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If blood didn't circulate through our bodies, carrying oxygenand nutrients, we wouldn't be able to live.
如果血液不能在我們身體內循環，為人體輸送氧氣和營養，我們便無法存活。
Blood is so important to life that the body constantlymakes new blood.
血液對生命十分重要，所以人體會不斷的制造新鮮血液。
To do this, the body must produce the liquid part of blood, called plasma, and the cells that floatin it.
人體在造血時必須要產生血液的液體部分，即血漿，以及浮在血漿上的細胞。
Plasma is made mostly of water and salts that we absorb through our digestive tracts everyday.
血漿主要由我們通過消化道每天吸收的水和鹽組成。
Its job is to deliver nutrients and water throughout the body.
它的作用就是輸送水和營養物質到全身各個部位。
Ninety-nine percent of the blood cells floating in plasma are red blood cells,
浮在血漿上99%的血細胞都是紅細胞，
which carry oxygen from the lungs to the rest of the body and give blood its red color.
它們會把肺部的氧氣輸送到全身各處，這樣血液才會呈現出紅色。
The average life of a red bloodcell is four months.
一個紅細胞的平均壽命是4個月。
What Happens after that?
接下來會發生什么呢？
The spleen continuously destroys millions of old red blood cells, recycling the iron to make new red cells.
脾臟不斷地將數百萬老化的紅細胞解體，循環利用其中的鐵元素來制造新的紅細胞。
White blood cells, which are part of the immune system, and platelets,
白細胞是人體免疫系統的一部分而血小板則在人體傷口處起到凝血作用，
which help with bloodclotting at a site of injury, also float in plasma.
這兩種血細胞也都漂浮在血漿上。
They have much shorter life spans than red cellsand also are replaced continuously.
但是它們的存活周期要比紅細胞短，同時也會被不斷地更新。
If you've ever seen a bone cut crosswise, with soft tissue called marrow inside, then you've seenwhere blood cells are made.
如果你看到過一塊被橫切的骨頭，你就會看到里面的軟組織，即骨髓，它是專門制造血細胞的。
Bone marrow contains special blood cells,
骨髓含有特殊的血細胞，
which constantly divide toproduce new cells to replace the ones that have been destroyed.
這種細胞會不斷分離產生新的細胞來取代那 些已被破壞的細胞。
Why all this bloody business inthe bone marrow?
為什么所有的造血工作都在骨髓中進行呢？
The body's process of forming new blood cells helps you recover if you loseblood to an injury,
答案是如果你因受傷而流血的話，人體生成新血細胞的過程便能幫助你復原，
and it also helps blood perform its many functions despite cell damage and loss.
同時，盡管存在細胞受 損或缺失，這個過程也能幫助血液發揮很多其它的功能。
