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  3. Up to the beginning of the twentieth century, the nervous system was thought to control all communication within the body and th

Up to the beginning of the twentieth century, the nervous system was thought to control all communication within the body and th

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问题     Up to the beginning of the twentieth century, the nervous system was thought to control all communication within the body and the resulting integration of behavior. Scientists had determined that nerves ran, essentially, on electrical impulses. These impulses were thought to be the engine for thought, emotion, movement, and internal processes such as digestion. However, experiments by William Bayliss and Ernest Starling on the chemical secretin, which is produced in the small intestine, secretin travels through the bloodstream to the pancreas. There, it stimulates the release of digestive chemicals. In this fashion, the intestinal cells that produce secretin ultimately regulate the production of different chemicals in a different organ, the pancreas.
    Such a coordination of processes had been thought to require control by the nervous system; Bayliss and Starling showed that it could occur through chemicals alone. This discovery spurred Starling to coin the term hormone to refer to secretin, taking it from the Greek word hormone, meaning "to excite" or "to set in motion. " A hormone is a chemical produced by one tissue to make things happen elsewhere.
    As more hormones were discovered, they were categorized, primarily according to the process by which they operated on the body. Some glands (which make up the endocrine system) secrete hormones directly into the bloodstream. Such glands include the thyroid and the pituitary. The exocrine system consists of organs and glands that produce substances that are used outside the bloodstream, primarily for digestion. The pancreas is one such organ, although it secretes some chemicals into the blood and thus is also part of the endocrine system.
    Much has been learned about hormones since their discovery. Some play such key roles in regulating bodily processes or behavior that their absence would cause immediate death. The most abundant hormones have effects that are less obviously urgent but can be more far-reaching and difficult to track: they modify moods and affect human behavior, even some behavior we normally think of as voluntary. Hormonal systems are very intricate. Even minute amounts of the right chemical can suppress appetite, calm aggression, and change the attitude of a parent toward a child. Certain hormones accelerate the development of the body, regulating growth and form; others may even define an individual’s personality characteristics. The quantities and proportions of hormones produced change with age, so scientists have given a great deal of study to shifts in the endocrine system over time in the hopes of alleviating ailments associated with aging.
    In fact, some hormone therapies are already very common. A combination of estrogen and progesterone has been prescribed for decades to women who want to reduce mood swings, sudden changes in body temperature, and other discomforts caused by lower natural levels of those hormones as they enter middle age. Known as hormone replacement therapy (HRT), the treatment was also believed to prevent weakening of the bones. At least one study has linked HRT with a heightened risk of heart disease and certain types of cancer. HRT may also increase the likelihood that blood clots—dangerous because they could travel through the bloodstream and block major blood vessels—will form. Some proponents of HRT have tempered their enthusiasm in the face of this new evidence, recommending it only to patients whose symptoms interfere with their abilities to live normal lives.
    Human growth hormone may also be given to patients who are secreting abnormally low amounts on their own. Because of the complicated effects growth hormone has on the body, such treatments are generally restricted to children who would be pathologically small in stature without it. Growth hormone affects not just physical size but also the digestion of food and the aging process. Researchers and family physicians tend to agree that it is foolhardy to dispense it in cases in which the risks are not clearly outweighed by the benefits.
The glands and organs mentioned in Paragraph 3 are categorized according to________.
选项 A、whether scientists understand their function
B、how frequently they release hormones into the body
C、whether the hormones they secrete influence the aging process
D、whether they secrete chemicals into the blood
答案D
解析 事实细节题。第三段第二句提到，构成内分泌系统的腺体直接向血流中分泌荷尔蒙，第四句提到，构成外分泌系统的器官和腺体分泌的物质作用于血流之外，即器官和腺体以分泌的荷尔蒙是否直接作用于血流中区分为内分泌系统和外分泌系统，故D项为答案。文中没有提到科学家、分泌荷尔蒙的频率、分泌的荷尔蒙是否会影响衰老，故排除其他三项。
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