Richard Kansel Jr., age 13, of Wichita, Kan., for his question:
WHAT MAKES THE HEART BEAT?
A healthy human heart beats lub dub, lub dub, all day and all night. It has its own built in pacemaker to start each pulsing beat. And the pacemaker has a partner to complete the cycle. These built in drummers are bundles of special nerves. And the whole thing works like a built in miracle.
The heart is a muscular fist that contracts and relaxes with each pulsing beat. Its hollow inside has a central wall which divides it into two separate chambers. Each of these chambers has an upstairs and a downstairs chamber with a sort of trapdoor in the downstairs ceiling.
The downstairs chambers are the ventricles. Those upstairs are the atria, the plural of atrium. The right atrium has a system of valves and vessels to let in used blood from the veins and pump it up to be refreshed by the lungs. The left atrium has a similar system to let in fresh blood from the lungs and pump it out through the arteries.
Obviously this neat operation needs a complex pumping system. To make it work, various muscles must relax to let the blood flow into the heart and contract to push it out. Each complex beat is triggered by the sinoauricular node, alias the S N node, alias the pacemaker.
The pacemaker is a tiny, C shape button. To start the beat, it sends a spurt of electrical impulses that cause the atria to contract and send their blood supplies through the trapdoor down into the relaxed ventricles. The spreading pacemaker impulses are captured and relayed by a nerve bundle called the atrioventricular node, alias the A N node.
The A N node has nerve branches to carry the signals through the thick muscular walls of the two ventricles. They contract from the bottom, pumping blood up through the trapdoors and out through the atria. After each beat, the heart takes a brief moment to rest before the pacemaker triggers the next round of activity.
The rate of the average heartbeat is about 70 times a minute. But the average is slower during rest periods and faster when the busy body needs more oxygen. These variations are signaled to the pacemaker by nerve networks in the spine and other parts of the body.