
The epidermis, the topmost layer of skin, is only 0.1 to 1.5 millimeters thick. It is made up of five layers: the basal cell layer, the squamous cell layer, the stratum granulosum, the stratum lucidum, and the stratum corneum. Working together, these layers continually rebuild the surface of the skin from within, maintaining the skin's strength and helping thwart wear and tear.
The process begins in the basal cell layer, the innermost layer of the epidermis. This layer houses small round cells called basal cells. These cells constantly divide, with the new cells constantly pushing older ones on a migration toward the surface of the skin. The basal cell layer is also called the stratum germinativum because it is constantly producing, or germinating, new cells.
The basal cell layer also contains melanocytes, specialized cells that produce a pigment called melanin. Melanin protects the skin against sun damage, and its rate of production determines skin colorthe more melanin produced in the skin, the darker the skin appears. Exposure to the sun causes the melanocytes to increase melanin production in an effort to shield the skin from damaging ultraviolet rays; the resulting effect is a suntan. Freckles, birthmarks, and age spots are also caused by patches of melanin within the skin.
Above the basal cell layer is the squamous cell layer, also called the stratum spinosum or “spiny layer” because the cells are held together with spiny projections. Here lie the basal cells that have been pushed upward; these maturing cells are now called squamous cells, or keratinocytes. They have begun to produce keratin, a tough, protective protein that makes up a large part of the structure of the skin, hair, and nails. (Horses' hooves, fish scales, and animal horns are also made of keratin.)
The squamous cell layer is the thickest layer of the epidermis. This is the layer of the skin that helps to move certain substances in and out of the body; it's also where blisters form when the skin is chafed. The squamous cell layer also contains cells called Langerhans cells. If the skin becomes damaged, these cells latch onto invading antigens, substances that are foreign to the body, 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 the cells migrate, they enlarge, flatten, and bond together, then eventually become dehydrated and die. The process fuses the cells into layers of tough, durable material, which continue to rise toward the skin's surface.
As the layers near the surface, they become part of the stratum corneum, the outermost, visible layer of the epidermis. The stratum corneum (or “horny layer,” because its cells are toughened like an animal's horn) is made up of 10 to 30 thin layers of these dead cells. External pressure or friction can cause thickened areas in the stratum corneum known as corns or calluses.
As the outermost cells give way to wear and tear, they are replaced from within by new layers of strong, long-wearing cells. In the average adult, it takes nearly a month for the stratum corneum to be completely replaced. The replacement process generally slows with age, though in some people it becomes abnormally accelerated, causing a flaky, scaly skin condition known as psoriasis.
The dermis, which lies just beneath the epidermis, is 1.5 to 4 millimeters thickthe thickest of the three layers of the skin. It's also home to most of the skin's structures, including sweat and oil glands (which secrete substances through openings in the skin called pores, or comedos), hair follicles, nerve endings, and blood and lymph vessels. But the main components of the dermis are collagen and elastin.
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, keeps the skin flexible. This is the substance that allows the skin to spring back into place when stretched: the scientific reason a funny face won't stay that way, no matter what your mother told you. The properties of collagen and elastin fade with age, giving rise to wrinkles and sagging skin.
In addition to collagen and elastin, you'll find water in the dermis. In fact, much of the body's water supply is stored there. When the amount of stored water is increasedfor example, when you're retaining waterthe skin becomes tight and stretched as it expands to accommodate the surplus. The dermis also contains scavenger cells from the immune system. In the event that a foreign organism makes it past the epidermis, these cells will engulf and destroy it.
Several structures can be found in the dermis. Sweat glands, numbering about 3 million in the average person, are the most numerous and are classified according to two types: the apocrine glands and the eccrine glands.
Apocrine glands are specialized sweat glands that can be found only in the armpits and pubic region. In animals, it is the apocrine glands that secrete the scents used to attract a mate; however, no one is sure of their function in humans. What we do know is that these glands secrete a milky sweat that encourages the growth of bacteria responsible for body odor. These glands are activated at puberty when stimulated by hormones.
The 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 releases heat. These glands respond to heat, exercise, and fever, and some eccrine glands, such as those on the palms, respond to emotional stress, as well. It's these glands that give you clammy hands when you're nervous.
Unlike apocrine glands, eccrine glands function from childhood, though they do increase their activity at puberty. Though these glands can produce up to two liters of sweat an hour when they're working at their full potential, they're not usually to blame for body odor. These glands secrete mostly water, which doesn't encourage the growth of odor-producing bacteria.
The dermis is also home to the sebaceous, or oil, glands, which are attached to hair follicles, cylindrical structures that house the roots of the hair. Sebaceous glands can be found everywhere on the body except for the palms of the hands and the soles of the feet. Usually called into action by hormones during puberty, 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. At times, these glands overproduce and cause acne, a condition in which pores become clogged and inflamed.
Nerve endings can also be found in the dermis. Of course, these structures are responsible for the sense of touch, relaying information to the brain for interpretation. They also signal temperature to the brain and, if necessary, trigger shivering, an involuntary contraction and relaxation of muscles. This muscle activity generates body heat.
Finally, blood and lymph vessels are found in the dermis. The blood vessels bring nutrients and oxygen to the skin and remove cell waste and cell products. The blood vessels also carry the vitamin D produced in the skin back to the rest of the body. Enlarged vessels that can be seen through the skin are known as spider veins or varicose veins. Broken blood vessels appear as bruises.
The lymph vessels bathe the tissues of the skin with lymph, a milky substance that contains infection-fighting immune system cells. The cells work to destroy any infection or invading organisms as the lymph gradually circulates back through the body's tissues to the lymph nodes.
The Subcutaneous Tissue
The subcutaneous tissue is the deepest layer of the skin. It is missing on parts of the body where the skin is especially thinthe eyelids, nipples, genitals, and shins. Subcutaneous tissue acts both as an insulator, conserving body heat, and as a shock absorber, protecting internal organs from injury. It also stores fat as an energy reserve in the event extra calories are needed to power the body. The blood vessels, nerves, lymph vessels, and hair follicles also cross through this layer.
Changes Throughout a Lifetime
As you can see, there's much more to the skin than meets the eye. But what you should also realize is that not only is the skin constantly at work, it's also constantly changing. As we grow, and grow older, our skin modifies itself to meet our needs. Let's take a look at how age affects the skin.
From the very start of life, the skin is working to protect the rest of the body from infection. A newborn's skin comes in contact with bacteria even before the child is out of the birth canal. In fact, throughout your life you will have bacteriausually harmlessfeeding, multiplying, and dying on the surface of your skin. For the most part, the stratum corneum bars entry for these bacteria, which settle into folds and creases in the skin for moisture, warmth, and protection.
At birth, the skin is thin and easily torn. A baby's skin does a poor job of protecting it against the elements. Because its blood vessels and sweat glands are not yet functioning well, the skin has no way to regulate temperature. The delicate skin is also very susceptible to damage from the sun's ultraviolet rays. Chemicals and medications are also easily absorbed through the skin. At this early age, sweat glands are barely functioning.
Many newborns have a blotchy look because their muscles and blood vessels are initially poorly developed, causing blood to pool under the skin's surfacea normal condition that soon disappears. The process of birth itself can also irritate an infant's skin, causing redness that usually fades within a few days. Some children are born with birthmarks, areas of pigmented skin. While a birthmark itself is harmless, it is a risk factor for skin cancer and should prompt precautions against exposure to ultraviolet rays.
Of course, as the infant matures, so does the skin. During childhood, the skin is often at its best. Typically, there are few signs of wear and tear, and the blotchy redness of infancy has disappeared. Though the eccrine sweat glands are now functioning normally, the apocrine glandsthose that produce body odorhave not yet reached maturity, nor have the oil-producing sebaceous glands. At this age, there are few instances of rashes and acne, and most children heal quickly with little scarring.
At adolescence, the trials typically begin. During puberty, the body begins to produce the hormones that will engineer the person's transformation into adulthood. This move toward sexual maturity, however, also means changes for the skin. The hormones being pumped into the body cause the sebaceous glands within the skin to enlarge and produce oil that brings on acne. The apocrine glands are also called into action by the hormones, creating the advent of body odor, and the eccrine glands begin to produce even more sweat.
Generally, after adolescence, changes to the skin cease, and its condition stabilizes. For women, this status quo may be thrown off by hormonal changes. Oral contraceptives, for example, which alter the body's hormonal balance, often cause changes in the skin. Pregnancy also brings with it physical and hormonal changes that may affect the skin. In about 75 percent of women, stretch marksraised, red, shiny lines, also called striaeappear on the breasts, thighs, and abdomen.
Changes in pigmentation also commonly occur during pregnancy. A woman's nipples may darken, and spotty pigmentation may appear around the eyes and forehead. This “mask of pregnancy” is called melasma. Also, a dark line, called the linea nigra, may appear between the navel and the middle of the lower abdomen during pregnancy. These changes disappear within a month or two of delivery
Spider veins and varicose veinspermanent purple tangles that lie close to the surface of the skinoften develop during pregnancy. For some women, pregnancy may mean a temporary reprieve from skin conditions such as eczema and psoriasis as hormonal activity seems to improve these diseases. However, other conditions, such as yeast infections, may flare up during pregnancy
As the years pass, the skin becomes subject to the effects of time. Though you might have no intention of slowing down wit age, your skin has a different plan. Through the years, the rate of replacement of skin cells in the epidermis gradually slows, and by the age of 65, it may take twice as long as the typical 28 days to rejuvenate the surface of the skin. The slower turnover of cells means thinner, rough-looking skin.
The melanocytes within the epidermis slow their production of pigments, making it more difficult to tan. This decrease is also responsible for graying or whitening hair. Brown or yellow spots may appear on the skin due to changes in pigmentation. These spots, called age spots, are common in older adults. While most are harmless, they may be removed for cosmetic reasons. (Any unusual spots that bleed or grow rapidlyespecially if they are irregular in shape or colorshould be examined by a practitioner for signs of skin cancer.)
Within the dermis, elastin and collagen fibers begin to break down, leaving the skin more fragile and vulnerable to sagging and wrinkles. The water content of the dermis diminishes, triggering dry skin and less flexibility. Aging makes the blood vessels more delicate, which results in more frequent bruising and slower healing after an injury. Nerves within the skin become less sensitive, diminishing the sense of touch.
Hormones can play a role during the later years, as well. For women, this occurs at menopause, the stage of life during which a woman's levels of naturally produced estrogen begin to wane. This decrease in hormones triggers hot flashes, sudden increases in body temperature, flushed skin, and profuse sweating. For men, changes in the way the hormone testosterone functions, as well as slight declines in hormone levels, can lead to drier, less flexible skin as well as hair loss.
Chronological aging is inevitable, but photoaging, which is caused by sun exposure, actually accounts for more skin problems. Taking steps to minimize sun damage throughout a lifetime can do a lot to prevent future problems. For those already seeing the effects of photoaging, there are also tips for preventing and improving the condition of your skin.