S4·E16 Heart

Your heart pumps your blood around your body, all hours, every day of the week, to keep you alive. Your heart is about the size of your fist, and it’s made of special muscle called “cardiac” (KAR-dee-ak) muscle. Cardiac muscle lets your heart keep the beat, it can also speed up or slow down, depending on what your body needs. Your heart works like an automatic pump – it squeezes, or contracts, and un-squeezes, or relaxes, to push blood through the four different sections of your heart. Valves, special one-way openings, are like little doors between the sections – making sure your blood moves in only one direction through your heart, to your lungs, back to your heart, and then around your body again.