where it might cause unwanted effects,

就不会导致药物产生预计外的效应
so that's a very good reason to target it.

这为药物靶向提供了很好的动因
Sometimes there are biological barriers to drugs

有时候药物进入人体不同部位时
entering different parts of the body.

会遇到生物屏障
We're going to start by talking about the body

我们将讨论一下
as sort of a single unit or compartment.

身体作为一个独立的单元
When I introduce drugs into them,

当药物引入体内后
those drugs are available everywhere,

就可以到达体内各处
but that's not true.

但实际并非如此
There are some parts of the body that are protected from entry of drugs,

身体有些部位受到保护使药物不能进入
and the brain is the most famous of those parts

在这些受到保护的部位中
of the body that are protected.

大脑是声名在外的