Your heart is an amazing, powerful muscle about the size of your fist that never stops working your entire life. It beats about 100,000 times every single day, which adds up to more than 35 million times each year! Your heart's job is to pump blood through your body. Blood is important because it carries oxygen and nutrients that every cell in your body needs to stay alive and healthy.
Your heart is divided into four separate sections called chambers, almost like four connected rooms. The two chambers on top are called atria, and the two chambers on the bottom are called ventricles. The right side of your heart receives dark red blood from your body that has already delivered its oxygen. It pumps this blood to your lungs, where the blood picks up fresh oxygen and gets rid of carbon dioxide when you breathe.
The blood turns bright red when it has lots of oxygen. Then the left side of your heart receives this oxygen-rich blood from your lungs and pumps it out forcefully through tubes called arteries to every part of your body. Your heart never gets to rest, even when you're sleeping peacefully at night. When you exercise or play hard, your heart beats faster to deliver more oxygen to your working muscles. Regular exercise actually makes your heart muscle stronger and more efficient, just like it makes your other muscles stronger when you use them.