来自《耶鲁大学开放课程：生物医学工程探索 第11集 第11集》

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  and being in the blood is not the same as being in those tissues,
  在血液中的药物浓度和在组织中并不相同
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  and you're right.
  那你是正确的
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  It's hard to measure those concentrations in individual tissues
  测定特定的组织中的药物含量比较困难
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  and so we define bioavailability as the fraction of a drug dose
  因此我们用进入血液的药物剂量
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  that gets into the blood.
  来定义生物利用度
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  Now, why wouldn't we do intravenous administration for everything?
  为什么不将静脉给药作为万能的给药方式
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  Because if it is fast, usually you don't want to wait for--
  因为它作用快 而通常你又不想等待
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  you want the drug to act as rapidly as possible
  你想要药物尽可能快地起效
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  and you'd like all the drug molecules to be bioavailable,
  你想要利用所有的药物分子
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  you would like all of them to be useful.
  你要它们都发挥作用
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  So, why don't we use intravenous administration all the time?
  那么我们为什么不总采用静脉给药方式
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  Well, it probably is obvious that not--that it's not so easy to do.
  很明显 静脉注射不是那么容易做到