Why does your heart beat?

In PE, the teacher had the class run two laps of the playground. At the finish, Na pressed a hand to her chest: her heart was pounding, much faster than in class.

Cú Bông was afraid a heart racing like that meant something was wrong. Na just grinned, a little out of breath.

They sat down under the almond tree, and Na gulped some water. After a while, her heart had slowed down to how it was in class. So why does it have to rush when you run?

Make a fist: your heart is about the size of it, and made of muscle. With each beat it squeezes, like your fist tightening, pushing blood all round your body. Blood carries oxygen and nutrients to your cells, then takes carbon dioxide to your lungs to be breathed out.

Your heart has two sides that work like two pumps: the right side sends blood to the lungs for oxygen. The left side takes the oxygen-rich blood back from the lungs and pumps it round your body. Valves keep the blood moving one way. Those valves closing helps make the lub-dub of a heartbeat.

Choose Asleep, Reading or Running, then look at the rows of dots. When does the body need the most blood to carry oxygen to its cells?

You might not know

Your heart doesn't keep one steady pace from morning to night. Run about and it speeds up; lie down with a book and it slows.
Blood comes back from the body to the heart, is pumped to the lungs, then returns to the heart. From there it is pumped round the body, so it passes through the heart twice on each full circuit.

Your heart is a muscular pump the size of your fist; each beat sends oxygen-rich blood to your cells. When you run, your body needs more oxygen, so your heart beats faster.

The chapter's sources

Everything this chapter tells you rests on at least two trustworthy sources, retold in the book's own words.

Your Heart & Circulatory System — Nemours KidsHealth
Heart — Cleveland Clinic
How the Heart Works — U.S. National Heart, Lung, and Blood Institute
How Blood Flows through the Heart — U.S. National Heart, Lung, and Blood Institute
Heart Valves — Cleveland Clinic
Heart-Healthy Living — Get Regular Physical Activity — U.S. National Heart, Lung, and Blood Institute
Does Your Heart Stop When You Sneeze? — Cleveland Clinic