George Abboud, age 12, of Ottawa, Ontario, for his question:
What makes the heart beat?
In the 1600s, William Harvey demonstrated how the heart works, beat by pulsing beat. But what makes it beat remained a mystery while generations of human hearts continued to follow their secret instructions. In the 1900s, a silver of chicken heart was kept beating in a solution for 34 years. However, modern medical scientists cannot explain fully what makes each beat begin though they are getting close to the secret.
The human heart normally beats about 70 to 75 times a minute. After each pulsing beat, it pauses to rest about 0.8 seconds. Then something happens to start the next beat through its rhythmical operation. This something is what makes the heart beat, for it sets each pulse in motion and controls its pace and rhythm. It is a small wad of tissue, located on the right side of the heart. We may call it the pacemaker, though medical scientists call it the S A node, short for the sinoatrial node.
When time comes to spark the next beat, the pacemaker sends out electrical signals. In a fraction of a second, these waves are conducted by two sets of nerve fibers that spread around the heart. This causes the heart muscle to contract as a single unit, starting the squeeze relax operation completed with each beat. The nerve fibers that conduct the starting signal can be described in great detail. We also know the chemical electrical method used to transmit the signal.
But the rest is somewhat like an old song from an old war. This weary soldier wants to silence the bugler who wakes him before dawn. Then he decides it might be wiser to find acid stop the fellow who wakes the bugler. Translated into matters of the heart, we can prove that each beat is started by the pacemaker. But nobody can identify what starts the pacemaker that starts the beat. Apparently this small node is made of tissue with special chemical electrical properties that make it perform its duties.
This does not mean that the pacemaker's pace cannot be controlled. It is regulated continuously. During periods of intense activity, it speeds up the heart beat to supply extra oxygen to the muscles. It slows down during periods of relaxation, when less oxygen is needed. Certain drugs and biochemicals also increase or decrease the pacemaker's pace.
Naturally medical scientists would like to explain what starts the pacemaker. To this end, researchers carry on numerous experiments with mammal hearts, similar to human hearts. The tissues can be kept alive and beating in suitable solutions, sectioned with or without the pacemaker node. Some experts suspect that the miraculous operation is controlled by mysterious electrical activities. But nobody has been able to prove it.