Functions of the Integumentary System

Functions of the Integumentary System

The skin is constantly exposed to abrasion, micro-organisms, chemicals, and extremes of temperature, and has three primary barriers that protect the body: chemical, physical, and biological barriers.

Chemical Barriers

Chemical barriers include melanin and secretions from the skin. There are numerous types of bacteria and other microorganisms on the skin. The low pH of skin secretions is described as the acid mantle, which slows the multiplication of microorganisms. Many bacteria are killed directly by contact with bac-tericidal substances from the sebum and the dermici-din from the sweat. Defensins are natural substances secreted by skin cells that create holes in bacteria, helping to kill them. Protective peptides or catheli-cidins are released by injured skin and are effective against many bacteria, but mostly against group A streptococci. Also, melanin is a chemical pigment barrier against UV light.

Physical Barriers

Physical barriers are created by skin continuity and hardened keratinized cells. In the stratum corneum, there are many layers of flat, dead cells surrounded by glycolipids along with the acid mantle and skin secre-tions to stop bacterial invasion. Water and water-soluble substances are largely kept from diffusing between cells by the glycolipids. Even so, a continual loss of small amounts of water occurs through the epidermis. When the skin is immersed in water, except for salt water, it swells slightly as it absorbs some of the water. In limited amounts, the following substances are able to penetrate the skin: lipid-soluble substances, oleoresins, organic solvents, heavy metal salts, certain drugs, and drug agents known as penetration enhancers. Lipid-soluble substances include carbon dioxide, oxygen, fat-soluble vitamins (A, D, E, and K), and steroids such as estrogens.

Examples of oleoresins (plant resins) include poison oak and poison ivy. Organic solvents, which dissolve cell lipids, include acetone, dry cleaning fluids, and paint thinner. Examples of heavy metal salts include lead and mercury. Drugs that can penetrate the skin include nic-otine, nitroglycerine, and medications used to stop sea-sickness. Also, for at least 24 hours after being ingested, drinks containing alcohol enhance skin permeability to a large degree. Penetration enhancers are drugs that help allow certain drugs to enter the body.

Biological Barriers

The body also has biological barriers that act for its protection: the dendritic cells in the epidermis, the macro-phages in the dermis, and the body’s DNA. Dendritic cells are active immune system components that patrol the epidermis for antigens (foreign substances). The dendritic cells play the same role in the epidermis as the lymphocytes in the blood. The second line of biologi-cal barriers consists of the dermal macrophages, which function in much the same way as the dendritic cells. They dispose of bacteria and viruses that get past the epidermis. DNA is a fairly potent biologic sunscreen, and its electrons absorb UV radiation. This is trans-ferred to the nuclei of DNA atoms, causing them to heat and vibrate quickly. The heat dissipates to nearby water molecules in an instant. Therefore, DNA con-verts possibly harmful UV radiation into heat, which has no negative effects in this example.

Body Temperature Regulation

As mentioned earlier, the skin is important in regulat-ing body temperature. The deeper body parts are nor-mally set at 98.6°F (which is equivalent to 37°C). Body heat is produced by cellular metabolism, with most body heat being produced by the skeletal and cardiac muscle cells as well as certain glandular cells (such as liver cells). As temperature rises, the body is stimulated to release body heat by relaxing dermal blood vessel walls. The eccrine sweat glands release sweat to the skin surface, which evaporates to cool the skin. The reverse process occurs when the body temperature drops. This holds heat in, and if the temperature continues to drop, the body signals certain muscles to contract. Shivering is the result, which helps to generate more body heat.

The sweat glands secrete about 17 ounces (500 mL) of sweat per day, when the body is at rest and when the environmental temperature is below 88–90°F (31–32°C). This normal, unnoticed sweating is described as insensible perspiration. The sweat glands greatly increase their activity when body temperature rises and the nervous system causes the dermal blood vessels to dilate. If the weather is hot, sweat becomes noticeable (sensible perspiration), accounting for up to three gallons (12 liters) of body water loss in a sin-gle day. As the sweat evaporates, the body is cooled and overheating is prevented. On cold days, dermal blood vessels constrict to cause the blood to temporar-ily bypass the skin. The skin temperature assumes the temperature of the external environment. Passive heat loss slows down, and the body conserves heat.

Cutaneous Sensation

Sensory receptors are located in the dermis and are actually parts of the nervous system. They initiate nerve impulses that can reach our conscious awareness and are classified as exteroceptors because they respond to ­stimuli from outside the body. Nerve fibers in the skin control blood flow, adjust gland secretion rates, and monitor sensory receptors in the dermis and deeper layers of the epidermis. The epidermis also contains the exten-sion of sensory neurons that provide sensations of pain and temperature. The dermis contains similar receptors and other more specialized receptors (e.g., sensations of touch and pressure). In the dermal papillae, Meissner’s (tactile) corpuscles and tactile discs sense light surface touching. In the deeper dermis or hypodermis, Pacinian or lamellar corpuscles sense harder contacts that involve deep pressure. Even hair follicle receptors play a part in the sense of touch. Painful stimuli are sensed by free nerve endings located throughout the skin.

Metabolic Functions

The skin also has many metabolic functions. It reacts to sunlight by converting modified cholesterol mol-ecules to vitamin D precursors. These are carried by the blood to other areas of the body for conversion to vitamin D, which aids in calcium metabolism. With-out vitamin D, calcium could not be absorbed from the gastrointestinal tract. Therefore, sunlight is essen-tial so the bones can absorb calcium via the presence of vitamin D. The epidermis carries out chemical con-versions that assist the liver. Keratinocyte enzymes play important roles related to cancer and inflam-mation. These enzymes can either convert harmless chemicals to carcinogens or disarm chemicals that are carcinogenic. They activate certain steroids, such as when they transform cortisone into hydrocortisone, which has strong anti-inflammatory properties. Other important proteins manufactured by skin cells include collagenase, which fights against wrinkling by stimu-lating the natural use and reuse of collagen.

Blood Reservoir

The skin also acts as a blood reservoir and plays a role in excretion. Nearly 5% of the body’s entire blood ­volume can be contained by the extensive vascular supply of the dermis. The nervous system constricts dermal blood vessels to supply more blood to other body organs that need this supply. More blood is moved into the general circulation by this constric-tion, and working muscles or other organs use it.

Excretion

Nitrogen-containing wastes are also eliminated through sweat, although most of this elimination goes through the urine. These nitrogen-containing wastes include ammonia, urea, and uric acid. When sweating is profuse, water and sodium chloride (salt) are lost in large quantities.

1. What are the three types of skin protection?

2. Explain metabolic skin functions.

3. Describe the chemical agents that protect the skin.

4. Why is sunlight essential for bone health?