HEALTH HORIZONS : MEDICINE : Layers of Protection: Facts About the Skin
An average-sized adult’s skin weighs about nine pounds, about twice that of the second-largest organ, the liver. If spread out, it would cover about six square feet.
Like an onion, human skin consists of multiple layers, including three major ones, that enable it to retain bodily heat and moisture and ward off health threats. Yet it is porous enough to allow the release of fluid from the sweat glands to cool the body.
The epidermis, or outer layer, is the thinnest. Yet it is packed with different cells, including melanocytes, which produce melanin--the pigment that gives the skin its color.
The epidermis is extremely active, with its innermost cells continuously dividing and moving outward in a cycle of about 28 days. The cells take about 14 days to travel to the surface, where they gradually die as they fill up with keratin, the hard protein that also makes up the hair and nails.
The dermis, or middle layer of skin, is the thickest and is filled with collagen and elastic fibers that give the skin its shape. It includes the shafts of the hair, which are lubricated by sebum, a fatty material produced by nearby sebaceous glands, located primarily in the face, scalp, upper back and chest. Acne is largely an inflammation of these glands. The dermis also is the home of the sweat (eccrine) glands, which funnel perspiration through the pores, and of specialized sweat or scent (apocrine) glands.
The hypodermis, or subcutaneous tissue, forms the bottom layer of skin. Though not everyone considers it a true component of the skin, it provides a fatty insulating cushion and an anchor for hair.
The skin is laced with nerve cells that constantly monitor the environment for changes in pressure, temperature and humidity, as well as with tiny blood vessels that bring nutrients to the skin cells and carry off wastes. When the skin’s tiny nerve receptors detect a rise in temperature, it is reported to the hypothalamus, the body’s thermostat in the brain. The latter activates the sweat glands, which release perspiration to the skin’s surface. As the salty fluid evaporates, the skin cools.
The hypothalamus also orders the skin’s blood vessels to dilate--open wider--increasing the flow of blood. This helps shed more heat from the body’s interior (and accounts for the flushed look that accompanies exertion). When the skin detects a drop in temperature, the hypothalamus signals these same vessels to constrict, reducing the flow of blood to the skin and retaining heat within the core of the body, thereby keeping vital internal organs warm, even at the risk of frostbite damage to the skin.
