When you think about a person who is very strong, you might picture someone who has large arms and six pack abs.  Maybe you picture a body builder or your favorite Ultimate Fighting Championship fighter.  In either case, you probably do not picture a person who runs for a long time, like your friend on the track team.  However, there is a pretty good chance that the runner has one muscle that is stronger than the body builder's same muscle.  This muscle is very important, even though it is only about the size of your fist; without it nobody could live.  Can you guess what it is?

If you said "heart," then you were correct.  Your heart is the muscle that makes life possible because it keeps blood pumping throughout your body.  Unlike the muscles in your arms and legs, your heart is actually always being used
; not once in your life does it get to rest.  In fact, your heart is so important that it works on its own; you never have to tell your heart to work.  It keeps going even when you are resting at night. 

Your heart has four different parts or chambers in it, just like a house or apartment has different rooms.  Since your heart also has two sides to it, the left side and the right side, it has two of these chambers on each side.  The right side of your heart pumps blood to your lungs.  The left side of your heart pumps blood to the rest of your body. 

An atrium is a chamber of the heart that brings blood into the heart from the rest of the body.  You can think of it as being like the part of a water gun that gets filled with water before shooting it out.  The two atriums are called atria. They form the top part of your heart.  You have one atrium on the right side of your heart and one on the left side. 

A ventricle is a part on the bottom of the heart that pushes blood out of the heart.  Just like you have two atria, you also have two ventricles "" one on each side of the bottom part of your heart.  Your blood flows to these parts after it goes through your atrium.  Then it is the job of these lower parts of your heart to push out this blood.  If the upper parts of your heart are like the part of a water gun that gets filled with water, then the lower heart parts are like the part of the water gun that shoots the water out of it after it has been filled.  The shape of your heart changes as the upper and lower parts work together to make sure that your blood gets to and from all of the cells in your body.

Your heart has a lot of moving parts.

When your blood is pumped out of your heart by your ventricles it goes into your arteries.  The aorta is the biggest artery in your body.  It carries the blood that is pumped out of the left ventricle of your heart to almost all the cells in your body.  This large artery then divides into smaller ones that carry your blood through the rest of your body.  Think of it as being like a river that divides into small creeks.

Just relax, this won't hurt a bit.

As you might have guessed, your heart has to use a lot of power to pump all of your blood through your body.  After all, it only takes about ten seconds for blood to get from your heart to your toes and back when you are running!  You can measure how hard your heart is working is by taking your blood pressure.  Your blood pressure tells you how much your blood is pushing on the walls of your blood vessels.  The harder your heart is working, the more power it is using to pump your blood.  The more power your heart uses, the higher your blood pressure number will be. 

Watch out, here I come.

To help yourself remember the direction of the flow of your blood through your heart, think of a slide at a water park.  To enjoy the slide, you have to start at the top of it, just like your blood enters your heart through the atrium, or top parts.  Then, just like you go down a slide, your blood gets pumped down through your heart to the ventricles, or bottom parts of your heart.  Next, your ventricles pump your blood out of your heart and into your arteries, such as your aorta.  You can picture this part to be like a set of fun tunnels that you would swim through after going down the water park slide.  Finally, just like you would want to climb up to the top of the slide to go down again, your blood goes back to the top of your heart.  It then gets pumped through your whole body all over again.