轻松学英语每日一练(277)
阅读下面短文，在空白处填入1个适当单词或括号内单词的正确形式。
Blood is part of the machinery of your body and pretty __1__ (wonder) stuff. It contains __2__ number of different things that __3__ made in different ways.
Let's talk about the red blood cells. They make __4__ almost half of the blood.
New red blood cells are __5__ (be) made all the time in your bone marrow. That's a spongy red tissue on the inside of your large bones. As they are made, the new cells are __6__ (fill) with hemoglobin, the stuff that makes them red __7__ does the big job of carrying oxygen.
Your bone marrow has to keep __8__ (work) all the time because some red blood cells are always breaking down and need to be replaced.
Just __9__ you were reading this, your bone marrow made several million new red cells. You can see that __10__ is pretty busy.
