Your heart is a muscular organ that pumps blood throughout your body. It’s about the size of your fist and sits in the middle of your chest. The heart has four chambers: two atria (upper chambers) and two ventricles (lower chambers). Thick walls of muscle separate the chambers. They contract and relax in a coordinated way to pump blood. Valves between the chambers open and close with each heartbeat. They prevent blood from flowing backwards. The rhythmic contractions of your heart keep blood moving through your body. Your heart is a muscular pump that works nonstop for your entire life. The average heart beats about 100,000 times a day. That’s about once every second! Your heart also works as a team with your lungs. They share the important job of getting oxygen-rich blood to all the parts of your body. The human heart is an amazing organ. It’s about the size of your fist and weighs around 10 ounces. This hard-working muscle pumps blood throughout your body 24/7. In an avera...