The heart pumps blood throughout the body. The walls of the heart are thick muscles that contract (beat) when they get an electrical signal, about once per second. Valves inside the heart open and close exactly on time to keep blood flowing smoothly in the right direction.
Blood picks up oxygen in the lungs. This oxygen-rich blood goes to the left side of the heart, which then pumps it out to the rest of the body. The blood returns to the right side of the heart, which then pumps it back to the lungs.
To work properly, the heart needs precise electrical timing of each beat, proper opening and closing of the valves, and plenty of energy from its own blood supply (called the "coronary" blood vessels).
Arteries are blood vessels that carry blood away from the heart. Because they are under high pressure, arteries have strong elastic walls. The largest artery from the heart is called the aorta. This artery branches into smaller and smaller vessels before finally reaching the tiny capillaries where oxygen and nutrients are delivered to organs and tissues. The other main artery from the heart goes to the lungs where its blood picks up oxygen. Veins are the blood vessels that carry blood back to the heart. They have much thinner walls than arteries and are under less pressure.