Picture this: You are playing soccer in the street with your best friend.  Suddenly you trip and fall down.  You scrape up your knee.  Little bits of rocks get into the cut.  You feel a lot of pain, but you also notice something else.  Not only does your new cut hurt, but it also has a red fluid coming out of it.  You know that this is blood, but what exactly IS blood?  Why do you even have blood?

Our bodies are made up of organs. These work together to make life possible.  The organs are made of many different tissues and cells that also work together.

Organs do not move from one place in the body to another.  Organs stay in one spot.  That means that our bodies need something that can move between all of the different cells in our organs.  This is what blood does.  Blood is a fluid that is pumped through our body by the heart.  Blood carries things our cells need to the cells and it carries away things they don't need.  The system that helps the blood run through the body is called the circulatory system.  Your body would not work without the circulatory system.

What does blood carry from cell to cell?  One of the most important things carried by blood is oxygenOxygen is a gas in the air that our bodies use to make energy.  We get oxygen into our bodies by breathing.  Then the blood carries it to all our organs.  Our bodies need oxygen to turn our food into energy.  We use that energy to run, dance, or do any kind of movement.  Without blood, oxygen would not be able to reach all of the cells in our body.

Blood has another job.  Carbon dioxide is a gas that is formed when energy is made in your cells.  Too much carbon dioxide is bad for our organs.  Our blood carries it away from our organs to our lungs.  We push it out of our bodies by breathing out.

Blood is made up of different kinds of cells.  One of the most important is the red blood cellRed blood cells carry the things you need to all of your cells and they carry the things you don't need away from them.  No other blood cells can do this job as well as the red blood cells.

Wow, they're really red.

In fact, red blood cells are so important that there are more of them than any other kind of cell in your body.  A single drop of blood has millions of red blood cells.  If you looked at a red blood cell through a microscope, you would see that it looks very much like a small, short round cake that has been punched down in the middle.  This shape helps the red blood cells hold onto the oxygen that they carry through your body.

Most red blood cells live for around 140 days.  This is about four to five months.  Don't worry about your red bloods cells dying after 4-5 months, though.  Your bones make new red blood cells all the time.  These new cells take the place of the old ones that die.  Your body will make sure that you a
lways have enough red blood cells, even if you fall down and cut your knee.