15.High School English Listening

If blood didn't circulate through our bodies, carrying oxygenand nutrients, we wouldn't be able to live.
Blood is soimportant to life that the body constantly makes new blood.
To do this, the body must produce the liquid part of blood, called plasma, and the cells that floatin it.
Plasma is made mostly of water and salts that we absorb through our digestive tracts everyday.
Its job is to deliver nutrients and water throughout the body.
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 bloodcell is four months.
What Happens After That?
The spleen continuously destroys millions of old red blood cells, recycling the iron to make new redcells.
White blood cells, which are part of the immune system, and platelets, which help with bloodclotting at a site of injury, also float in plasma.
They have much shorter life spans than red cellsand also are replaced continuously.
If you've ever seen a bone cut crosswise, with soft tissue called marrow inside, then you've seenwhere blood cells are made.
Bone marrow contains special blood cells, which constantly divide toproduce new cells to replace the ones that have been destroyed.
Why all this bloody business inthe 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.
译文：
如果血液不在我们的身体里循环，不携带氧气和营养物质，我们就无法生存。
血液对生命是如此重要，以至于身体不断制造新的血液。
要做到这一点，身体必须产生血液的液体部分，称为血浆，以及漂浮在上面的细胞。
血浆主要由我们每天通过消化道吸收的水和盐组成。
它的工作是将营养物质和水输送到整个身体。
漂浮在血浆中的99%的血细胞是红细胞，它们将氧气从肺部带到身体的其他部位，并使血液呈现红色。
一个红细胞的平均寿命是四个月。
此后会发生什么？
脾脏不断地破坏数以百万计的旧红细胞，回收铁来制造新的红细胞。
作为免疫系统一部分的白细胞和帮助受伤部位凝血的血小板，也漂浮在血浆中。
它们的寿命比红细胞短得多，也会不断被替换。
如果你曾见过被横切的骨头，里面有被称为骨髓的软组织，那么你就看到了血细胞的制造过程。
骨髓中含有特殊的血细胞，它们不断分裂，产生新的细胞，以取代被破坏的细胞。
为什么骨髓里会有这些血腥的东西？
身体形成新血细胞的过程有助于你在受伤失血后的恢复，也有助于血液在细胞受损和丧失的情况下发挥其许多功能。
重点词汇：
be able to 能够 ; 能做到…的 ; 可以…的
new blood 新鲜血液;新生力量
part of 部分
average life 平均寿命
After That 然后
millions of 数百万