Four rooms inside
Inside, your heart is hollow, with four spaces called chambers. You can think of them as four rooms in a little house.
The two top rooms are called atria (AY-tree-uh). Just one is called an atrium. They are the waiting rooms. Blood flows into them and waits for its turn.
The two bottom rooms are called ventricles (VEN-trih-kulz). They are the pumping rooms. Their thick muscle walls squeeze hard to push blood out of the heart.
So there is a right atrium and a left atrium on top, and a right ventricle and a left ventricle below.
The wall in the middle
A strong wall of muscle runs down the middle of your heart, from top to bottom. It is called the septum (SEP-tum).
The septum keeps the two sides apart. The right side holds blood that has used up most of its oxygen. The left side holds fresh blood full of oxygen.
Keeping them apart means the blood going out to your body is as full of oxygen as it can be.
The strongest room
The left ventricle has the thickest wall of all four rooms. It has the biggest job: pushing blood all the way to your head, your hands and your toes.
The right ventricle only has to push blood next door to your lungs, which is a short trip. So its wall is thinner.
The atria have the thinnest walls of all. They only need to nudge blood down into the rooms below them.
Follow one drop of blood
Let's follow one drop of blood through the heart. It starts out in your body, where it has given away its oxygen. It flows up a big vein into the right atrium, the top right room.
Next it drops through a little door into the right ventricle. The ventricle squeezes, and the drop shoots out to your lungs. There it lets go of carbon dioxide, a waste gas you breathe out, and grabs fresh oxygen.
Now bright red, the drop flows back into the left atrium, the top left room. Then it slips through another door into the left ventricle.
Squeeze! The strong left ventricle sends the drop out through the aorta to the rest of your body. Then the whole trip starts again.
Top rooms first, then bottom rooms
Each heartbeat happens in two quick steps. First the two top rooms squeeze together, pushing blood down. A split second later, the two bottom rooms squeeze together, pushing blood out.
Your heart's own electric spark makes this happen in the right order. The spark starts at the top of the heart and travels down, like a wave.
It all happens so fast that it feels like one beat to you.