326 résultats pour "producir"

• La digestión - (exposé gratuit en espagnol).

  Lo que queda de papilla pasa al intestino grueso, que está formado por el colon y el recto. Allí termina de producirse la absorción y se forman las heces. El colon mide alrededor de 1 metro. Es más corto que el intestino delgado, pero más grueso. En su interior se termina de absorber el agua que contiene el quilo. Lo que queda de los alimentos no puede ser utilizado por tu organismo y forma las heces. Las hecesabandonan el colon y recorren el recto, un tubo pequeño que termina en un orifici...

• Endocrine System.

  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 thefurnace off. Usually, the change that a hormone produces also serves to regulate that hormone's secretion. For example, parathyroid hormone causes the body toincrease the level of calcium in the blood. As calcium levels rise, the secretion of parathyroid hormone then decreases. This feedback mechanism allows for tight controlover hormon...

• Deoxyribonucleic Acid - biology.

  bacterium Escherichia coli . In 1998 scientists achieved the milestone of sequencing the complete genome of a multicellular organism—a roundworm identified as Caenorhabditis elegans . The Human Genome Project, an international research collaboration, was established to determine the sequence of all of the 3 billion nucleotide base pairs that make up thehuman genetic material. In 2003 scientists completed the sequencing of the human genome. The project identified nearly all of the estimated 20...

• Cáncer (medicina) - ciencias de la naturaleza.

  Tumores del aparato reproductor femeninoEl aparato reproductor femenino puede verse afectado por diversas formas tumorales, algunas de ellas de desarrollo maligno. Enparticular, el tumor del cuello uterino representa la primera causa de muerte por tumor en las mujeres, después del cáncer de mama.Los fibromas y tumores de los ovarios suelen tener un pronóstico más favorable. Lo que resulta de fundamental importancia en eltratamiento de estas patologías es la precocidad del diagnóstico y, por tant...

• Cloning - biology.

  found that such embryo cells are totipotent (able to give rise to all the different cell types in the body). Exploiting this characteristic, scientists developed three techniques to clone embryo cells: blastomere separation, blastocyst division, and somatic cell nuclear transfer. A Blastomere Separation In blastomere separation, scientists fertilize an egg cell with a sperm cell in a laboratory dish. The resulting embryo is allowed to divide until it forms a mass of aboutfour cells. Scientist...

• Bacteria - biology.

  A2 b Bacterial Killers Some dramatic infectious diseases result from exposure to bacteria that are not part of our normal bacterial community. Cholera, one of the world’s deadliest diseasestoday, is caused by the bacterium Vibrio cholerae . Cholera is spread in water and food contaminated with the bacteria, and by people who have the disease. After entering the body, the cholera bacteria grow in the intestines, often along the surface of the intestinal wall, where they secrete a toxin (poiso...

• Heat (physics) I INTRODUCTION Heat Loss from a House A thermograph shows the large amount of heat lost through a house's windows during winter.

  Theoretically, the molecules of a substance would exhibit no activity at the temperature termed absolute zero. See Molecule. III TEMPERATURE SCALES Five different temperature scales are in use today: the Celsius scale, known also as the Centigrade scale, the Fahrenheit scale, the Kelvin scale, the Rankine scale, andthe international thermodynamic temperature scale ( see Thermometer). The Celsius scale, with a freezing point of 0° C and a boiling point of 100° C, is widely used throughout the...

• Animal Courtship and Mating - biology.

  activity. These animals frequently copulate in water, and the penis helps transfer sperm before it is carried away by the currents Ostriches and rheas also contain apenis within the cloaca. Virtually all snakes and lizards display yet another variation of internal fertilization. Located in the tail are two penises, called hemepenes, which are covered by spinesand knobs that help lock it into place during mating. The male uses one hemepene at a time for copulation. Male salamanders secrete large...

• Medio ambiente - ciencias de la naturaleza.

  aproximadamente. Lo significativo de este cambio es que puede provocar un aumento de la temperatura de la Tierra a través del proceso conocido como efecto invernadero.El dióxido de carbono atmosférico tiende a impedir que la radiación de onda larga escape al espacio exterior; dado que se produce más calor y puede escapar menos, latemperatura global de la Tierra aumenta. Un calentamiento global significativo de la atmósfera tendría graves efectos sobre el medio ambiente. Aceleraría la fusión de l...

• Spider (arthropod) - biology.

  The spider’s abdomen is soft and saclike. On the underside of the tip of the abdomen are three pairs of spinnerets. Each spinneret is studded with many fine, hairliketubes called spigots, which produce a variety of silk threads. The spigots lead to several large silk glands inside the abdomen. Silk is formed as a liquid inside theseabdominal glands. As the silk is drawn out through the spigots, protein molecules within the silk line up parallel to one another, causing the silk to harden and form...

• Culture.

  form of knowledge, such as scientific discoveries; objects, such as works of art; and traditions, such as the observance of holidays. C1 Ethnocentrism and Cultural Relativism Self-identity usually depends on culture to such a great extent that immersion in a very different culture—with which a person does not share common ways of life orbeliefs—can cause a feeling of confusion and disorientation. Anthropologists refer to this phenomenon as culture shock. In multicultural societies —societies s...

• Immune System.

  The humoral immune response involves a complex series of events after antigens enter the body. First, macrophages take up some of the antigen and attach it to classII MHC molecules, which then present the antigen to T helper cells. The T helper cells bind the presented antigen, which stimulates the T helper cells to divide andsecrete stimulatory molecules called interleukins. The interleukins in turn activate any B lymphocytes that have also bound the antigen. The activated B cells then divide...

• Genética - ciencias de la naturaleza.

  5 HERENCIA CUANTITATIVA Los caracteres que se expresan como variaciones en cantidad o extensión, como el peso, la talla o el grado de pigmentación, suelen depender de muchos genes, así comode las influencias del medio. Con frecuencia, los efectos de genes distintos parecen ser aditivos, es decir, parece que cada gen produce un pequeño incremento o descensoindependiente de los otros genes. Por ejemplo, la altura de una planta puede estar determinada por una serie de cuatro genes: A, B, C y D....

• Acid Rain.

  the Adirondack Mountains of New York State, a quarter of the lakes and ponds are acidic, and many have lost their brook trout and other fish. In the middleAppalachian Mountains, over 1,300 streams are afflicted. All of Norway’s major rivers have been damaged by acid rain, severely reducing salmon and trout populations. E Plants and Animals The effects of acid rain on wildlife can be far-reaching. If a population of one plant or animal is adversely affected by acid rain, animals that feed on tha...

• Light - astronomy.

  Each different frequency or wavelength of visible light causes our eye to see a slightly different color. The longest wavelength we can see is deep red at about 700 nm.The shortest wavelength humans can detect is deep blue or violet at about 400 nm. Most light sources do not radiate monochromatic light. What we call white light,such as light from the Sun, is a mixture of all the colors in the visible spectrum, with some represented more strongly than others. Human eyes respond best to greenlight...

• Frog (animal).

  which means that their body temperature depends on the temperature of the surrounding environment. Few species can tolerate temperatures below 4°C (40°F) orabove 40°C (104°F), and many species can survive only within a narrower range of temperatures. In addition, frogs’ thin, moist skin offers little protection againstwater loss, and when on land the animals must guard against drying out. Many frogs are active at night because temperatures are cooler and humidity is higher thanduring the day. In...

• Frog (animal) - biology.

  which means that their body temperature depends on the temperature of the surrounding environment. Few species can tolerate temperatures below 4°C (40°F) orabove 40°C (104°F), and many species can survive only within a narrower range of temperatures. In addition, frogs’ thin, moist skin offers little protection againstwater loss, and when on land the animals must guard against drying out. Many frogs are active at night because temperatures are cooler and humidity is higher thanduring the day. In...

• Weather.

  hours, and the snow can be much deeper in places where the wind piles it up in drifts. Extraordinarily deep snows sometimes accumulate on the upwind side ofmountain slopes during severe winter storms or on the downwind shores of large lakes during outbreaks of polar air. VI WIND Wind is the horizontal movement of air. It is named for the direction from which it comes—for example, a north wind comes from the north. In most places near theground, the wind speed averages from 8 to 24 km/h (from 5...

• Cell (biology) - biology.

  proteins, or other proteins required by the cell. While relatively simple in construction, prokaryotic cells display extremely complex activity. They have a greater range of biochemical reactions than those found in theirlarger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria andthe bacteria, whose habitats include polar ice, deserts, and hydrothermal vents—deep regions of the ocean un...

• Reproduction - biology.

  E Gestation After fertilization of the egg, the resulting zygote undergoes cell division and differentiation to form the embryo. In most higher plants, the embryo is enclosed in a layerof nutritive material surrounded by a hard outer covering, forming the seed. In most lower animals the embryo, surrounded by the nutritive material of the formerovum, is enveloped by a leathery or calcareous shell and is extruded from the body of the female. Animals, such as birds, that lay their eggs before the...

• Chemical Reaction - chemistry.

  Many groups of elements occur so often as ions that they are given names: nitrate, NO 3-; sulfate, SO 42-; and phosphate, PO 43-. The suffix -ate usually indicates the presence of oxygen. The positive ion, NH 4+, is called ammonium, as in NH 4Cl, ammonium chloride, or (NH 4)3PO4, ammonium phosphate. Rules for naming more complicated compounds exist, but many compounds have been given trivial names—for example, Na 2B4O7·10 H 2O, borax—or proprietary names—F(CF 2)nF, Teflon. These nonsystemat...

• Brazil - country.

  occasional droughts. Brazil contains a wealth of mineral and plant resources that have not yet been fully explored. It possesses some of the world’s largest deposits of iron ore and containsrich deposits of many other minerals, including gold and copper. Brazil’s fossil fuel resources are modest, but this limitation is offset by the considerable hydroelectricpotential of the nation’s many rivers. Although Brazil is an important producer of tropical crops, areas of highly fertile land are limited...

• Fecundación - ciencias de la naturaleza.

  evolutiva lógica de los hábitos reproductivos. Mientras que grupos muy diferentes de animales emplean los mismos métodos de fecundación, otros relacionadosestrechamente difieren mucho en ellos. En general, la fecundación externa se produce con más frecuencia en los seres acuáticos y anfibios que en los terrestres, ya que lafecundación debe tener lugar en un medio húmedo, y el embrión resultante debe ser protegido de la sequedad. En esencia, la reproducción sexual en las plantas es similar a la d...

• Contaminación atmosférica - ciencias de la naturaleza.

  nitrógeno, y de un 30 a un 40% de las partículas en suspensión emitidos a la atmósfera en Estados Unidos proceden de las centrales eléctricas que queman combustiblesfósiles, las calderas industriales y las calefacciones. Un 80% del monóxido de carbono y un 40% de los óxidos de nitrógeno e hidrocarburos emitidos proceden de lacombustión de la gasolina y el gasóleo en los motores de los coches y camiones. Otras importantes fuentes de contaminación son la siderurgia y las acerías, las fundicionesde...

• Star (astronomy) - astronomy.

  absorbing the missing colors of light. For example, the set of dark lines made by hydrogen includes a dark red line, the set of dark lines made by sodium includes a pairof dark yellow lines, and the set of dark lines made by iron includes lines of nearly every color. Each element in the gaseous outer layer of a star produces its ownparticular pattern of dark spectrum lines, depending on the temperature and pressure of the gas. Astronomers have observed spectrum lines, or spectra, for hundredsof...

• Colorado - geography.

  Although the rivers of Colorado are navigable only by small boats, they are important as a source of irrigation water for use in Colorado and adjoining states. However,the water level of the rivers fluctuates seasonally and from year to year. The level is generally low in winter and high in spring and summer, during the runoff of meltedsnow from the mountains. Colorado has no large lakes of natural origin, but there are numerous small lakes in the mountains. The largest bodies of water in Colora...

• Colorado - USA History.

  Although the rivers of Colorado are navigable only by small boats, they are important as a source of irrigation water for use in Colorado and adjoining states. However,the water level of the rivers fluctuates seasonally and from year to year. The level is generally low in winter and high in spring and summer, during the runoff of meltedsnow from the mountains. Colorado has no large lakes of natural origin, but there are numerous small lakes in the mountains. The largest bodies of water in Colora...

• Hazardous Wastes.

  A Source Reduction The best way to eliminate hazardous wastes is not to generate them in the first place. For example, improvements have been made in the production of integratedcircuits: The toxic chlorinated hydrocarbons commonly used in the 1970s were replaced in the 1980s by less toxic glycol ethers and in the 1990s by low-toxicity estersand alcohols. B Recycling Recycling is the recovery or reuse of usable materials from waste. About 5 percent of hazardous waste in the United States is re...

• Hormone.

  women is called a follicle-stimulating hormone. When a follicle-stimulating hormone binds to an ovary cell, it stimulates the enzymes needed for the synthesis ofestradiol, a female sex hormone. Another gonadotropin called luteinizing hormone regulates the production of eggs in women and the production of the male sexhormone testosterone. Produced in the male gonads, or testes, testosterone regulates changes to the male body during puberty, influences sexual behavior, and playsa role in growth. T...

• Electronics.

  electronic circuits. The resistors are made from carbon mixtures, metal films, or resistance wire and have two connecting wires attached. Variable resistors, with anadjustable sliding contact arm, are often used to control volume on radios and television sets. E Capacitors Capacitors consist of two metal plates that are separated by an insulating material ( see Capacitor). If a battery is connected to both plates, an electric charge will flow for a short time and accumulate on each plate. If th...

• Observatory - astronomy.

  Ultraviolet radiation, X rays, and gamma rays have shorter wavelengths than visible light has. These types of radiation tell astronomers about the hottest and mostviolent phenomena in the universe. Earth’s atmosphere blocks most of this radiation, so astronomers must send their observatories above the atmosphere aboardballoons, rockets, or satellites. Ultraviolet telescopes are much like visible light telescopes, but X-ray telescopes must have special nested cylindrical mirrors to prevent Xrays...

• Conservation.

  cancers, Hodgkin’s disease and leukemia. Unfortunately, human activities have greatly reduced biodiversity around the world. The 20th century encompasses one of the greatest waves of extinction, orelimination of species, to occur on the planet. The greatest threat to biodiversity is loss of habitat as humans develop land for agriculture, grazing livestock, industry,and habitation. The most drastic damage has occurred in the tropical rain forests, which cover less than seven percent of the Earth’...

• Tiger - biology.

  have a simple digestive system designed to process meat so that the nutrients can be readily absorbed into the bloodstream. With the exception of white tigers, which have blue eyes, all tigers have yellow eyes. Tigers mainly use vision to find prey. Although tigers see about as well as humansduring the day, their large eye openings gather more light than do human eyes, making tiger night vision far superior to that of humans. In addition, a special structurein the tiger’s eye, called the tapetu...

• Pottery I INTRODUCTION Pottery, clay that is chemically altered and permanently hardened by firing in a kiln.

  basket, or a clay or plaster form. Liquid clay can be poured into plaster molds. A pot can be coil built: Clay is rolled between the palms of the hands and extended intolong coils, a coil is formed into a ring, and the pot is built up by superimposing rings. Also, a ball of clay can be pinched into the desired shape. The most sophisticatedpottery-making technique is wheel throwing. The potter's wheel, invented in the 4th millennium BC, is a flat disk that revolves horizontally on a pivot. Both...

• Radar.

  gathers the weak returning radar signals and converts them into an electric current. Because a radar antenna may both transmit and receive signals, the duplexerdetermines whether the antenna is connected to the receiver or the transmitter. The receiver determines whether the signal should be reported and often does furtheranalysis before sending the results to the display. The display conveys the results to the human operator through a visual display or an audible signal. B1 The Antenna The rec...

• Insect - biology.

  they almost always have six legs. In some insects, such as beetles, the legs are practically identical, but in other insects each pair is a slightly different shape. Still otherinsects have specialized leg structures. Examples are praying mantises, which have grasping and stabbing forelegs armed with lethal spines, and grasshoppers andfleas, which have large, muscular hind legs that catapult them into the air. Mole crickets’ front legs are modified for digging, and backswimmers have hind legs de...

• Prokaryote - biology.

  earliest organisms to evolve, an estimated 3.4 billion to 3.5 billion years ago. The environment of the early Earth lacked oxygen, and cyanobacteria probably usedfermentation (a chemical process performed without the presence of oxygen) to produce the energy molecule adenosine triphosphate (ATP). Cyanobacteria introduced oxygen into the atmosphere through the process of photosynthesis. As the oxygen content in the atmosphere increased over the centuries, bacteria evolved that usedthis oxygen in...

• Steven Spielberg I INTRODUCTION Steven Spielberg The imaginative films of Steven Spielberg are known for their technical creativity and memorable characters.

  Courtesy of Everett Collection Spielberg teamed up with writer-producer George Lucas in the 1980s to make the action-adventure Indiana Jones film series: Raiders of the Lost Ark (1981), Indiana Jones and the Temple of Doom (1984), and Indiana Jones and the Last Crusade (1989). Other directorial projects during this period included the science-fiction fantasy E.T.—The Extra-Terrestrial , at the time the highest-grossing film ever made; The Color Purple (1985), a drama based on the novel...

• Aparato circulatorio - ciencias de la naturaleza.

  es completa. Sus paredes musculares se contraen e impulsan todo su contenido a través de los orificios auriculoventriculares hacia los ventrículos. Este proceso es rápido yse produce casi de forma simultánea en ambas aurículas. La masa de sangre en las venas hace imposible el reflujo. La fuerza del flujo de la sangre en los ventrículos no eslo bastante poderosa para abrir las válvulas semilunares, pero distiende los ventrículos, que se encuentran aún en un estado de relajación. Las válvulas mitr...

• George Frideric Handel I INTRODUCTION Handel's Water Music In addition to his popular operas and oratorios, German-born composer George Frideric Handel wrote music in the 1700s for the church and for royal celebrations.

  During the 1720s and 1730s Handel worked primarily as a composer and producer of operas for the London stage. This extremely productive phase of his career beganwith the opening of the Royal Academy of Music in London in 1719. The Royal Academy was founded with the support of the king and aristocratic subscribers for theproduction of Italian operas. Its directors sent Handel to continental Europe to hire some of the world’s greatest singers. Handel was not the only composer writingoperas for Aca...

• Deforestación - ciencias de la naturaleza.

  las áreas boscosas de Inglaterra ya estaban deforestadas. A mediados del siglo XVII en la península Ibérica había desaparecido el 75% de los bosques. En la Europacontinental y en América del Norte, la deforestación se aceleró durante los siglos XVIII y XIX, con el fin de despejar tierras y dedicarlas a cultivos alimentarios paraabastecer a las ciudades industriales y hacer frente a las necesidades de combustible y de materiales de construcción. Desde entonces, la creciente productividad agrícola...

• Corazón - ciencias de la naturaleza.

  Implantación quirúrgica de una válvula cardiacaLos cirujanos realizan intervenciones a corazón abierto para abrir las arterias obstruidas y reparar las lesiones físicas del corazónoriginadas por algún proceso lesivo, como un infarto. Este tipo de cirugía también se puede utilizar para corregir cardiopatíascongénitas, incluyendo la reposición de válvulas cardiacas defectuosas.David Leah/Science Source/Photo Researchers, Inc. Los trastornos del corazón son responsables de mayor número de muertes q...

• Obesity.

  of a day, contributing to the development of obesity. V TREATMENTS FOR OBESITY Obesity can become a chronic lifelong condition caused by overeating, physical inactivity, and even genetic makeup. No matter what the cause, however, obesity can beprevented or managed with a combination of diet, exercise, behavior modification, and in severe cases, weight-loss medications and surgery. A Diets The most common and conservative treatment for obesity utilizes a nutritionally balanced, low calorie diet...

• Guyana - country.

  European patterns of living. People of mixed African and European ancestry form a distinct group in Guyana, maintaining closer social ties to the European communitythan to the African Guyanese community. Asians from the Indian subcontinent began to arrive in the 19th century, following the abolition of slavery in Guyana, to work as indentured and contract laborers. Theycontinued to arrive until 1917, when Britain outlawed indentured servitude. Thousands of Indians chose to remain in Guyana after...

• Water Pollution.

  Cryptosporidium in the water supply of Milwaukee, Wisconsin, sickened more than 400,000 people and killed more than 100. H Thermal Pollution Water is often drawn from rivers, lakes, or the ocean for use as a coolant in factories and power plants. The water is usually returned to the source warmer than when itwas taken. Even small temperature changes in a body of water can drive away the fish and other species that were originally present, and attract other species in placeof them. Thermal pol...

• Bahrain - country.

  A Religion Almost all Bahrainis and the majority of nonnatives are followers of Islam (Muslims). About 70 percent of all native Bahrainis belong to the Shia branch of Islam, whilethe remainder, including the ruling al-Khalifa clan, are adherents of the Sunni branch. Non-Muslims, including Hindus, Buddhists, Christians, and Jews, account for 15percent of the total population. High unemployment among the Shia population has caused considerable discontent on the part of this group toward the Sunni...

• Syria - country.

  D Education Primary education is free and compulsory for all children aged 6 through 12. Some 78 percent of the adult Syrian population was estimated to be literate in 2005.Primary schools enrolled 2.8 million pupils in the 2000 school year, and 1.1 million students attended secondary schools and vocational institutes. In 1998, 94,110 Syrian students were enrolled in institutes of higher education. Syria has universities in Damascus, Ḩalab, Ḩim ş, and Al L ādhiq īyah. Also in Damascus isthe Ar...

• Radioactivity I INTRODUCTION Marie Curie Working with her husband, Pierre Curie, French physicist Marie Curie discovered the radioactive elements polonium and radium in 1898.

  Rutherford when he allowed an alpha-emitting substance to decay near an evacuated thin-glass vessel. The alpha particles were able to penetrate the glass and werethen trapped in the vessel, and within a few days the presence of elemental helium was demonstrated by use of a spectroscope. Beta particles were subsequentlyshown to be electrons, and gamma rays to consist of electromagnetic radiation of the same nature as X rays but of considerably greater energy. A The Nuclear Hypothesis Rutherford...

• Capitalism.

  Physiocracy is the term applied to a school of economic thought that suggested the existence of a natural order in economics, one that does not require direction from the state for people to be prosperous. The leader of the physiocrats, the economist François Quesnay, set forth the basic principles in his Tableau économique (1758), in which he traced the flow of money and goods through the economy. Simply put, this flow was seen to be both circular and self-sustaining. More important, however...

• Metalwork I INTRODUCTION Metalwork, in the fine arts, objects of artistic, decorative, and utilitarian value made of one or more kinds of metal--from precious to base--fashioned by either casting, hammering, or joining or a combination of these techniques.

  Early Bronze DiskThis disk with the head of Acheloos, an Etruscan river god, was made of bronze, an alloy of copper and tin, sometime inthe early 5th century bc. It comes from the necropolis of Monte Quaglieri in Tarquinia. Alloys are made by smelting twodifferent metals together.Scala/Art Resource, NY Knowledge of smelting ultimately led to knowledge of mixing different ores together in the smelting process to produce simple alloys. This followed an intermediateperiod, about 3000 BC, when comp...