C2 Expansion Processes
Thermosetting and thermoplastic resins can be expanded by injecting gases (often nitrogen or methyl chloride) into the plastic melt. As the resin cools, tiny bubbles of gas are trapped inside, forming a cellular plastic structure. This process is used to make foam products such as cushions, pillows, sponges, egg cartons, and polystyrene cups.
Foam plastics can be classified according to their bubble, or cell, structure. Sponges and carpet pads are examples of open-celled foam plastics, in which the bubbles are interconnected. Flotation devices are examples of closed-celled foam plastics, in which the bubbles are sealed like tiny balloons. Foam plastics can also be classified by density (ratio of plastic to cells), by the type of plastic resin used, and by flexibility (rigid or flexible foam). For example, rigid, closed-celled polyurethane plastics make excellent insulation for refrigerators and freezers.
VII IMPORTANT TYPES OF PLASTICS
A wide variety of both thermoplastics and thermosetting plastics are manufactured. These plastics have a spectrum of properties that are derived from their chemical compositions. As a result, manufactured plastics can be used in applications ranging from contact lenses to jet body components.
A Thermoplastics
Thermoplastic materials are in high demand because they can be repeatedly softened and remolded. The most commonly manufactured thermoplastics are presented in this section in order of decreasing volume of production.
A1 Polyethylene
]n (where n denotes that the chemical formula inside the brackets repeats itself to form the plastic molecule) is made in low- and high-density forms. Low-density polyethylene (LDPE) has a density ranging from 0.91 to 0.93 g/cm3 (0.60 to 0.61 oz/cu in). The molecules of LDPE have a carbon backbone with side groups of four to six carbon atoms attached randomly along the main backbone. LDPE is the most widely used of all plastics, because it is inexpensive, flexible, extremely tough, and chemical-resistant. LDPE is molded into bottles, garment bags, frozen food packages, and plastic toys.CH2CH2CH2). Polyethylene, with the chemical formula [(PE) resins are milky white, translucent substances derived from ethylene (CH2
High-density polyethylene (HDPE) has a density that ranges from 0.94 to 0.97 g/cm3 (0.62 to 0.64 oz/cu in). Its molecules have an extremely long carbon backbone with no side groups. As a result, these molecules align into more compact arrangements, accounting for the higher density of HDPE. HDPE is stiffer, stronger, and less translucent than low-density polyethylene. HDPE is formed into grocery bags, car fuel tanks, packaging, and piping.
A2 Polyvinyl Chloride
CHCl). PVC is the most widely used of the amorphous plastics. PVC is lightweight, durable, and waterproof. Chlorine atoms bonded to the carbon backbone of its molecules give PVC its hard and flame-resistant properties.Polyvinyl chloride (PVC) is prepared from the organic compound vinyl chloride (CH2
In its rigid form, PVC is weather-resistant and is extruded into pipe, house siding, and gutters. Rigid PVC is also blow molded into clear bottles and is used to form other consumer products, including compact discs and computer casings.
PVC can be softened with certain chemicals. This softened form of PVC is used to make shrink-wrap, food packaging, rainwear, shoe soles, shampoo containers, floor tile, gloves, upholstery, and other products. Most softened PVC plastic products are manufactured by extrusion, injection molding, or casting.
A3 Polypropylene
CH3) branching off of every other carbon along the molecular backbone. Because the most common form of polypropylene has the methyl groups all on one side of the carbon backbone, polypropylene molecules tend to be highly aligned and compact, giving this thermoplastic the properties of durability and chemical resistance. Many polypropylene products, such as rope, fiber, luggage, carpet, and packaging film, are formed by injection molding.CH2) and has a methyl group (CHis polymerized from the organic compound propylene (CH3
A4 Polystyrene
CH2), has phenyl groups (six-member carbon ring) attached in random locations along the carbon backbone of the molecule. The random attachment of benzene prevents the molecules from becoming highly aligned. As a result, polystyrene is an amorphous, transparent, and somewhat brittle plastic. Polystyrene is widely used because of its rigidity and superior insulation properties. Polystyrene can undergo all thermoplastic processes to form products such as toys, utensils, display boxes, model aircraft kits, and ballpoint pen barrels. Polystyrene is also expanded into foam plastics such as packaging materials, egg cartons, flotation devices, and styrofoam. (For more information, see the Expansion Processes section of this article.), produced from styrene (C6H5CH
A5 Polyethylene Terephthalate
]n. PET molecules are highly aligned, creating a strong and abrasion-resistant material that is used to produce films and polyester fibers. PET is injection molded into windshield wiper arms, sunroof frames, gears, pulleys, and food trays. This plastic is used to make the trademarked textiles Dacron, Fibre V, Fortrel, and Kodel. Tough, transparent PET films (marketed under the brand name Mylar) are magnetically coated to make both audio and video recording tape.CH2CH2COOC6H4OOCCH2OH), which produces the PET monomer [COOH) and ethylene glycol (HOCH2C6H4Polyethylene terephthalate (PET) is formed from the reaction of terephthalic acid (HOOC
A6 Acrylonitrile Butadiene Styrene
] n, which allows these monomers to form chains by attaching to the rubber molecules.CHCHCHCHCH2). Acrylonitrile and styrene are dissolved in polybutadiene rubber [Acrylonitrile butadiene styrene (ABS) is made by copolymerizing (combining two or more monomers) the monomers acrylonitrile (CH2CHCN) and styrene (C6H5CH
The advantage of ABS is that this material combines the strength and rigidity of the acrylonitrile and styrene polymers with the toughness of the polybutadiene rubber. Although the cost of producing ABS is roughly twice the cost of producing polystyrene, ABS is considered superior for its hardness, gloss, toughness, and electrical insulation properties. ABS plastic is injection molded to make telephones, helmets, washing machine agitators, and pipe joints. This plastic is thermoformed to make luggage, golf carts, toys, and car grills. ABS is also extruded to make piping, to which pipe joints are easily solvent-cemented.
A7 Polymethyl Methacrylate
Polymethyl methacrylate (PMMA), more commonly known by the generic name acrylic, is polymerized from the hydrocarbon compound methyl methacrylate (C4O2H8). PMMA is a hard material and is extremely clear because of the amorphous arrangement of its molecules. As a result, this thermoplastic is used to make optical lenses, watch crystals, aircraft windshields, skylights, and outdoor signs. These PMMA products are marketed under familiar trade names, including Plexiglas, Lucite, and Acrylite. Because PMMA can be cast to resemble marble, it is also used to make sinks, countertops, and other fixtures.
A8 Polyamide
Polyamides (PA), known by the trade name Nylon, consist of highly ordered molecules, which give polyamides high tensile strength. Some polyamides are made by reacting dicarboxylic acid with diamines (carbon molecules with the ion –NH2 on each end), as in nylon-6,6 and nylon-6,10. (The two numbers in each type of nylon represent the number of carbon atoms in the diamine and the dicarboxylic acid, respectively.) Other types of nylon are synthesized by the condensation of amino acids.
Polyamides have mechanical properties such as high abrasion resistance, low coefficients of friction (meaning they are slippery), and tensile strengths comparable to the softer of the aluminum alloys. Therefore, nylons are commonly used for mechanical applications, such as gears, bearings, and bushings (see Engineering: Mechanical Engineering). Nylons are also extruded into millions of tons of synthetic fibers every year. The most commonly used nylon fibers, nylon-6,6 and nylon-6 (single number because this nylon forms by the self-condensation of an amino acid) are made into textiles, ropes, fishing lines, brushes, and other items.
B Thermosetting Materials
Because thermosetting plastics cure, or cross-link, after being heated, these plastics can be made into durable and heat-resistant materials. The most commonly manufactured thermosetting plastics are presented below in order of decreasing volume of production.
B1 Polyurethane
]n, where R may represent a different alkyl group than R’. Alkyl groups are chemical groups obtained by removing a hydrogen atom from an alkane—a hydrocarbon containing all carbon-carbon single bonds. Most types of polyurethane resin cross-link and become thermosetting plastics. However, some polyurethane resins have a linear molecular arrangement that does not cross-link, resulting in thermoplastics.R’OOCNHRPolyurethane is a polymer consisting of the repeating unit [
Thermosetting polyurethane molecules cross-link into a single giant molecule. Thermosetting polyurethane is widely used in various forms, including soft and hard foams. Soft, open-celled polyurethane foams are used to make seat cushions, mattresses, and packaging. Hard polyurethane foams are used as insulation in refrigerators, freezers, and homes.
Thermoplastic polyurethane molecules have linear, highly crystalline molecular structures that form an abrasion-resistant material. Thermoplastic polyurethanes are molded into shoe soles, car fenders, door panels, and other products.
B2 Phenolics
Phenolic (phenol-formaldehyde) resins, first commercially available in 1910, were some of the first polymers made. Today phenolics are some of the most widely produced thermosetting plastics. They are produced by reacting phenol (C6H5OH) with formaldehyde (HCOH). Phenolic plastics are hard, strong, inexpensive to produce, and they possess excellent electrical resistance. Phenolic resins cure (cross-link) when heat and pressure are applied during the molding process. Phenolic resin-impregnated paper or cloth can be laminated into numerous products, such as electrical circuit boards. Phenolic resins are also compression molded into electrical switches, pan and iron handles, radio and television casings, and toaster knobs and bases.
B3 Melamine-Formaldehyde and Urea-Formaldehyde
Urea-formaldehyde (UF) and melamine-formaldehyde (MF) resins are composed of molecules that cross-link into clear, hard plastics. Properties of UF and MF resins are similar to the properties of phenolic resins. As their names imply, these resins are formed by condensation reactions between urea (H2NCONH2) or melamine (C3H6N6) and formaldehyde (CH2O).
Melamine-formaldehyde resins are easily molded in compression and special injection molding machines. MF plastics are more heat-resistant, scratch-proof, and stain-resistant than urea-formaldehyde plastics are. MF resins are used to manufacture dishware, electrical components, laminated furniture veneers, and to bond wood layers into plywood.
Urea-formaldehyde resins form products such as appliance knobs, knife handles, and plates. UF resins are used to give drip-dry properties to wash-and-wear clothes as well as to bond wood chips and wood sheets into chip board and plywood.
B4 Unsaturated Polyesters
CH2]n. Unsaturated polyesters (an unsaturated compound contains multiple bonds) cross-link when the long molecules are joined (copolymerized) by the aromatic organic compound styrene (see Aromatic Compounds).CH2COOC6H4OOCUnsaturated polyesters (UP) belong to the polyester group of plastics. Polyesters are composed of long carbon chains containing [
Unsaturated polyester resins are often premixed with glass fibers for additional strength. Two types of premixed resins are bulk molding compounds (BMC) and sheet molding compounds (SMC). Both types of compounds are doughlike in consistency and may contain short fiber reinforcements and other additives. Sheet molding compounds are preformed into large sheets or rolls that can be molded into products such as shower floors, small boat hulls, and roofing materials. Bulk molding compounds are also preformed to be compression molded into car body panels and other automobile components.
B5 Epoxy
Epoxy (EP) resins are named for the epoxide groups (cycl-CH2OCH; cycl or cyclic refers to the triangle formed by this group) that terminate the molecules. The oxygen along epoxy’s carbon chain and the epoxide groups at the ends of the carbon chain give epoxy resins some useful properties. Epoxies are tough, extremely weather-resistant, and do not shrink as they cure (dry).
Epoxies cross-link when a catalyzing agent (hardener) is added, forming a three-dimensional molecular network. Because of their outstanding bonding strength, epoxy resins are used to make coatings, adhesives, and composite laminates. Epoxy has important applications in the aerospace industry. All composite aircraft are made of epoxy. Epoxy is used to make the wing skins for the F-18 and F-22 fighters, as well as the horizontal stabilizer for the F-16 fighter and the B-1 bomber. In addition, almost 20 percent of the Harrier jet’s total weight is composed of reinforcements bound with an epoxy matrix (see Airplane). Because of epoxy’s chemical resistance and excellent electrical insulation properties, electrical parts such as relays, coils, and transformers are insulated with epoxy.
B6 Reinforced Plastics
Reinforced plastics, called composites, are plastics strengthened with fibers, strands, cloth, or other materials. Thermosetting epoxy and polyester resins are commonly used as the polymer matrix (binding material) in reinforced plastics. Due to a combination of strength and affordability, glass fibers, which are woven into the product, are the most common reinforcing material. Organic synthetic fibers such as aramid (an aromatic polyamide with the commercial name Kevlar) offer greater strength and stiffness than glass fibers, but these synthetic fibers are considerably more expensive.
The Boeing 777 aircraft makes extensive use of lightweight reinforced plastics. Other products made from reinforced plastics include boat hulls and automobile body panels, as well as recreation equipment, such as tennis rackets, golf clubs, and jet skis.
VIII HISTORY OF PLASTICS
Humankind has been using natural plastics for thousands of years. For example, the early Egyptians soaked burial wrappings in natural resins to help preserve their dead. People have been using animal horns and turtle shells (which contain natural resins) for centuries to make items such as spoons, combs, and buttons.
During the mid-19th century, shellac (resinous substance secreted by the lac insect) was gathered in southern Asia and transported to the United States to be molded into buttons, small cases, knobs, phonograph records, and hand-mirror frames. During that time period, gutta-percha (rubberlike sap taken from certain trees in Malaya) was used as the first insulating coating for electrical wires.
In order to find more efficient ways to produce plastics and rubbers, scientists began trying to produce these materials in the laboratory. In 1839 American inventor Charles Goodyear vulcanized rubber by accidentally dropping a piece of sulfur-treated rubber onto a hot stove. Goodyear discovered that heating sulfur and rubber together improved the properties of natural rubber so that it would no longer become brittle when cold and soft when hot. In 1862 British chemist Alexander Parkes synthesized a plastic known as pyroxylin, which was used as a coating film on photographic plates. The following year, American inventor John W. Hyatt began working on a substitute for ivory billiard balls. Hyatt added camphor to nitrated cellulose and formed a modified natural plastic called celluloid, which became the basis of the early plastics industry. Celluloid was used to make products such as umbrella handles, dental plates, toys, photographic film, and billiard balls.
These early plastics based on natural products shared numerous drawbacks. For example, many of the necessary natural materials were in short supply, and all proved difficult to mold. Finished products were inconsistent from batch to batch, and most products darkened and cracked with age. Furthermore, celluloid proved to be a very flammable material.
Due to these shortcomings, scientists attempted to find more reliable plastic source materials. In 1909 American chemist Leo Hendrik Baekeland made a breakthrough when he created the first commercially successful thermosetting synthetic resin, which was called Bakelite (known today as phenolic resin). Use of Bakelite quickly grew. It has been used to make products such as telephones and pot handles.
The chemistry of joining small molecules into macromolecules became the foundation of an emerging plastics industry. Between 1920 and 1932, the I.G. Farben Company of Germany synthesized polystyrene and polyvinyl chloride, as well as a synthetic rubber called Buna-S. In 1934 Du Pont made a breakthrough when it introduced nylon—a material finer, stronger, and more elastic than silk. By 1936 acrylics were being produced by German, British, and U.S. companies. That same year, the British company Imperial Chemical Industries developed polyethylene. In 1937 polyurethane was invented by the German company Friedrich Bayer & Co. (see Bayer AG), but this plastic was not available to consumers until it was commercialized by U.S. companies in the 1950s. In 1939 the German company I.G. Farbenindustrie filed a patent for polyepoxide (epoxy), which was not sold commercially until a U.S. firm made epoxy resins available to the consumer market almost four years later.
After World War II (1939-1945), the pace of new polymer discoveries accelerated. In 1941 a small English company developed polyethylene terephthalate (PET). Although Du Pont and Imperial Chemical Industries produced PET fibers (marketed under the names Dacron and Terylene, respectively) during the postwar era, the use of PET as a material for making bottles, films, and coatings did not become widespread until the 1970s. In the postwar era, research by Bayer and by General Electric resulted in production of plastics such as polycarbonates, which are used to make small appliances, aircraft parts, and safety helmets. In 1965 introduced a linear, heat-resistant thermoplastic known as polysulfone, which is used to make face shields for astronauts and hospital equipment that can be sterilized in an autoclave (a device that uses high pressure steam for sterilization).
Today, scientists can tailor the properties of plastics to numerous design specifications. Modern plastics are used to make products such as artificial joints, contact lenses, space suits, and other specialized materials. As plastics have become more versatile, use of plastics has grown as well. By the year 2005, annual global demand for plastics is projected to exceed 200 million metric tons (441 billion lb).
IX PLASTICS AND THE ENVIRONMENT
Every year in the United States, consumers throw millions of tons of plastic away—of the estimated 210 million metric tons (232 short tons) of municipal waste produced annually in the United States, 10.7 percent are plastics. As municipal landfills reach capacity and additional landfill space diminishes across the United States, alternative methods for reducing and disposing of wastes—including plastics—are being explored. Some of these options include reducing consumption of plastics, using biodegradable plastics, and incinerating or recycling plastic waste.
A Source Reduction
Source reduction is the practice of using less material to manufacture a product. For example, the wall thickness of many plastic and metal containers has been reduced in recent years, and some European countries have proposed to eliminate packaging that cannot be easily recycled.
B Biodegradable Plastics
Due to their molecular stability, plastics do not easily break down into simpler components. Plastics are therefore not considered biodegradable (see Solid Waste Disposal). However, researchers are working to develop biodegradable plastics that will disintegrate due to bacterial action or exposure to sunlight. For example, scientists are incorporating starch molecules into some plastic resins during the manufacturing process. When these plastics are discarded, bacteria eat the starch molecules. This causes the polymer molecules to break apart, allowing the plastic to decompose. Researchers are also investigating ways to make plastics more biodegradable from exposure to sunlight. Prolonged exposure to ultraviolet radiation from the sun causes many plastics molecules to become brittle and slowly break apart. Researchers are working to create plastics that will degrade faster in sunlight, but not so fast that the plastic begins to degrade while still in use.
C Incineration
Some wastes, such as paper, plastics, wood, and other flammable materials can be burned in incinerators. The resulting ash requires much less space for disposal than the original waste would. Because incineration of plastics can produce hazardous air emissions and other pollutants, this process is strictly regulated.
D Recycling Plastics
All plastics can be recycled. Thermoplastics can be remelted and made into new products. Thermosetting plastics can be ground, commingled (mixed), and then used as filler in moldable thermoplastic materials. Highly filled and reinforced thermosetting plastics can be pulverized and used in new composite formulations.
Chemical recycling is a depolymerization process that uses heat and chemicals to break plastic molecules down into more basic components, which can then be reused. Another process, called pyrolysis, vaporizes and condenses both thermoplastics and thermosetting plastics into hydrocarbon liquids.
Collecting and sorting used plastics is an expensive and time-consuming process. While about 27 percent of aluminum products, 45 percent of paper products, and 23 percent of glass products are recycled in the United States, only about 5 percent of plastics are currently recovered and recycled. Once plastic products are thrown away, they must be collected and then separated by plastic type. Most modern automated plastic sorting systems are not capable of differentiating between many different types of plastics. However, some advances are being made in these sorting systems to separate plastics by color, density, and chemical composition. For example, x-ray sensors can distinguish PET from PVC by sensing the presence of chlorine atoms in the polyvinyl chloride material.
If plastic types are not segregated, the recycled plastic cannot achieve high remolding performance, which results in decreased market value of the recycled plastic. Other factors can adversely affect the quality of recycled plastics. These factors include the possible degradation of the plastic during its original life cycle and the possible addition of foreign materials to the scrap recycled plastic during the recycling process. For health reasons, recycled plastics are rarely made into food containers. Instead, most recycled plastics are typically made into items such as carpet fibers, motor oil bottles, trash carts, soap packages, and textile fibers.
To promote the conservation and recycling of materials, the U.S. federal government passed the Resource Conservation and Recovery Act (RCRA) in 1976. In 1988 the Plastic Bottle Institute of the Society of the Plastics Industry established a system for identifying plastic containers by plastic type. The purpose of the "chasing arrows" symbol that appears on the bottom of many plastic containers is to promote plastics recycling. The chasing arrows enclose a number (such as a 1 indicating PET, a 2 indicating high density polyethylene (HDPE), and a 3 indicating PVC), which aids in the plastics sorting process.
By 1994, 40 states had legislative mandates for litter control and recycling. Today, a growing number of communities have collection centers for recyclable materials, and some larger municipalities have implemented curbside pickup for recyclable materials, including plastics, paper, metal, and glass.
I INTRODUCTION
Earthquake, shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the Earth by earthquake waves. The study of earthquakes and the waves they create is called seismology (from the Greek seismos, “to shake”). Scientists who study earthquakes are called seismologists.
The destruction an earthquake causes depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage the structure incurs. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Earthquakes can deform the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Lives may be lost in the resulting destruction.
Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs (machines that record movements of the Earth) detects about 1 million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which caused millions of dollars in damage, occur worldwide once every few years. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the 1995 tremor in Kōbe, Japan, occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people.
In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.
II ANATOMY OF AN EARTHQUAKE
Seismologists examine the parts of an earthquake, such as what happens to the Earth’s surface during an earthquake, how the energy of an earthquake moves from inside the Earth to the surface, how this energy causes damage, and the slip of the fault that causes the earthquake. Faults are cracks in Earth’s crust where rocks on either side of the crack have moved. By studying the different parts and actions of earthquakes, seismologists learn more about their effects and how to predict and prepare for their ground shaking in order to reduce damage.
A Focus and Epicenter
The point within the Earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter. The point on the Earth’s surface directly above the focus is called the epicenter. Earthquake waves begin to radiate out from the focus and subsequently form along the fault rupture. If the focus is near the surface—between 0 and 70 km (0 and 40 mi) deep—shallow-focus earthquakes are produced. If it is intermediate or deep below the crust—between 70 and 700 km (40 and 400 mi) deep—a deep-focus earthquake will be produced. Shallow-focus earthquakes tend to be larger, and therefore more damaging, earthquakes. This is because they are closer to the surface where the rocks are stronger and build up more strain.
Seismologists know from observations that most earthquakes originate as shallow-focus earthquakes and most of them occur near plate boundaries—areas where the Earth’s crustal plates move against each other (see Plate Tectonics). Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subducts, or moves under another plate. See also Geology; Earth.
B Faults
Stress in the Earth’s crust creates faults, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an angle into the Earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the footwall, the fault is called a reverse fault. Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike-slip faults are another type of fault that produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past each other.
C Waves
The sudden movement of rocks along a fault causes vibrations that transmit energy through the Earth in the form of waves. Waves that travel in the rocks below the surface of the Earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, move the ground back and forth.
Earthquakes also contain surface waves that travel out from the epicenter along the surface of the Earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures.
Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km (about 4 mi) per second. P waves are followed by S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km (about 2 mi) per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists to locate the focus and epicenter of an earthquake. As P and S waves move through the interior of the Earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by glass. Seismologists examine this bending to determine where the earthquake originated.
On the surface of the Earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach. The second type of surface wave, the Love wave, causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements. Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves.
III CAUSES
Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the Earth’s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the Earth’s crust.
A Elastic Rebound Theory
In 1911 American seismologist Harry Fielding Reid studied the effects of the April 1906 California earthquake. He proposed the elastic rebound theory to explain the generation of certain earthquakes that scientists now know occur in tectonic areas, usually near plate boundaries. This theory states that during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault slips, movement in the crustal rock causes vibrations. The slip changes the local strain out into the surrounding rock. The change in strain leads to aftershocks (smaller earthquakes that occur after the initial earthquake), which are produced by further slips of the main fault or adjacent faults in the strained region. The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface. On each side of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations that propagate as seismic waves. After the earthquake, strain begins to build again until it is greater than the forces holding the rocks together, then the fault snaps again and causes another earthquake.
B Human Activities
Fault rupture is not the only cause of earthquakes; human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and firing underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily. While earthquakes caused by human activities may be harmful, they can also provide useful information. Prior to the Nuclear Test Ban treaty, scientists were able to analyze the travel and arrival times of P waves from known earthquakes caused by underground nuclear test blasts. Scientists used this information to study earthquake waves and determine the interior structure of the Earth.
Scientists have determined that as water level in a reservoir increases, water pressure in pores inside the rocks along local faults also increases. The increased pressure may cause the rocks to slip, generating earthquakes. Beginning in 1935, the first detailed evidence of reservoir-induced earthquakes came from the filling of Lake Mead behind Hoover Dam on the Nevada-Arizona state border. Earthquakes were rare in the area prior to construction of the dam, but seismographs registered at least 600 shallow-focus earthquakes between 1936 and 1946. Most reservoirs, however, do not cause earthquakes.
IV DISTRIBUTION
Seismologists have been monitoring the frequency and locations of earthquakes for most of the 20th century. Seismologists generally classify naturally occurring earthquakes into one of two categories: interplate and intraplate. Interplate earthquakes are the most common; they occur primarily along plate boundaries. Intraplate earthquakes occur where the crust is fracturing within a plate. Both interplate and intraplate earthquakes may be caused by tectonic or volcanic forces.
A Tectonic Earthquakes
Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a fault. The released energy is produced by the strain on the rocks due to movement within the Earth, called tectonic deformation. The effect is like the sudden breaking and snapping back of a stretched elastic band.
B Volcanic Earthquakes
Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and at isolated hot spots such as Hawaii.
V LOCATIONS
Seismologists use global networks of seismographic stations to accurately map the focuses of earthquakes around the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake types, and the movement of the Earth’s rocky crust, scientists proposed that plate tectonics, or the shifting of the plates as they move over another weaker rocky layer, was the main underlying cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories and discoveries. Scientists now use the plate tectonics theory to describe the movement of the Earth’s plates and how this movement causes earthquakes. They also use the knowledge of plate tectonics to explain the locations of earthquakes, mountain formation, and deep ocean trenches, and to predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes occur most frequently in areas with features that are found at plate boundaries: high mountain ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare compared with the thousands of earthquakes that occur at plate boundaries each year, but they can be very large and damaging.
Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of 80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1,000 tremors greater than 3.5 in magnitude each year. The western coasts of North and South America are very also active earthquake zones, with several thousand small to moderate earthquakes each year.
Intraplate earthquakes are less frequent than plate boundary earthquakes, but they are still caused by the internal fracturing of rock masses. The New Madrid, Missouri, earthquakes of 1811 and 1812 were extreme examples of intraplate seismic events. Scientists estimate that the three main earthquakes of this series were about magnitude 8.0 and that there were at least 1,500 aftershocks.
VI EFFECTS
Ground shaking leads to landslides and other soil movement. These are the main damage-causing events that occur during an earthquake. Primary effects that can accompany an earthquake include property damage, loss of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of food and clean water, also occur after a large earthquake.
A Ground Shaking and Landslides
Earthquake waves make the ground move, shaking buildings and causing poorly designed or weak structures to partially or totally collapse. The ground shaking weakens soils and foundation materials under structures and causes dramatic changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil beneath structures and buildings weakens. Shaking may also dislodge large earth and rock masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or further property damage.
B Fire
Another post-earthquake threat is fire, such as the fires that happened in San Francisco after the 1906 earthquake and after the devastating 1923 Tokyo earthquake. In the 1923 earthquake, about 130,000 lives were lost in Tokyo, Yokohama, and other cities, many in firestorms fanned by high winds. The amount of damage caused by post-earthquake fire depends on the types of building materials used, whether water lines are intact, and whether natural gas mains have been broken. Ruptured gas mains may lead to numerous fires, and fire fighting cannot be effective if the water mains are not intact to transport water to the fires. Fires can be significantly reduced with pre-earthquake planning, fire-resistant building materials, enforced fire codes, and public fire drills.
C Tsunami Waves and Flooding
Along the coasts, sea waves called tsunamis that accompany some large earthquakes centered under the ocean can cause more death and damage than ground shaking. Tsunamis are usually made up of several oceanic waves that travel out from the slipped fault and arrive one after the other on shore. They can strike without warning, often in places very distant from the epicenter of the earthquake. Tsunami waves are sometimes inaccurately referred to as tidal waves, but tidal forces do not cause them. Rather, tsunamis occur when a major fault under the ocean floor suddenly slips. The displaced rock pushes water above it like a giant paddle, producing powerful water waves at the ocean surface. The ocean waves spread out from the vicinity of the earthquake source and move across the ocean until they reach the coastline, where their height increases as they reach the continental shelf, the part of the Earth’s crust that slopes, or rises, from the ocean floor up to the land. Tsunamis wash ashore with often disastrous effects such as severe flooding, loss of lives due to drowning, and damage to property.
Earthquakes can also cause water in lakes and reservoirs to oscillate, or slosh back and forth. The water oscillations are called seiches (pronounced saysh). Seiches can cause retaining walls and dams to collapse and lead to flooding and damage downstream.
D Disease
Catastrophic earthquakes can create a risk of widespread disease outbreaks, especially in underdeveloped countries. Damage to water supply lines, sewage lines, and hospital facilities as well as lack of housing may lead to conditions that contribute to the spread of contagious diseases, such as influenza (the flu) and other viral infections. In some instances, lack of food supplies, clean water, and heating can create serious health problems as well.
VII REDUCING DAMAGE
Earthquakes cannot be prevented, but the damage they cause can be greatly reduced with communication strategies, proper structural design, emergency preparedness planning, education, and safer building standards. In response to the tragic loss of life and great cost of rebuilding after past earthquakes, many countries have established earthquake safety and regulatory agencies. These agencies require codes for engineers to use in order to regulate development and construction. Buildings built according to these codes survive earthquakes better and ensure that earthquake risk is reduced.
Tsunami early warning systems can prevent some damage because tsunami waves travel at a very slow speed. Seismologists immediately send out a warning when evidence of a large undersea earthquake appears on seismographs. Tsunami waves travel slower than seismic P and S waves—in the open ocean, they move about ten times slower than the speed of seismic waves in the rocks below. This gives seismologists time to issue tsunami alerts so that people at risk can evacuate the coastal area as a preventative measure to reduce related injuries or deaths. Scientists radio or telephone the information to the Tsunami Warning Center in Honolulu and other stations.
Engineers minimize earthquake damage to buildings by using flexible, reinforced materials that can withstand shaking in buildings. Since the 1960s, scientists and engineers have greatly improved earthquake-resistant designs for buildings that are compatible with modern architecture and building materials. They use computer models to predict the response of the building to ground shaking patterns and compare these patterns to actual seismic events, such as in the 1994 Northridge, California, earthquake and the 1995 Kōbe, Japan, earthquake. They also analyze computer models of the motions of buildings in the most hazardous earthquake zones to predict possible damage and to suggest what reinforcement is needed. See also Engineering: Civil Engineering.
A Structural Design
Geologists and engineers use risk assessment maps, such as geologic hazard and seismic hazard zoning maps, to understand where faults are located and how to build near them safely. Engineers use geologic hazard maps to predict the average ground motions in a particular area and apply these predicted motions during engineering design phases of major construction projects. Engineers also use risk assessment maps to avoid building on major faults or to make sure that proper earthquake bracing is added to buildings constructed in zones that are prone to strong tremors. They can also use risk assessment maps to aid in the retrofit, or reinforcement, of older structures.
In urban areas of the world, the seismic risk is greater in nonreinforced buildings made of brick, stone, or concrete blocks because they cannot resist the horizontal forces produced by large seismic waves. Fortunately, single-family timber-frame homes built under modern construction codes resist strong earthquake shaking very well. Such houses have laterally braced frames bolted to their foundations to prevent separation. Although they may suffer some damage, they are unlikely to collapse because the strength of the strongly jointed timber-frame can easily support the light loads of the roof and the upper stories even in the event of strong vertical and horizontal ground motions.
B Emergency Preparedness Plans
Earthquake education and preparedness plans can help significantly reduce death and injury caused by earthquakes. People can take several preventative measures within their homes and at the office to reduce risk. Supports and bracing for shelves reduce the likelihood of items falling and potentially causing harm. Maintaining an earthquake survival kit in the home and at the office is also an important part of being prepared.
In the home, earthquake preparedness includes maintaining an earthquake kit and making sure that the house is structurally stable. The local chapter of the American Red Cross is a good source of information for how to assemble an earthquake kit. During an earthquake, people indoors should protect themselves from falling objects and flying glass by taking refuge under a heavy table. After an earthquake, people should move outside of buildings, assemble in open spaces, and prepare themselves for aftershocks. They should also listen for emergency bulletins on the radio, stay out of severely damaged buildings, and avoid coastal areas in the event of a tsunami.
In many countries, government emergency agencies have developed extensive earthquake response plans. In some earthquake hazardous regions, such as California, Japan, and Mexico City, modern strong motion seismographs in urban areas are now linked to a central office. Within a few minutes of an earthquake, the magnitude can be determined, the epicenter mapped, and intensity of shaking information can be distributed via radio to aid in response efforts.
VIII STUDYING EARTHQUAKES
Seismologists measure earthquakes to learn more about them and to use them for geological discovery. They measure the pattern of an earthquake with a machine called a seismograph. Using multiple seismographs around the world, they can accurately locate the epicenter of the earthquake, as well as determine its magnitude, or size, and fault slip properties.
A Measuring Earthquakes
An analog seismograph consists of a base that is anchored into the ground so that it moves with the ground during an earthquake, and a spring or wire that suspends a weight, which remains stationary during an earthquake. In older models, the base includes a rotating roll of paper, and the stationary weight is attached to a stylus, or writing utensil, that rests on the roll of paper. During the passage of a seismic wave, the stationary weight and stylus record the motion of the jostling base and attached roll of paper. The stylus records the information of the shaking seismograph onto the paper as a seismogram. Scientists also use digital seismographs, computerized seismic monitoring systems that record seismic events. Digital seismographs use rewriteable, or multiple-use, disks to record data. They usually incorporate a clock to accurately record seismic arrival times, a printer to print out digital seismograms of the information recorded, and a power supply. Some digital seismographs are portable; seismologists can transport these devices with them to study aftershocks of a catastrophic earthquake when the networks upon which seismic monitoring stations depend have been damaged.
There are more than 1,000 seismograph stations in the world. One way that seismologists measure the size of an earthquake is by measuring the earthquake’s seismic magnitude, or the amplitude of ground shaking that occurs. Seismologists compare the measurements taken at various stations to identify the earthquake’s epicenter and determine the magnitude of the earthquake. This information is important in order to determine whether the earthquake occurred on land or in the ocean. It also helps people prepare for resulting damage or hazards such as tsunamis. When readings from a number of observatories around the world are available, the integrated system allows for rapid location of the epicenter. At least three stations are required in order to triangulate, or calculate, the epicenter. Seismologists find the epicenter by comparing the arrival times of seismic waves at the stations, thus determining the distance the waves have traveled. Seismologists then apply travel-time charts to determine the epicenter. With the present number of worldwide seismographic stations, many now providing digital signals by satellite, distant earthquakes can be located within about 10 km (6 mi) of the epicenter and about 10 to 20 km (6 to 12 mi) in focal depth. Special regional networks of seismographs can locate the local epicenters within a few kilometers.
All magnitude scales give relative numbers that have no physical units. The first widely used seismic magnitude scale was developed by the American seismologist Charles Richter in 1935. The Richter scale measures the amplitude, or height, of seismic surface waves. The scale is logarithmic, so that each successive unit of magnitude measure represents a tenfold increase in amplitude of the seismogram patterns. This is because ground displacement of earthquake waves can range from less than a millimeter to many meters. Richter adjusted for this huge range in measurements by taking the logarithm of the recorded wave heights. So, a magnitude 5 Richter measurement is ten times greater than a magnitude 4; while it is 10 x 10, or 100 times greater than a magnitude 3 measurement.
Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1906 San Francisco earthquake was 7.6; the Alaskan earthquake of 1964, about 9.0; and the 1995 Kōbe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitudes were 8.3, 9.2, and 6.8, respectively for these tremors.
Earthquake size can be measured by seismic intensity as well, a measure of the effects of an earthquake. Before the advent of seismographs, people could only judge the size of an earthquake by its effects on humans or on geological or human-made structures. Such observations are the basis of earthquake intensity scales first set up in 1873 by Italian seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were later superseded by the Mercalli scale, created in 1902 by Italian seismologist Giuseppe Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann adapted the standards set up by Giuseppe Mercalli to California conditions and created the Modified Mercalli scale. Many seismologists around the world still use the Modified Mercalli scale to measure the size of an earthquake based on its effects. The Modified Mercalli scale rates the ground shaking by a general description of human reactions to the shaking and of structural damage that occur during a tremor. This information is gathered from local reports, damage to specific structures, landslides, and peoples’ descriptions of the damage.
B Predicting Earthquakes
Seismologists try to predict how likely it is that an earthquake will occur, with a specified time, place, and size. Earthquake prediction also includes calculating how a strong ground motion will affect a certain area if an earthquake does occur. Scientists can use the growing catalogue of recorded earthquakes to estimate when and where strong seismic motions may occur. They map past earthquakes to help determine expected rates of repetition. Seismologists can also measure movement along major faults using global positioning satellites (GPS) to track the relative movement of the rocky crust of a few centimeters each year along faults. This information may help predict earthquakes. Even with precise instrumental measurement of past earthquakes, however, conclusions about future tremors always involve uncertainty. This means that any useful earthquake prediction must estimate the likelihood of the earthquake occurring in a particular area in a specific time interval compared with its occurrence as a chance event.
The elastic rebound theory gives a generalized way of predicting earthquakes because it states that a large earthquake cannot occur until the strain along a fault exceeds the strength holding the rock masses together. Seismologists can calculate an estimated time when the strain along the fault would be great enough to cause an earthquake. As an example, after the 1906 San Francisco earthquake, the measurements showed that in the 50 years prior to 1906, the San Andreas fault accumulated about 3.2 meters (10 feet) of displacement, or movement, at points across the fault. The maximum 1906 fault slip was 6.5 meters (21 feet), so it was suggested that 50 years x 6.5 meters/3.2 meters (21 feet/10 feet), about 100 years, would elapse before sufficient energy would again accumulate to produce a comparable earthquake.
Scientists have measured other changes along active faults to try and predict future activity. These measurements have included changes in the ability of rocks to conduct electricity, changes in ground water levels, and changes in variations in the speed at which seismic waves pass through the region of interest. None of these methods, however, has been successful in predicting earthquakes to date.
Seismologists have also developed field methods to date the years in which past earthquakes occurred. In addition to information from recorded earthquakes, scientists look into geologic history for information about earthquakes that occurred before people had instruments to measure them. This research field is called paleoseismology (paleo is Greek for “ancient”). Seismologists can determine when ancient earthquakes occurred.
C The Earth’s Interior
Seismologists also study earthquakes to learn more about the structure of the Earth’s interior. Earthquakes provide a rare opportunity for scientists to observe how the Earth’s interior responds when an earthquake wave passes through it. Measuring depths and geologic structures within the Earth using earthquake waves is more difficult for scientists than is measuring distances on the Earth’s surface. However, seismologists have used earthquake waves to determine that there are four main regions that make up the interior of the Earth: the crust, the mantle, and the inner and outer core.
The intense study of earthquake waves began during the last decades of the 19th century, when people began placing seismographs at observatories around the world. By 1897 scientists had gathered enough seismograms from distant earthquakes to identify that P and S waves had
traveled through the deep Earth. Seismologists studying these seismograms later in the late 19th and early 20th centuries discovered P wave and S wave shadow zones—areas on the opposite side of the Earth from the earthquake focus that P waves and S waves do not reach. These shadow zones showed that the waves were bouncing off some large geologic interior structures of the planet.
Seismologists used these measurements to begin interpreting the paths along which the earthquake waves traveled. In 1904 Croatian seismologist Andrija Mohoroviić showed that the paths of P and S waves indicated a rocky surface layer, or crust, overlying more rigid rocks below. He proposed that inside the Earth, the waves are reflected by discontinuities, chemical or structural changes of the rock. Because of his discovery, the interface between the crust and the mantle below it became known as the Mohoroviić, or Moho Discontinuity.
In 1906 Richard Dixon Oldham of the Geological Survey of India used the arrival times of seismic P and S waves to deduce that the Earth must have a large and distinct central core. He interpreted the interior structure by comparing the faster speed of P waves to S waves, and noting that P waves were bent by the discontinuities such as the Moho Discontinuity. In 1914 German American seismologist Beno Gutenberg used travel times of seismic waves reflected at this boundary between the mantle and the core to determine the value for the radius of the core to be about 3,500 km (about 2,200 mi). In 1936 Danish seismologist Inge Lehmann discovered a smaller center structure, the inner core of the Earth. She estimated it to have a radius of 1,216 km (755 mi) by measuring the travel times of waves produced by South Pacific earthquakes. As the waves passed through the Earth and arrived at the Danish observatory, she determined that their speed and arrival times indicated that they must have been deflected by an inner core structure. In further studies of earthquake waves, seismologists found that the outer core is liquid and the inner core is solid.
IX EXTRATERRESTRIAL QUAKES
Seismic events similar to earthquakes also occur on other planets and on their satellites. Scientific missions to Earth’s moon and to Mars have provided some information related to extraterrestrial quakes. The current Galileo mission to Jupiter’s moons may provide evidence of quakes on the moons of Jupiter.
Between 1969 and 1977, scientists conducted the Passive Seismic Experiment as part of the United States Apollo Program. Astronauts set up seismograph stations at five lunar sites. Each lunar seismograph detected between 600 and 3,000 moonquakes every year, a surprising result because the Moon has no tectonic plates, active volcanoes, or ocean trench systems. Most moonquakes had magnitudes less than about 2.0 on the Richter scale. Scientists used this information to determine the interior structure of the Moon and to examine the frequency of moonquakes.
Besides the Moon and the Earth, Mars is the only other planetary body on which seismographs have been placed. The Viking 1 and 2 spacecraft carried two seismographs to Mars in 1976. Unfortunately, the instrument on Viking 1 failed to return signals to Earth. The instrument on Viking 2 operated, but in one year, only one wave motion was detected. Scientists were unable to determine the interior structure of Mars with only this single event.
Q15:
Endocrine System
I INTRODUCTION
Endocrine System, group of specialized organs and body tissues that produce, store, and secrete chemical substances known as hormones. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another. Because of the hormones they produce, endocrine organs have a great deal of influence over the body. Among their many jobs are regulating the body's growth and development, controlling the function of various tissues, supporting pregnancy and other reproductive functions, and regulating metabolism.
Endocrine organs are sometimes called ductless glands because they have no ducts connecting them to specific body parts. The hormones they secrete are released directly into the bloodstream. In contrast, the exocrine glands, such as the sweat glands or the salivary glands, release their secretions directly to target areas—for example, the skin or the inside of the mouth. Some of the body's glands are described as endo-exocrine glands because they secrete hormones as well as other types of substances. Even some nonglandular tissues produce hormone-like substances—nerve cells produce chemical messengers called neurotransmitters, for example.
The earliest reference to the endocrine system comes from ancient Greece, in about 400 BC. However, it was not until the 16th century that accurate anatomical descriptions of many of the endocrine organs were published. Research during the 20th century has vastly improved our understanding of hormones and how they function in the body. Today, endocrinology, the study of the endocrine glands, is an important branch of modern medicine. Endocrinologists are medical doctors who specialize in researching and treating disorders and diseases of the endocrine system.
II COMPONENTS OF THE ENDOCRINE SYSTEM
The primary glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal body, and reproductive glands—the ovary and testis. The pancreas, an organ often associated with the digestive system, is also considered part of the endocrine system. In addition, some nonendocrine organs are known to actively secrete hormones. These include the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta. Almost all body cells can either produce or convert hormones, and some secrete hormones. For example, glucagon, a hormone that raises glucose levels in the blood when the body needs extra energy, is made in the pancreas but also in the wall of the gastrointestinal tract. However, it is the endocrine glands that are specialized for hormone production. They efficiently manufacture chemically complex hormones from simple chemical substances—for example, amino acids and carbohydrates—and they regulate their secretion more efficiently than any other tissues.
The hypothalamus, found deep within the brain, directly controls the pituitary gland. It is sometimes described as the coordinator of the endocrine system. When information reaching the brain indicates that changes are needed somewhere in the body, nerve cells in the hypothalamus secrete body chemicals that either stimulate or suppress hormone secretions from the pituitary gland. Acting as liaison between the brain and the pituitary gland, the hypothalamus is the primary link between the endocrine and nervous systems.
Located in a bony cavity just below the base of the brain is one of the endocrine system's most important members: the pituitary gland. Often described as the body’s master gland, the pituitary secretes several hormones that regulate the function of the other endocrine glands. Structurally, the pituitary gland is divided into two parts, the anterior and posterior lobes, each having separate functions. The anterior lobe regulates the activity of the thyroid and adrenal glands as well as the reproductive glands. It also regulates the body's growth and stimulates milk production in women who are breast-feeding. Hormones secreted by the anterior lobe include adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and prolactin. The anterior lobe also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain.
The posterior lobe of the pituitary gland contains the nerve endings (axons) from the hypothalamus, which stimulate or suppress hormone production. This lobe secretes antidiuretic hormones (ADH), which control water balance in the body, and oxytocin, which controls muscle contractions in the uterus.
The thyroid gland, located in the neck, secretes hormones in response to stimulation by TSH from the pituitary gland. The thyroid secretes hormones—for example, thyroxine and three-iodothyronine—that regulate growth and metabolism, and play a role in brain development during childhood.
The parathyroid glands are four small glands located at the four corners of the thyroid gland. The hormone they secrete, parathyroid hormone, regulates the level of calcium in the blood.
Located on top of the kidneys, the adrenal glands have two distinct parts. The outer part, called the adrenal cortex, produces a variety of hormones called corticosteroids, which include cortisol. These hormones regulate salt and water balance in the body, prepare the body for stress, regulate metabolism, interact with the immune system, and influence sexual function. The inner part, the adrenal medulla, produces catecholamines, such as epinephrine, also called adrenaline, which increase the blood pressure and heart rate during times of stress.
The reproductive components of the endocrine system, called the gonads, secrete sex hormones in response to stimulation from the pituitary gland. Located in the pelvis, the female gonads, the ovaries, produce eggs. They also secrete a number of female sex hormones, including estrogen and progesterone, which control development of the reproductive organs, stimulate the appearance of female secondary sex characteristics, and regulate menstruation and pregnancy.
Located in the scrotum, the male gonads, the testes, produce sperm and also secrete a number of male sex hormones, or androgens. The androgens, the most important of which is testosterone, regulate development of the reproductive organs, stimulate male secondary sex characteristics, and stimulate muscle growth.
The pancreas is positioned in the upper abdomen, just under the stomach. The major part of the pancreas, called the exocrine pancreas, functions as an exocrine gland, secreting digestive enzymes into the gastrointestinal tract. Distributed through the pancreas are clusters of endocrine cells that secrete insulin, glucagon, and somastatin. These hormones all participate in regulating energy and metabolism in the body.
The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle. Research has shown that disturbances in the secretion of melatonin are responsible, in part, for the jet lag associated with long-distance air travel.
III HOW THE ENDOCRINE SYSTEM WORKS
Hormones from the endocrine organs are secreted directly into the bloodstream, where special proteins usually bind to them, helping to keep the hormones intact as they travel throughout the body. The proteins also act as a reservoir, allowing only a small fraction of the hormone circulating in the blood to affect the target tissue. Specialized proteins in the target tissue, called receptors, bind with the hormones in the bloodstream, inducing chemical changes in response to the body’s needs. Typically, only minute concentrations of a hormone are needed to achieve the desired effect.
Too much or too little hormone can be harmful to the body, so hormone levels are regulated by a feedback mechanism. Feedback works something like a household thermostat. When the heat in a house falls, the thermostat responds by switching the furnace on, and when the temperature is too warm, the thermostat switches the furnace off. Usually, the change that a hormone produces also serves to regulate that hormone's secretion. For example, parathyroid hormone causes the body to increase the level of calcium in the blood. As calcium levels rise, the secretion of parathyroid hormone then decreases. This feedback mechanism allows for tight control over hormone levels, which is essential for ideal body function. Other mechanisms may also influence feedback relationships. For example, if an individual becomes ill, the adrenal glands increase the secretions of certain hormones that help the body deal with the stress of illness. The adrenal glands work in concert with the pituitary gland and the brain to increase the body’s tolerance of these hormones in the blood, preventing the normal feedback mechanism from decreasing secretion levels until the illness is gone.
Long-term changes in hormone levels can influence the endocrine glands themselves. For example, if hormone secretion is chronically low, the increased stimulation by the feedback mechanism leads to growth of the gland. This can occur in the thyroid if a person's diet has insufficient iodine, which is essential for thyroid hormone production. Constant stimulation from the pituitary gland to produce the needed hormone causes the thyroid to grow, eventually producing a medical condition known as goiter.
IV DISEASES OF THE ENDOCRINE SYSTEM
Endocrine disorders are classified in two ways: disturbances in the production of hormones, and the inability of tissues to respond to hormones. The first type, called production disorders, are divided into hypofunction (insufficient activity) and hyperfunction (excess activity). Hypofunction disorders can have a variety of causes, including malformations in the gland itself. Sometimes one of the enzymes essential for hormone production is missing, or the hormone produced is abnormal. More commonly, hypofunction is caused by disease or injury. Tuberculosis can appear in the adrenal glands, autoimmune diseases can affect the thyroid, and treatments for cancer—such as radiation therapy and chemotherapy—can damage any of the endocrine organs. Hypofunction can also result when target tissue is unable to respond to hormones. In many cases, the cause of a hypofunction disorder is unknown.
Hyperfunction can be caused by glandular tumors that secrete hormone without responding to feedback controls. In addition, some autoimmune conditions create antibodies that have the side effect of stimulating hormone production. Infection of an endocrine gland can have the same result.
Accurately diagnosing an endocrine disorder can be extremely challenging, even for an astute physician. Many diseases of the endocrine system develop over time, and clear, identifying symptoms may not appear for many months or even years. An endocrinologist evaluating a patient for a possible endocrine disorder relies on the patient's history of signs and symptoms, a physical examination, and the family history—that is, whether any endocrine disorders have been diagnosed in other relatives. A variety of laboratory tests—for example, a radioimmunoassay—are used to measure hormone levels. Tests that directly stimulate or suppress hormone production are also sometimes used, and genetic testing for deoxyribonucleic acid (DNA) mutations affecting endocrine function can be helpful in making a diagnosis. Tests based on diagnostic radiology show anatomical pictures of the gland in question. A functional image of the gland can be obtained with radioactive labeling techniques used in nuclear medicine.
One of the most common diseases of the endocrine systems is diabetes mellitus, which occurs in two forms. The first, called diabetes mellitus Type 1, is caused by inadequate secretion of insulin by the pancreas. Diabetes mellitus Type 2 is caused by the body's inability to respond to insulin. Both types have similar symptoms, including excessive thirst, hunger, and urination as well as weight loss. Laboratory tests that detect glucose in the urine and elevated levels of glucose in the blood usually confirm the diagnosis. Treatment of diabetes mellitus Type 1 requires regular injections of insulin; some patients with Type 2 can be treated with diet, exercise, or oral medication. Diabetes can cause a variety of complications, including kidney problems, pain due to nerve damage, blindness, and coronary heart disease. Recent studies have shown that controlling blood sugar levels reduces the risk of developing diabetes complications considerably.
Diabetes insipidus is caused by a deficiency of vasopressin, one of the antidiuretic hormones (ADH) secreted by the posterior lobe of the pituitary gland. Patients often experience increased thirst and urination. Treatment is with drugs, such as synthetic vasopressin, that help the body maintain water and electrolyte balance.
Hypothyroidism is caused by an underactive thyroid gland, which results in a deficiency of thyroid hormone. Hypothyroidism disorders cause myxedema and cretinism, more properly known as congenital hypothyroidism. Myxedema develops in older adults, usually after age 40, and causes lethargy, fatigue, and mental sluggishness. Congenital hypothyroidism, which is present at birth, can cause more serious complications including mental retardation if left untreated. Screening programs exist in most countries to test newborns for this disorder. By providing the body with replacement thyroid hormones, almost all of the complications are completely avoidable.
Addison's disease is caused by decreased function of the adrenal cortex. Weakness, fatigue, abdominal pains, nausea, dehydration, fever, and hyperpigmentation (tanning without sun exposure) are among the many possible symptoms. Treatment involves providing the body with replacement corticosteroid hormones as well as dietary salt.
Cushing's syndrome is caused by excessive secretion of glucocorticoids, the subgroup of corticosteroid hormones that includes hydrocortisone, by the adrenal glands. Symptoms may develop over many years prior to diagnosis and may include obesity, physical weakness, easily bruised skin, acne, hypertension, and psychological changes. Treatment may include surgery, radiation therapy, chemotherapy, or blockage of hormone production with drugs.
Thyrotoxicosis is due to excess production of thyroid hormones. The most common cause for it is Graves' disease, an autoimmune disorder in which specific antibodies are produced, stimulating the thyroid gland. Thyrotoxicosis is eight to ten times more common in women than in men. Symptoms include nervousness, sensitivity to heat, heart palpitations, and weight loss. Many patients experience protruding eyes and tremors. Drugs that inhibit thyroid activity, surgery to remove the thyroid gland, and radioactive iodine that destroys the gland are common treatments.
Acromegaly and gigantism both are caused by a pituitary tumor that stimulates production of excessive growth hormone, causing abnormal growth in particular parts of the body. Acromegaly is rare and usually develops over many years in adult subjects. Gigantism occurs when the excess of growth hormone begins in childhood.
Last edited by Last Island; Sunday, December 30, 2007 at 07:17 PM.
