32 Your Hardworking Heart-Mid

Your Hardworking Heart FOCUS Question How does the human heart work?

A Mighty Muscle What's the most important muscle in your body? It's your heart! That's right—your heart is a muscle, and it has a very important job. It pumps blood throughout your body. Most other muscles in your body get to rest now and then, but your heart never stops. It started working long before you were born. It will keep working nonstop for your whole life, without you even thinking about it. Amazing. The heart, blood and blood vessels together make up the circulatory system. That's because the heart causes blood to travel, or circulate, throughout the body. Wowser! Your heart beats about 100,000 times each day. including while you sleep.

In and Around the Heart Blood enters and leaves the heart through blood vessels. Arteries carry blood away from the heart, and tiny capillaries take it to individual cells. Veins bring blood back to the heart. The heart has four sections, or chambers, that fill with blood at different times. The top two chambers, called atriums, receive blood coming back from the body and lungs. The bottom two chambers, called ventricles, pump the blood out to the body and lungs. The septum is a thick wall of muscle. It separates the left. and right sides of the heart.

One Heartbeat Like other muscles, your heart produces force and motion. It does this by contracting (tightening) and relaxing over and over. When a chamber of the heart relaxes, it fills with blood. When it contracts, it squeezes and pushes the blood out. A single heartbeat 1 The right atrium relaxes and fills with blood from all parts of the body. The left atrium also relaxes and fills with blood from the lungs. 2 The right and left atriums contract and force blood into the ventricles. 3 The right ventricle contracts and pushes blood to the lungs. The left ventricle also contracts and sends blood out to all parts of the body.

The four heart chambers work together to keep blood moving through your body all the time. Blood rushes into the atriums and is immediately sent to the ventricles. As the ventricles pump blood out, other blood flows into the atriums. This cycle continues, causing heartbeat after heartbeat. You can actually feel your heartbeat in certain places where blood flows close to the skin. This is called checking your pulse. An average pulse for humans is about 70 beats per minute. Math Moment People often check their pulse for only 15 seconds. then multiply by 4 to figure the beats per minute. Jane counted 16 beats in 15 seconds, and Doug counted 18 beats. What is the difference between their total beats in one minute?

Which Way? Your heart is a busy place, like a big city where many freeways meet. How does the blood know which way to go? Four valves inside the heart act like security gates. They keep blood moving in the right direction. Two valves direct blood from the atriums to the ventricles. Two other valves direct blood as it leaves the ventricles. All the valves close after blood moves through them to prevent backward flow.