Summary:
- The heart is a muscular organ in the chest.
- It pumps blood through blood vessels by repeated, rhythmic contractions.
- The heart has four chambers: two upper atria (singular, atrium) and two lower ventricles.
- Valves between chambers keep blood flowing through the heart in just one direction.
- The left side of the heart carries the oxygen-rich blood back from the lungs and pumps it to the rest of the body.
- The right side collects oxygen-poor blood from the body and pumps the blood to the lungs.