The Integumentary System - Workings: how the integumentary system functions
The integumentary system is essential to the body's homeostasis, or ability to maintain the internal balance of its functions regardless of outside conditions. The system works to protect underlying tissues and organs from infections and injury. It also prevents the loss of body fluids.
Receiving about one-third of the blood pumped from the heart every minute, the skin and its glands help maintain normal body temperature. The system also acts as a mini-excretory system, secreting salts, water, and wastes in the form of sweat. Cells in the skin utilize sunlight to create vitamin D, which is necessary for normal bone growth and function. Finally, the skin contains sensory receptors or specialized nerve endings that allow an individual to "feel" sensations such as touch, pain, pressure, and temperature.
Protection
The outermost epidermal layer of the skin is a barrier between the internal environment of the body and the external world. Keratin, in abundance in this outer layer, waterproofs the body. Without it, handling household chemicals, swimming in a pool, or taking a shower (a necessary everyday activity) would be disastrous to the underlying cells of the body. Not only does keratin keep water out, it also keeps water in. Excessive evaporation or loss of body fluids would result in dehydration and eventual death.
Karen Wetterhahn (1948–1997) was a chemistry professor at Dartmouth College in Hanover, New Hampshire, where she conducted environmental research projects. During an experiment in August 1996, Wetterhahn spilled a tiny drop of dimethyl mercury (a highly toxic chemical) on her hand. Less than a year later, she was dead.
Wetterhahn had been conducting research to determine the effects that heavy metals (metals such as mercury having a high specific gravity) produce on the environment. During her experiment, she was transferring some dimethyl mercury to a tube when she spilled a tiny amount. Although Wetterhahn was wearing latex gloves, the mercury permeated the thin latex and soaked into her skin, passing through its waterproof layers within seconds.
Dimethyl mercury is deadly. Once in the body, it seeps from the bloodstream into brain tissues, causing fatal damage to the central nervous system and the brain. Symptoms of mercury poisoning include loss of motor (movement) control, numbness in the arms and legs, blindness, hearing and speech loss.
Wetterhahn did not feel the effects of the mercury until six months after the accident. Within three months, she was dead. After her death, the U.S. Occupational Safety and Health Administration urged scientists to wear highly resistant laminate gloves (consisting of several bonded layers) under a pair of heavy-duty neoprene gloves when handling compounds such as dimethyl mercury.
The thickness of the outer layer of the epidermis, combined with the toughness provided by keratin, also prevents microorganisms and viruses from entering the body. In addition, sebum secreted by the sebaceous glands helps prevent microorganisms from living and growing on the skin surface. Since it is slightly acidic, sebum creates a condition in which many microorganisms cannot exist. Sebum serves a further protective function by keeping the skin and hair moist; dry skin would crack, allowing viruses and bacteria to enter.
If the protective outer layer of the skin is broken because of an injury and microorganisms enter the body, the many blood vessels in the dermis help prevent the microorganisms from reaching internal tissues. As an immune response, the vessels dilate or expand. This increases the amount of blood flowing to the area, which in turn brings in more white blood cells and other protein factors to battle the infection.
Even though the skin forms a protective barrier, it is still slightly permeable or allows certain substances to pass through it. Vitamins A, D, E, and K all pass through the skin and are absorbed in the capillaries in the dermis. Steroid hormones such as estrogen and chemicals such as nicotine also pass through and are absorbed. With this in mind, medical researchers have developed therapeutic patches that are attached to the skin to deliver chemicals or medication (nicotine patches for those individuals trying to quit smoking are an example).
Nails protect the exposed tips of fingers and toes from physical injury. Fingernails also aid the fingers in picking up small objects.
Hair serves a protective function, although it is limited. On the head, hair protects the scalp from damaging ultraviolet (UV) radiation from the Sun, cushions the head from physical blows, and insulates the scalp to a degree. On the eyelids, eyelashes prevent airborne particles and insects from entering the eyes. Hairs in the nostrils and the external ear canals perform a similar function.
When stimulated by cold or an emotion such as fear, the arrector pili muscles contract, pulling hair follicles upright. In animals (and in our evolutionary ancestors, who had much more body hair), this action adds warmth by adding a layer of insulating air to the fur. In present-day humans, who have very little body hair, this action seems to serve no purpose other than to create dimples or "goose bumps" in the skin.
The body is protected against the Sun's harmful UV radiation by melanin, produced by melanocytes in the epidermis. Melanin accumulates within the cells of the epidermis. It then absorbs UV radiation before that radiation can destroy the cells' DNA or deoxyribonucleic acid (large, complex molecules found in the nuclei of cells that carries genetic or hereditary information for an organism's development). Increased exposure to the Sun causes melanocytes to increase their production of melanin. The temporary result is that the skin becomes darker or tanned and is able to withstand further exposure to UV rays.
The protection afforded by melanin, however, is limited. Prolonged or excessive exposure to UV radiation eventually damages the skin. It causes elastic fibers in the dermis to clump, and the skin takes on a leathery appearance. Overexposure can also result in melanoma, a tumor composed of melanocytes.
Body temperature
Normal internal body temperature averages approximately 98.6°F (37°C). The heat-regulating functions of the body are extremely important. If the internal temperature varies more than a few degrees from normal, life-threatening changes take place in the body.
Eccrine glands play an important part in maintaining normal body temperature. When the temperature of the body rises due to physical exercise or environmental conditions, the hypothalamus (region of the brain containing many control centers for body functions and emotions) sends signals to the eccrine glands to secrete sweat. When sweat evaporates on the skin surface, it carries large amounts of body heat with it and the skin surface cools.
Because blood carries heat (a form of energy), blood flow is another regulator of body temperature. Under warm conditions, the hypothalamus signals blood vessels in the dermis to dilate or expand. This increases blood flow (and carries excess heat) to the body's surface. Like a radiator, the skin then gives off heat to the surrounding environment.
During cold conditions, the hypothalamus signals eccrine glands to stop secreting sweat. It also signals blood vessels in the dermis to constrict or close, which reduces blood flow to the skin surface. As a result, heat is kept within the core of the body.
Excretion and vitamin D formation
Excretion is a very minor function of the skin. Sweat does contain salt and urea (a compound produced when the liver breaks down amino acids), but the amounts of these wastes are slight. The kidneys are mainly responsible for removing waste products from the blood.
For years, workers have cleaned old stone and concrete structures by blasting their surfaces with a spray of fine sand. In the late 1990s, dermatologists and beauty salon owners in the United States began using a similar technique to remove the signs of aging on people's faces.
The new treatment, already used in Europe since the early 1990s, is called microdermabrasion. A machine blows tiny sterile sand crystals onto the skin of the face, then suctions them off. The crystals rub off the top layer of the skin, helping remove wrinkles.
The procedure is relatively painless and quick. However, its effects are not permanent, and it only removes fine lines. Deep lines around the mouth, crow's feet around the eyes, and deep lines on the forehead remain, although they are softened.
As explained earlier, too much sunlight is harmful to the body. A limited amount, however, is beneficial. In the lower layers of the epidermis, cells contain a form of cholesterol (fatlike substance produced by the liver that is an essential part of cell membranes and body chemicals). When exposed to UV radiation, that cholesterol changes into vitamin D, which the body uses to absorb calcium and phosphorus from food in the small intestine. Those two minerals are then used to build and maintain bones and teeth, among other functions.
Sensory reception
The main function of the sensory receptors in the dermis is to provide the brain with information about the external world and its effect on the skin. Thus, they alert the body to the possible tissue-damaging effects of extreme heat or cold or something that is pressing hard against the skin. They also transmit pleasant sensations, such as a gentle breeze blowing across the face or the soft caress of a loved one.
The receptors differ in their sensitivity. Touch receptors are the most sensitive, responding to the slightest contact. Found mainly in the fingers, tongue, and lips, they number about 500,000. Pain receptors, however, do not react unless the stimulus is strong enough. Located all over the body, pain receptors number between three and four million. Their high numbers indicate their importance to the body.
Receptors send their information to the brain to be interpreted. The brain then directs the body to respond, whether to remove itself from the situation or remain. Sensation, therefore, is a function of the brain and the nervous system.
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