David Kidd, age 9, of Shreveport, Louisiana, for his question:
What keeps the heart beating?
The beat beat beat of your heart goes on day and night and most of the time you do not notice it. It works by itself and you do not have to tell it what to do or even to think about it. But surely there is something that keeps it steadily pulsing along. And so there is.
Each beating pulse of your heart takes less than a second. When one pulse ends the next one begins and the busy heart beats 70 to 80 times a minute. It works like a big fist that keeps relaxing a little and then squeezing tight again and again and again. Each beat helps to keep the blood streaming around through the veins and arteries. The heart acts like a clever pump and it knows all the things it should do without being told. After each beat, it takes a moment's rest. But it knows just when the next pulsing beat must begin.
The heart has a sort of built in starter that tells it when to begin. the next beat. The starter has two little buttons and a small net of stringy nerves. One of the buttons is called the pacesetter. It starts each beat with a prod and keeps the heart beating at a steady pace. The prod from the pacemaker reaches the second button and spreads through the strands of stringy nerves. The prodding is a nervous impulse that makes the heart squeeze up to start its next beat.
The heart is made of meaty muscle and muscles move when nerves of the body prod them to move. Nerves can make your muscles move, even without your permission. If you touch a hot stove, your nerves order you to move and your body moves away before you even have time to think about it. The heart must keep beating day and night. If you had to order every beat, you would have time for nothing else. So the heart has this built in starter of nerves and buttons to keep reminding it.
Sometimes the heart must beat faster to pump more blood through the body. It beats faster when you run or get excited. When you sleep, the beating slows down and the heart takes it easy. The prodding starter makes these changes in the heartbeat happen. Its pacesetter is in contact with other nerves in the body. They tell it when more blood is needed and it prods the heartbeat to go faster. They tell it when the resting body is asleep and the little built in starter makes the heart slow down its pulsing beat.
The nerves and buttons of the starter have fancy names. If you like big words, you will want to learn them. The pacesetter that starts the starter is called the sinoatrial node. Experts sometimes call it the S A node, which is easier to say. The second button is the atrioventricular node but you may call it the A V node. The little network of stringy nerves is the atrioventricular bundle also called the bundle of "His." The "His" is said like the hiss of a snake.