题型：阅读选择 题类：常考题 难易度：困难
北京市海淀区2019届九年级下学期英语5月期中考试试卷
You're not the same person you were as a child. You're not even the same person you were five years ago. We don't just mean your personality. While we'd love to start a never-ending discussion over the "true self", we're here to talk about the cells(细胞)in your body.
Do you know how your fingernails and hair are constantly growing and replacing themselves? The same thing happens almost everywhere in your body. Your outer layer of skin makes way for fresher skin, and inside your veins(血管), blood cells turn over to make way for fresh blood. It's probably no surprise that these types of cells are short-lived, even some of the most seemingly permanent(永久的)parts of your body replace themselves over time-including your bones.
Strong as your bones might seem, the cells they're made of won't last a lifetime. Bones are masters at rebuilding themselves, which is why most broken bones can heal on their own into the right shape.
Bones have special cells called osteoclasts(破骨细胞)whose job is to break down bone, even if it isn't broken, and send the minerals into the blood stream. This may sound frighteningly like your bones are wearing away, but your body knows what it's doing. As osteoclasts break down bone, osteoblasts(成骨细胞)build up new bone to replace the old. By the end of each year, about 10 percent of your entire adult skeleton will have been rebuilt through this process.
If 10 percent of your bone breaks down every year, it must take about ten years to renew 100 percent of your skeleton. That would be a great guess, and it lines up nicely with the medical myth that the human body's cells are fully replaced every seven years, but the body is a bit more complex. That 10 percent is just an average-some bone grows faster than that, and some grows slower. While some parts of your bone will turn over quickly, other parts will stick around for decades. In fact, most bone cells could live as long as 50 years.
So before you get wrapped up in an identity crisis(危机)about the cells in your body disappearing on you, know that you can lose some hair and you can cut your nails, but the bones of you aren't going anywhere-at least not very soon and not all at once.
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