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Human Skin In human anatomy skin can be defined as the covering, or integument, of the body's surface that both provides protection and receives sensory stimuli from the external environment. The skin consists of three layers of tissue: the epidermis, an outermost layer that contains the primary protective structure, the stratum corneum; the dermis, a fibrous layer that supports and strengthens the epidermis; and the sub cutis, a subcutaneous layer of fat beneath the dermis that supplies nutrients to the other two layers and that cushions and insulates the body. (Goldsmith 67) The apparent lack of body hair immediately distinguishes human beings from all other large land mammals. Regardless of individual or racial differences, the human body seems to be more or less hairless, in the sense that the hair is so vestigial as to seem absent; yet in certain areas hair grows profusely. These may be referred to as epigamicareas, and they are concerned with social and sexual communication, either visually or by scent from glands associated with the hair follicles. (Goldsmith 68) The characteristic features of skin change from the time of birth to old age. In infants and children it is velvety, dry, soft, and largely free of wrinkles and blemishes. Children younger than two years sweat poorly and irregularly; their sebaceous glands function minimally.
At adolescence hair becomes longer, thicker, and more pigmented, particularly in the scalp, axillae, pubic eminence, and the male face. General skin pigmentation increases, localized pigmented foci appear mysteriously, and acne lesions often develop. Hair growth, sweating, and sebaceous secretion begin to blossom. (Goldsmith 69 z 0 As a person ages, anatomical and physiological alterations, as well as exposure to sunlight and wind, leave skin, particularly that not protected by clothing, dry, wrinkled, and flaccid. Human skin, more than that of any other mammal, exhibits striking topographic differences.
An example is the dissimilarity between the palms and the backs of the hands and fingers. The skin of the eyebrows is thick, coarse, and hairy; that on the eyelids is thin, smooth, and covered with almost invisible hairs. The face is seldom visibly haired on the forehead and cheekbones. (Goldsmith 69) It is completely hairless in the vermilion border of the lips, yet coarsely hairy over the chin and jaws of males. The surfaces of the forehead, cheeks, and nose are normally oily, in contrast with the relatively greaseless lower surface of the chin and jaws. The skin of the chest, pubic region, scalp, axillae, abdomen, soles of the feet, and ends of the fingers varies as much structurally and functionally as it would if the skin in these different areas belonged to different animals.
The skin achieves strength and pliability by being composed of numbers of layers oriented so that each complements the others structurally and functionally. To allow communication with the environment, countless nerves some modified as specialized receptor end organs and others more or less structureless come as close as possible to the surface layer, and nearly every skin organ is enwrapped by skeins of fine sensory nerves. The dermis makes up the bulk of the skin and provides physical protection. It is composed of an association of fibres, mainly collagen, with materials known as glycosaminoglycans, which are capable of holding a large amount of water, thus maintaining the turgidity of the skin. A network of extendable elastic fibres keeps the skin taut and restores it after it has been stretched. (Goldsmith 71) The hair follicles and skin glands are derived from the epidermis but are deeply embedded in the dermis. The dermis is richly supplied with blood vessels, although none penetrates the living epidermis.
The epidermis receives materials only by diffusion from below. The dermis also contains nerves and sense organs at various levels. Human skin is enormously well supplied with blood vessels; it is pervaded with a tangled, though apparently orderly, mass of arteries, veins, and capillaries. Such a supply of blood, far in excess of the maximum biologic needs of the skin itself, is evidence that the skin is at the service of the blood vascular system, functioning as a cooling device.
To aid in this function, sweat glands pour water upon its surface, the evaporation of which absorbs heat from the skin. If the environment is cold and body heat must be conserved, cutaneous blood vessels contract in quick, successive rhythms, allowing only a small amount of blood to flow through them. When the environment is warm, they contract at long intervals, providing a free flow of blood. During muscular exertion, when great quantities of generated heat must be dissipated, blood flow through the skin is maximal. (Goldsmith 71) In addition to its control of body temperature, skin also plays a role in the regulation of blood pressure. Much of the flow of blood can be controlled by the opening and closing of certain sphincter like vessels in the skin. These vessels allow the blood to circulate through the peripheral capillary beds or to bypass them by being shunted directly from small arteries to veins.
Human skin is permeated with an intricate mesh of lymph vessels. In the more superficial parts of the dermis, minute lymph vessels that appear to terminate in blind sacs function as affluents of a superficial lymphatic net that in turn opens into vessels that become progressively larger in the deeper portions of the dermis. The deeper, larger vessels are embedded in the loose connective tissue that surrounds the veins. The walls of lymph vessels are so flabby and collapsed that they often escape notice in specimens prepared for microscopic studies.
Their abundance, however, has been demonstrated by injecting vital dyes inside the dermis and observing the clearance of the dye. Because lymph vessels have minimal or no musculature in their walls, the circulation of lymph is sluggish and largely controlled by such extrinsic forces as pressure, skeletal muscle action, massaging, and heat. Any external pressure exerted, even from a fixed dressing, for example, interferes with its flow. Since skin plays a major role in immunologic responses of the body, its lymphatic drainage is as significant as its blood vascular system. The intact surface of the skin is pitted by the orifices of sweat glands and hair follicles the so-called pores and is furrowed by intersecting lines that delineate characteristic patterns. All individuals have roughly similar markings on any one part of the body, but the details are unique.
The lines are oriented in the general direction of elastic tension. Countless numbers of them, deep and shallow, together with the pores, give every region of the body a characteristic topography. Like the deeper furrows and ridges on the palms and soles, the skin lines are mostly established before birth. The fine details of each area of body surface are peculiar to each individual.
Fingerprints are used as a means of personal identification because they have a high relief, more evident patterning's, and can be easily obtained. Some of the lines on the surface of the skin are acquired after birth as a result of use or damage. For example, furrows on the forehead are an accentuation of preexisting congenital lines that become strongly emphasized in old age. As the skin becomes less firm with aging, it also forms wrinkles. Certain occupations leave skin marks that, depending upon duration and severity, may be transient or permanent. (Goldsmith 73) The ventral surfaces of the hands and feet are etched by distinct alternating ridges and grooves that together constitute dermatoglyphics. The ridges follow variable courses, but their arrangement in specific areas has a consistent structural plan.
Though apparently continuous, the ridges have many interruptions and irregularities, branching and varying in length. Every small area of surface has ridge details not matched anywhere in the same individual or in any other individual, even in an identical twin. This infallible signature makes dermatoglyphics the best-known physical characteristic for personal identification. The epidermis is thicker on the palms and soles than it is anywhere else and is usually thicker on dorsal than on ventral surfaces. Omitting the fine details, it is divisible everywhere into a lower layer of living cells and a superficial layer of compact dead cells. All the cells, living or dead, are attached to one another by a series of specialized surfaces called attachment plaques, or desmosomes.
Thus, instead of being completely fused, the membranes of adjacent cells make a zipper like contact, with fluid-filled spaces between the contact areas. This structural pattern ensures a concatenation of cells to one another so that they cannot be sloughed off easily; at the same time it allows nutrient fluids to seep in from the vessels in the dermis. Epidermal cells, which multiply chiefly at the base in contact with the dermis, gradually ascend to the surface, manufacturing keratin as they go. They finally die in the upper part, forming a horny layer. (Goldsmith 74) The epidermis is thickest on friction surfaces and thinnest over the eyelids, on the lower parts of the abdomen, and around the external genitalia. Unlike that of most other mammals, it has an intricately sculptured underside and does not lie flat upon the dermis.
Seen from beneath, there are straight and branching ridges and valleys, columns and pits, all finely punctuated. Because of this unevenness, it is almost impossible to state the exact thickness of epidermal tissue. Furthermore, individual differences, sex, and age have an enormous influence on the structure of the underside. Such labyrinthine patterns give human epidermis two unique advantages: it attains a more intimate connection with the subjacent dermis than if the surface were flat, and its source of dividing cells, the building blocks of the horny layer, is greatly increased. The clear stratification of the epidermis is the result of well-defined changes in its major constituent cells the keratinocytes, or corneocytesas they move peripherally from the basal layer, where they are continuously formed by mitosis, to the skin surface, where they are lost. In essence, the epidermis consists of a living malpighi an layer, in contact with the basement membrane (which is attached to the dermis), and a superficial cornified (horny) layer of dead cells.
The innermost cells of the malpighi an layer, next to the basement membrane, make up the basal layer, or stratum base. Immediately peripheral to the basal layer is the spinous, or prickle-cell, layer the stratum spinous. Its cells have a spiny appearance due to the numerous desmosomes on their surface. Studies with the electron microscope have revealed that desmosomes are symmetrical, laminated structures in which some layers are contributed by the plasma membranes of adjoining cells and some form an intercellular component. The spinous layer is succeeded by the granular layer, or stratum granulosum, with granules of keratohyalin contained in the cells.
These small particles are of irregular shape and occur in random rows or lattices. The cells of the outer spinous and granular layers also contain much larger, lamellate d bodies the membrane-coating granules. They are most numerous within the cells of the spinous layer. In the granular layer they appear to migrate toward the periphery of each cell and to pass into the intercellular spaces, where they discharge their waxy lipid components. Peripheral to the granular layer is the stratum corneum, or horny layer, in which the keratinocytes have lost their nuclei and most of their organelles and contents, including the keratohyalin granules.
They become progressively flattened and filled with keratin and are ultimately desquamated. Between the granular layer and stratum corneum, an unattainable stratum lucidum, or hyaline layer, can be recognized in palmar and plantar epidermis and some other regions. Physicians have traditionally categorized burns as first-, second-, or third-degree injuries, according to the depth of skin damage (see illustration). In a first-degree burn, only the epidermis is affected. (Rook 81) These injuries are characterized by redness and pain; there are no blisters, and edema (swelling due to the accumulation of fluids) in the wounded tissue is minimal. A classic example of a first-degree burn is moderate sunburn.
The damage in a second-degree burn extends through the entire epidermis and part of the dermis. These injuries are characterized by redness and blisters. The deeper the burn the more prevalent the blisters, which increase in size during the hours immediately following the injury. Like first-degree burns, second-degree injuries may be extremely painful.
The development of complications and the course of healing in a second-degree burn depend on the extent of damage to the dermis. Unless they become infected, most superficial second-degree burns heal without complications and with little scarring in 10 to 14 days. Third-degree, or full-thickness, burns destroy the entire thickness of the skin. The surface of the wound is leathery and may be brown, tan, black, white, or red.
There is no pain, because the pain receptors have been obliterated along with the rest of the dermis. Blood vessels, sweat glands, sebaceous glands, and hair follicles are all destroyed in skin that suffers a full-thickness burn. Fluid losses and metabolic disturbances associated with these injuries are grave. (Rook 84) Occasionally burns deeper than a full thickness of the skin are incurred, as when part of the body is entrapped in a flame and not immediately extricated. Electrical burns are usually deep burns. These deep burns frequently go into the subcutaneous tissue and, at times, beyond and into the muscle, fascia, and bone. Such burns are of the fourth degree, also called black (because of the typical color of the burn), or char, burns.
Fourth-degree burns are of grave prognosis, particularly if they involve more than a small portion of the body. In these deep burns toxic materials may be released into the bloodstream. If the char burn involves only a small part of the body, it should be excised down to healthy tissue. If an extremity is involved, amputation may be necessary. Surgeons measure the area of a burn as a percentage of the body's total skin area.
The skin area on each arm is roughly 9 percent of the body total, as is the skin covering the head and neck. The percentage on each leg is 18, and the percentage on the trunk is 18 on the front and 18 on the back. The percentage of damaged skin affects the chances of survival. Most people can survive a second-degree burn affecting 70 percent of their body area, but few can survive a third-degree burn affecting 50 percent. If the area is down to 20 percent, most people can be saved, though elderly people and infants may fail to survive a 15 percent skin loss. (Rook 85) Severe burns cause immediate nervous shock. The victim grows pale and is confused, anxious, and frightened by the pain and may faint.
Much more dangerous is the secondary shock that comes a few hours later. Its chief features are a dramatic fall in blood pressure that leads to pallor, cold extremities, and eventual collapse. This secondary shock is precipitated by loss of fluid from the circulation, not just the fluid lost in the destroyed tissue but fluid that leaks from the damaged area that has lost its protective covering of skin. Burns kill not just by damaging tissue but by allowing this leakage of fluid and salts. If more than a fifth of the blood volume is lost to the circulation, insufficient blood returns to the heart for it to maintain blood pressure. And the loss of salts, particularly sodium and potassium salts, not only disturbs their balance in the body but changes the osmotic balance of the blood and body fluids.
The significance of these physiological changes was understood in 1905, but not until the 1930 s were doctors able to correct them with transfusions of blood or plasma. The treatment of a burn is, of course, dependent upon the severity of the injury. In general, first-degree burns can be adequately treated with proper first-aid measures. Second-degree burns that cover more than 15 percent of an adult's body or 10 percent of a child's, or that affect the face, hands, or feet, should receive prompt medical attention, as should all third-degree burns, regardless of size. Word Count: 2633 Bibliography: The Structure and Function of Skin, 3 rd ed. (1999), a textbook; Lowell A. Goldsmith (ed. ), Biochemistry and Physiology of the Skin, 2 vol. (2000), Arthur Rook et al. (eds. ), Textbook of Dermatology, 4 th ed. , 3 vol. (1999), Howard I.
Mai bach and Edward K. Boisits (eds. ), Neonatal Skin: Structure and Function (2001) R. Marks and P. A.
Payne (eds. ), Bioengineering and the Skin (1999). "Real-time, Photo-realistic, Physically Based Rendering of Fine Scale Human Skin Structure", A. Haro, B. Guenter, and I. Essa, Proceedings 12 th Euro graphics Workshop on Rendering, London, England, June 2001.
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