Your body is made up of many, many cells, all of which need oxygen to survive. When the cells use oxygen they produce a waste product called carbon dioxide that then must be removed from your body or it will poison the cells. The only way your body can get oxygen or remove carbon dioxide is from the air which enters and leaves your lungs when you breathe. The heart's main function is to move blood from the lungs, where the oxygen enters your blood, to every other part of the body that needs the oxygen. The blood also carries carbon dioxide from the cells to the lungs where you then breathe the carbon dioxide out.

The heart is basically a big muscle wrapped around four compartments. Two of those compartment are responsible for directing the flow of the high-oxygen blood from the lungs toward the rest of the body. The other two compartments are responsible for returning the blood that has already delivered its oxygen back to the lungs so that it can get more oxygen and get rid of carbon dioxide that has built up. When you hold your breath, after a while your blood runs out of oxygen and your brain sends a message to your lungs that you need to take a breath. What triggers your brain to send that message is the high level of carbon dioxide in your blood, not the low level of oxygen!

The four compartments in the heart are called the left ventricle, the right ventricle, the left atrium and the right atrium. The left atrium and the left ventricle (the atrium is on top, the ventricle is on bottom) pump high-oxygen blood from the lungs which is delivered to the rest of the body by the arteries. The right atrium and the right ventricle pump low-oxygen, high-carbon dioxide blood from the body to back to the lungs by way of the veins. Low-oxygen blood is blue while high-oxygen blood is red, so the blue
veins you see under your skin are blood that is returing to the heart to pick up more blood (the red arteries are usually too deep to be seen though the skin). When you cut yourself, that blue blood turns red again when it comes into contact with air and picks up oxygen.

Blood only flows in one direction though the four different chambers of the heart (in or out) because valves prevent backwards flow. Why do you think this is? Human hearts have a natural pacemaker that sets the speed at which the heart beats. When your body releases certain chemicals, the pacemaker will speed up or slow down your heartbeat. Why would you want your heart to be able to change the speed at which it pumps (faster or slower)?

It sounds like you already have a good idea about how to find out about things by using the Internet. The human heart is actually a big muscle that works very hard, all of the time. It would be interesting for you to find out how many times a human heart beats in a day, a week, a month and a year. You can do this by looking up treh information OR you can do your own calculation by monitoring your own heartbeat! Also, I bet that you can find out how much blood a human heart pumps in a day. It is a big job! Another good project might be to explain why smoking or eating fatty foods is bad for your heart. You can learn about how the heart develops by visiting a website on Developmental Biology and then linking to other sites from there. The website address is: http://sdb.bio.purdue.edu

Have fun learning about the heart!

One way to understand how the heart works is to put a bit of water in a balloon. Don't use a lot of water. Now squeeze one end. What happens to the water? This is how your heart works. It is like having two balloons side-by-side, plus tubes that come in and go out carrying blood, and one-way doors to keep blood from backing up. The basic idea is the same though. The heart is really two side-by-side pumps. Blood comes into the top places (left and righ atria). Then the heart (which is a big muscle) squeezes the blood into the bottom spaces (ventricles). Then the ventricles squeeze the blood out to the arteries. Where does the blood go from there?

There are nice articles on the heart at:
http://whyfiles.news.wisc.edu/028heart/how_works1.html.
and
http://sln.fi.edu/biosci/heart.html

Thanks for asking,

Here are some good websites with lots of pictures to help you learn about the heart through seeing.

http://sln.fi.edu/biosci/structure/structure.html

http://www.atlcard.com/pump.html

http://www.gene.com/ae/AE/AEC/CC/heart_background.html

This last site has lots of nice pictures, and lots of links at the bottom of the page that lead to even more good, labelled pictures of the heart. Concentrate on the pictures and try reading what you can. Write me back if you have more specific questions or to tell me your answer to this question:
Why do we need a heart?
Good luck!