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If blood didn’t circulate through our bodies, carrying oxygen and nutrients¹, we wouldn’t be able to live. Blood is so important to life that the body constantly makes new blood.
如果血液不能在我们身体内循环，为人体输送氧气和营养，我们便无法存活。血液对生命十分重要，所以人体会不断的制造新鲜血液。

To do this, the body must produce the liquid part of blood, called plasma², and the cells that float in it. Plasma is made mostly of water and salts that we absorb through our digestive tracts³ every day. Its job is to deliver nutrients and water throughout the body.
人体在造血时必须要产生血液的液体部分，即血浆，以及浮在血浆上的细胞。血浆主要由我们通过消化道每天吸收的水和盐组成。它的作用就是输送水和营养物质到全身各个部位。

Ninety-nine percent of the blood cells floating in plasma are red blood cells, which carry oxygen from the lungs to the rest of the body and give blood its red color. The average life of a red blood cell is four months.
浮在血浆上99%的血细胞都是红细胞，它们会把肺部的氧气输送到全身各处，这样血液才会呈现出红色。一个红细胞的平均寿命是4个月。

What Happens After That?
接下来会发生什么呢？

The spleen continuously destroys millions of old red blood cells, recycling the iron to make new red cells. White blood cells, which are part of the immune system, and platelets, which help with blood clotting⁴ at a site of injury, also float in plasma. They have much shorter life spans than red cells and also are replaced continuously.
脾脏不断地将数百万老化的红细胞解体，循环利用其中的铁元素来制造新的红细胞。白细胞是人体免疫系统的一部分而血小板则在人体伤口处起到凝血作用，这两种血细胞也都漂浮在血浆上。但是它们的存活周期要比红细胞短，同时也会被不断地更新。

If you’ve ever seen a bone cut crosswise, with soft tissue called marrow⁵ inside, then you’ve seen where blood cells are made. Bone marrow contains special blood cells, which constantly divide to produce new cells to replace the ones that have been destroyed. Why all this bloody⁶ business in the bone marrow? The body’s process of forming new blood cells helps you recover if you lose blood to an injury, and it also helps blood perform its many functions despite cell damage and loss.
如果你看到过一块被横切的骨头，你就会看到里面的软组织，即骨髓，它是专门制造血细胞的。骨髓含有特殊的血细胞，这种细胞会不断分离产生新的细胞来取代那 些已被破坏的细胞。为什么所有的造血工作都在骨髓中进行呢？答案是如果你因受伤而流血的话，人体生成新血细胞的过程便能帮助你复原，同时，尽管存在细胞受 损或缺失，这个过程也能帮助血液发挥很多其它的功能。