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板蓝根除清热解毒、利咽外，还有的功效是
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六味地黄丸主治
瞿麦除利尿通淋外，还有的功效是
HUMAN MIGRATION 1 The long-term movement of individuals, families, or larger groups to a new location outside their community of origin is known as migration. Human migration occurs on various geographic scales: from one continent or country to another, between regions within a single country, and from one city neighborhood to another. Several factors stimulate migration, including economic conditions, political conflict, war, cultural circumstances, and environmental factors. 2 People migrate from source to destination in well-defined streams. Many migration streams actually consist of a series of stages, a phenomenon known as step migration. For example, a peasant family from the countryside is likely to move first to a village, then to a nearby town, later to a city, and finally to a metropolis--the capital or the largest city in the region. The intensity of a migration stream depends on such factors as the physical distance and the degree of difference between the source and the destination. It also depends on the flow of information from the destination back to the source. People are likely to have more complete and accurate information about nearby places than about places that are farther away. 3 The decision to move is the result of various stimuli, which social scientists classify as "push" and "pull" factors. Push factors are the conditions that impel people to leave their home communities. The lack of jobs or educational opportunities, political fear, ethnic or religious discrimination, and natural disasters are all examples of push factors. Pull factors are the circumstances that attract people to certain destinations, such as better living standards, the chance of getting a job, and family connections. The circumstances that induce people to move from one part of the world to another--economic, political, and environmental conditions--usually involve a combination of push and pull factors. Because people are usually more familiar with their home community than with a desired destination, they are likely to understand push factors more accurately than pull factors. Pull factors tend to be more vague, and people often have overly optimistic expectations of their destination. 4 Economic conditions are a leading factor in human migration. Throughout history, poverty has driven millions of people from their homelands. Industrialization has attracted populations to urban areas in search of economic opportunity. The flow from farms or villages to the expanding metropolitan and industrial centers has occurred both within and between countries. During the twentieth century, Russians moved into the new industrial centers in Siberia, Chinese migrated to Manchuria and Southeast Asia, and Africans moved from tribal areas into the mining regions of South Africa and Congo. Today, perceived opportunities in destinations such as Western Europe and North America encourage numerous migrants to search for a better life. Some workers migrate only seasonally or temporarily. Especially in newly industrializing areas, workers tend to retain their village roots and return home after a period of earning in a factory or mine. However, most migrants relocate permanently, and the growing urban populations worldwide are composed of people who have cut themselves off from their roots. 5 The twentieth century saw an increase in migratory flows caused by the push factors of political oppression, revolution, and war. Refugees fled from Russia after the 1917 revolution, from Germany and Italy during the Nazi and Fascist regimes, and from Eastern Europe after the Second World War. Millions of people were uprooted as a result of political, cultural, and religious conflict. The partition of India and Pakistan in 1947 led to the uprooting and resettlement of around 14 million Muslims and Hindus--the largest single movement of people in a short period. The armed conflict in the former Yugoslavia during the 1990s drove as many as three million people from their homes. In the same decade, a civil war in Rwanda forced more than two million Rwandans from their homeland. 6 Reasons for migration include environmental conditions, often in combination with economic and political problems. A major historical example is the Irish famine of the 1840s, when prolonged rains and blight destroyed the potato crop. The resulting famine, along with the oppressive political system, caused hundreds of thousands of peasants to migrate from Ireland to North America. In recent decades, a series of droughts resulting from successive rainless seasons in sub-Saharan Africa, combined with such factors as ethnic strife and civil war, have caused large-scale migrations and a growing refugee crisis in the region. The word vague in paragraph 3 is closest in meaning to
ECTOTHERMY AND ENDOTHERMY 1 Many biological systems are based on the process of homeostasis, which means "steady state." Homeostasis is the ability to maintain balance. Homeostatic mechanisms enable animals to survive changes in their external environment by regulating conditions within their bodies. Conditions in the external environment, such as temperature, may vary widely, but conditions in the internal environment can vary only within a narrow range necessary for survival. 2 Temperature is a constraint for animals, all of which must maintain biochemical stability. When an animal’s body temperature drops too low, its metabolism slows, thus reducing the amount of energy the animal can use for activity. If body temperature rises too high, metabolic reactions become unbalanced, and enzyme activity is hindered. Animals can succeed only in a limited range of body temperatures, and for most, this is between 0 and 40 degrees Celsius. 3 One way to classify animals is to emphasize their source of body heat. For instance, "cold-blooded" animals are those that must warm their body with heat from the surrounding environment, and "warm-blooded" animals are those that can heat themselves. However, these traditional terms are inaccurate and misleading. Some "cold-blooded" animals, such as lizards, have higher body temperatures when active than many "warm-blooded" animals have when hibernating. Physiologists prefer the terms "ectotherm" and "endotherm" because they reflect the fact that an animal’s body temperature is a balance between heat loss and heat gain. 4 All animals produce heat from cellular metabolism, but in most the heat is conducted away as fast as it is produced, so the amount of heat obtained from metabolism is very small. In these animals, the ectotherms, body temperature is determined almost entirely by their surroundings. Most invertebrates, fishes, amphibians, and reptiles are ectotherms. In contrast, some animals are able to generate and retain enough heat from metabolism to elevate their own body temperature to the optimum level. These animals are called the endotherms because the source of their body heat is internal. Mammals, birds, some fishes, and numerous insects are endotherms. 5 Ectotherms warm their body mainly by absorbing heat from their environment. Ectotherms cannot control their body temperature physiologically, yet many are able to regulate it behaviorally by selecting areas of the environment with a more favorable temperature. Some, such as desert lizards, exploit hour-to-hour changes in solar radiation to keep their body temperature relatively constant. In the morning, the lizard absorbs the sun’s heat through its head, while keeping the rest of its body protected from the cool air. Later, the lizard will emerge to bask in the sun. At noon, with its body temperature high, it seeks shade under a rock. When the air temperature drops in the late afternoon, it emerges and lies parallel to the sun’s rays. 6 Endotherms, on the other hand, derive most or all of their body heat from their own metabolism. A consistently warm body temperature requires active metabolism, which includes oxidation of foods, cellular respiration, and muscular contraction. Conversely, a warm body temperature contributes to the high levels of metabolism required for extended periods of intense physical activity. This is one reason endotherms can generally endure vigorous activity longer than ectotherms. However, being endothermic is energetically expensive, especially in a cold environment. Because much of an endotherm’s daily intake of calories is used to generate heat, the endotherm must eat more food than an ectotherm of the same size. Endothermy allows birds and mammals to stabilize their internal temperature so that biochemical processes and nervous system functions can proceed at steady levels of activity. These animals maintain a constant body temperature through a delicate balance between heat production and heat loss. This is why endotherms can remain active in winter and exploit habitats denied to ectotherms. If the animal becomes too cool, it can generate heat by increasing muscular activity (exercise or shivering), or it can decrease heat loss by increasing insulation. In general, birds and mammals are warmer than their surroundings, but they also have mechanisms for cooling the body in a hot environment. If the animal becomes too warm, it decreases heat production and increases heat loss by evaporative cooling (sweating or panting). All of the following are mentioned as advantages of endothermy EXCEPT
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