You are running and your heart is beating fast now. Do you know why? Scientists say that when you are quiet or resting, you heart beats about seventy times a minute. Your heart is a kind of pump that pumps blood to all parts of the body. The blood carries oxygen and food. You are running now, so your muscles are working hard. They are using the food that the blood carries to them. The muscles need oxygen, too. So your brain sends a signal to the heart. The signal means that the muscles need more food and oxygen. Then the heart beats fast (ninety to one hundred forty times a minute) and sends blood quickly to the muscles. That’s why. (Adapted from “curiosity”, in Challenge, Robert L.Saitz)
