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  3. It has been known since ancient times that certain plants regularly open their leaves in daytime and close them at night. It was

It has been known since ancient times that certain plants regularly open their leaves in daytime and close them at night. It was

admin2010-07-14  57
问题     It has been known since ancient times that certain plants regularly open their leaves in daytime and close them at night. It was assumed this periodic change was triggered by the alternating sunlight and darkness. It was then discovered that such leaves stick to their daily routine even when kept in constant darkness.
    What is a circadian rhythm? Many biological processes follow a 24-hour schedule. These cycles are called circadian rhythms. These innate rhythms are governed by sophisticated chemical mechanisms which can be described as biological clocks. Such rhythms are important for humans too! We sleep through most of the hours of darkness and awake through most of the hours of daylight. Pulse rate and blood pressure rise sharply on awakening. Body temperature increases during the day and decreases during the night. The secretion (分泌) of various hormones (激素) that control life’s processes rises and falls to a circadian rhythm.
    How does this relate to health? Circadian rhythms are important for optimal functioning of the many processes vital to health. This is most effectively illustrated when the normal circadian rhythms are disrupted by lack of sleep or by crossing time zones. It may take days or even weeks for the circadian rhythms to readjust appropriately. The turmoil that results from the readjustment process when time zones are crossed is experienced as the phenomenon of jet lag. That is also experienced by people who do shift work.
    Understanding circadian rhythms may be relevant to the understanding and treatment of disease. Therapy can be more effective and toxic effects reduced when drugs are administered at carefully selected times of the day—chronotherapy. A simple example of chronotherapy is the use of antihistamines (抗阻胺剂) and decongestants (解充血药) for the treatment of allergic symptoms. Cardiovascular disease (心血管疾病) is the single biggest killer of adults and is also influenced by circadian rhythms. The immediate cause of a heart problem is inadequate blood flow to the heart muscle. They may result in no symptoms, or it may cause angina (心绞痛), or, most severely, a heart attack may occur. It has been shown that heart attacks are much more common in the morning than at other times of the day.
The best time to take medicine for heart disease patients is ______.
选项 A、at noon
B、at night
C、in the morning
D、in the afternoon
答案B
解析 推理题。根据文章最后一句，心脏病和心肌局部缺血的易发期是在早晨，因此预先在晚间服用药物其治疗效果最好。
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