大黄的泻下机制如何？主要成分及其作用部位是什么？
参考答案：
大黄口服后，结合型蒽苷大部分未经小肠吸收而抵达大肠，在大肠被细菌酶水解，还原成泻下作用较强的蒽酮或蒽酚刺激肠黏膜，加快肠蠕动；此外，蒽酮具有胆碱样作用，兴奋肠平滑肌上M-胆碱受体，加快肠蠕动；同时又抑制平滑肌上Na+-K+-ATP酶，阻断Na+从肠腔转移至细胞内，使肠腔内渗透压升高，保留大量水分，使肠管扩张，机械性刺激肠壁，促使蠕动增强。大黄致泻的主要成分为结合型蒽苷，其中以番泻苷A的泻下作用最强。作用部位在大肠，对小肠营养物质的吸收无明显影响。
| 阅读理解。 |
| A man walked into a doctor's examining room. "Put out your tongue," the doctor said. The man put out his tongue, and the doctor looked at it quickly. "OK. You can put your tongue back now," the doctor said. "It's clear what's wrong with you. You need more exercise." "But, doctor," the man said, "I don't think…" "Don't tell me what you think," the doctor said. "I am the doctor, not you. I know what you need. I see hundreds of people like you. None of them get any exercise. They sit in offices all day and in front of the TV in the evening. What you need is to walk quickly for at least 20 minutes a day." "Doctor, you don't understand," the patient said. "I…" "I don't want to hear any excuses," the doctor said. "You must find time for exercise. If you don't, you will get fat and have health problems when you are older." "But I walk every day," the patient said. "Ｏh, yes, and I know what kind of walking that is. You walk a few feet to the train station from your house, a few more feet from the station to your office, and a few more feet from your office to a restaurant for lunch and back. That is not real walking. I'm talking about a walk in the park for twenty minutes every day." "Ｗill you listen to me, doctor!" the patient shouted, getting angry with his doctor who thought he knew everything. "I'm a mailman," the patient went on, "and I walk for seven hours every day!" For a moment the doctor was silent, then he said quietly, "Put your tongue out again, will you?" |
| 1. At first the doctor asked the man . |
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| A. to walk to work B. to exercise C. to show his tongue D. to see hundreds of other patients |
| 2. The doctor would not . |
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| A. tell the patient what was wrong with him B. let the patient speak C. look at the man's tongue D. let the patient watch TV |
| 3. The doctor told the patient that he . |
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| A. should get more exercise B. should stop making excuses C. should watch more TV D. was already too fat |
| 4. The patient was very when he heard what the doctor told him to do. |
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| A. angry B. happy C. interested D. tired |
| 5. The doctor . |
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| A. gave him some good advice B. walked seven hours a day C. was really a mailman D. knew that he was wrong at last |