The Skin as Our Integument Help (page 2)
Introduction to The Skin as Our Integument
“How is the human body like a peach?” In reply to this odd question, you might comically retort, “Well, we’re just a bunch of fruits!” But more seriously, both a peach and the human body are surrounded by an integument (in- TEG -you-ment) – a thin “covering.” And lying deep to this integument is a much thicker layer of flesh. Hence, it is the integument or “skin” (derm) that encloses and protects the flesh.
And the skin or integument of both the peach and the human body has an outermost layer, the epidermis. You might remember from our study of plants (Chapter 9) that the epidermis is literally “(something) present upon the skin,” and that in ferns and seedbearing plants (such as peach trees), the epidermis is an extremely thin layer of outer protective cells. But from here, the similarity between peach integument and human integument begins to fade (Figure 13.1).
Keratinized Epithelial Strata and Skin Coloration By Melanin
Keratinized Epithelial Strata
One key difference between the epidermis of humans and most other animals, from those of plants, is the presence of keratin. This “horn substance” is found in the epidermis and hair and nails of humans, as well as in the claws and horns of various other animals. Since keratin is essentially waterproof, so is human skin. Our epidermis consists of a multiple series of thin, overlapping strata ( STRAT -uh) of keratinized epithelial cells. When viewed under a light microscope, this highly orderly arrangement gives the epidermis the appearance of a collection of thin “layers or bed covers” (strat), heaped one upon the other over a bed.
The outer surface stratum (STRAT - um) is really not living, at all! Rather, it consists of a series of stacked squamae (SKWAY - me) – dead, keratinstuffed “scales” (squam). This means that you and I face the world with dead scales (squamae) showing at our body surface! But this has the definite advantage of protecting us from dirt, fungi, and bacteria, which cannot easily penetrate the multiple layers of lifeless squamae.
Skin Coloration By Melanin
In addition to containing keratin, the epithelial cells in our epidermis are also rich in melanin ( MEL -uh-nin) or “black” ( melan ) “substance” (- in ). Melanin is a brownish-black pigment produced by melanocytes ( MEL -uh-nuh- sights ). These “black cells” are large, octopus-shaped cells with several long arms of cytoplasm. After they produce the melanin granules, the melanocytes appear to penetrate the membranes of adjacent cells, and inject some of their melanin granules into them. This results in a darkening of the epidermis. Melanin’s chief function is absorption of ultraviolet ( ul -truh- VEYE -uh-lit) rays that strike the surface of the skin. These ultraviolet (UV) rays are invisible rays whose wavelengths lie “beyond” ( ultra -) those of X-rays, but below those of visible violet light. The benefits are dual: reduction in the risk of suffering skin cancer (due to mutation of skin cell DNA by UV light), and reduction in skin wrinkling.
The Dermis As Our Tough Main “skin”
True to its name, the epidermis literally lies “upon” ( epi -) the dermis ( DER -mis). The dermis is the main, fiber-rich, connective tissue portion of the skin. The dermis is especially rich in a dense network of collagen ( CALL -uh-jen) fibers . The word collagen translates to mean “glue” ( coll ) “producer” (- gen ). Collagen fibers are thick, tough, unbranched fibers that have a high tensile ( TEN -sil) strength ; that is, they have a great ability to resist pulling or “tension” ( tens ) forces. [ Study suggestion: Place two fingers on the skin of your forearm. Now, gently try to stretch your skin between your fingertips. Feel the resistance to stretching, the tensile strength, that is being exerted? This glue-like function is mainly the result of the thousands of collagen fibers running like a tough, woven basket throughout your dermis.]
One important type of structure found within the dermis is the hair follicle ( FAHL -uh-kul). As evident from Figure 13.1, the hair follicle is a “little bag” lined by a membrane, and containing a hair. The base of the hair follicle lies in the dermis. The hair, itself, is basically a flexible rod of tightly packed, keratin-stuffed squamae. Arranged around the base of the hair follicle is a sensory nerve basket . [ Study suggestion: Without touching your skin, gently stroke the hairs on your forearm. What do you feel – a tickling, tingling sensation? This reveals the main function of hairs: touch sensations.]
There are many other types of sensory receptors located within the dermis. Besides receptors for the sense of touch, there are those for pressure, pain, cold, heat, and vibration.
One of the most critical functions of the dermis is its role in thermoregulation ( THER -moh-reg-you- LAY -shun). By thermoregulation, we mean the “regulation” or control of “heat” (therm): specifically, the control of internal body temperature. Way back in Chapter 1, we talked about the homeostasis or relative constancy of oral body temperature, measured in units of degrees Fahrenheit. This homeostasis (relative constancy) of oral body temperature is another term for thermoregulation. It was represented symbolically by an S-shaped curve shown back in Figure 1.2 (A):
Figure 13.2 reveals what happens when the oral body temperature of a human or other homeothermic animal rises towards the upper limit of its normal range. (This temperature boost often occurs during heavy exercising.) In the dermis, two critical events kick in. The sweat glands increase their secretion of sweat into the sweat ducts, which then moves up and out onto the surface of the skin through sweat pores. The excess body heat essentially boils the watery sweat from the skin surface, causing it to evaporate into the air. This net heat loss helps to lower the oral body temperature.
A second chain of physiological events involve vasodilation ( vase -oh-die- LAY -shun) – the “process of” (-tion) blood “vessel” (vas) “widening” (dilat). As the body gets hotter, the blood vessels in the dermis vasodilate ( vase -oh- DIE -late), becoming wider. This allows more hot blood to circulate from the deep core of the body, and flow more freely into the vessels of the skin. Much more heat is then lost by radiation, the movement of heat waves or rays from the hot blood in the skin out into the cooler air surrounding the body.
By both of these means combined (increased evaporation from sweat + increased heat loss by radiation from the blood), oral body temperature is eventually brought back down to its average, long-term level (about 98.6 degrees Fahrenheit in most human beings). And thermoregulation, or homeostasis of oral body temperature, is thereby achieved.
Practice problems for these concepts can be found at: Skins And Skeletons Test
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