简要说明血脑屏障。
参考答案：
脑组织毛细血管的通透性与一般的不同，它只允许某些物质（葡萄糖、氧、CO2、和脂溶性物质）透入脑组织，而阻挡另外一些物质（胆盐、氢离子、非脂溶性物质和青霉素）透入脑组织。而且毛细血管内的血液与神经细胞间的物质交换，除通过毛细血管外都要通过胶质细胞的终足才能完成。这些特点就形成了血—脑之间的屏障作用，叫血—脑屏障。
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| Several weeks ago, our headmaster, Mr. Gaudi, told usthat our school had been chosen to compete on Smart Aleck-a game show which my best friend Carla and I both love. Carla has a good imagination, and I know a lot of history, so we thought we’d be good 1 for the team. A few days later, Mr. Gaudi held a meeting with everyone interested in the 2 . He told us that Smart Aleck uses a five member team. He also said that three more people would be needed as backup（替补）. Unluckily four times that many students were at the meeting. So Mr. Gaudi said we would have a 3 and the top eight scorers would become the team and the backup. Everybody agreed that this was a 4 way to decide. Yesterday, all of the hopefuls went to the dining hall after school. We would have to decide the starting line-up（阵容）.Mr. Gaudi acted as the host. After a couple of hours, all the 5 on the team were set except one. Carla and I 6 for that last position.We had to hold a one-on-one competition between the two of us.The winner would be on the team and the loser would be the backup. Midway through our two-person competition,I was starting to pull ahead. 7 I saw the look on Carla's face.I waited for a second and knew what I had to do.As much as I wanted to be on Smart Aleck,Carla wanted 8 even more.So,starting with my next question,I held back and le  tCarla 9 ， just by a little bit.Her expression as she 10 she would be on the show made up for my not being on the team. And since I will go as backup, I'll still have the fun of seeing the show up close. Was I 11 by not trying as hard as I could?I don't think so.No one noticed,and Carla and I didn't 12 it.It won't hurt the team because Carla will be a very good competitor on the real show.I think that making my best friend happy was the right thing to do. | ||||
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