Your heart is a muscle about the size of your fist that pumps blood through your whole body. Blood carries oxygen and nutrients to every part of you. Your heart beats every second of every day, and it beats faster when you exercise.
Have children feel their pulse on their wrist or neck. Count heartbeats for 15 seconds and multiply by 4. Compare resting heart rate to heart rate after jumping jacks. Use a stethoscope if available so children can hear their heartbeat.
Children often think the heart is shaped like a Valentine heart. The real heart looks more like a fist. Some children think the heart stops when they are asleep, when it actually keeps beating 24 hours a day. Others think blood is only in cuts and scrapes, not realizing it flows through the entire body all the time.
Put your hand on the left side of your chest. Can you feel it? That steady thump-thump is your heart beating. Your heart is a muscle about the size of your fist, and it sits in the middle of your chest, just slightly to the left. Its job is simple but incredibly important: it pumps blood. Every single beat pushes blood out through tubes called blood vessels that reach every part of your body — your toes, your fingers, your brain, everywhere.
Why does blood need to go everywhere? Because blood is your body's delivery system. It carries oxygen (the air you breathe in) and nutrients (the useful stuff from the food you eat) to every cell in your body. Your cells need these things to work, grow, and repair themselves. Blood also picks up waste that your cells do not need anymore and carries it away to be removed. Your heart keeps this delivery system running non-stop.
Try this: sit quietly and count your heartbeats for fifteen seconds. Then do twenty jumping jacks and count again. Your heart is beating much faster after exercise. That is because your muscles are working hard and need more oxygen and energy, so your heart speeds up to deliver more blood to them. When you rest, your heart slows back down. Your heart does this adjusting automatically — you never have to think about it. It beats about 100,000 times every single day, whether you are running, reading, or sleeping. It is the hardest-working muscle in your whole body.