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Your heart's electrical system controls the timing of your
heartbeat by regulating your:
Your heart's electrical system should maintain:
See a picture of the
heart and its electrical system.
Your heart muscle is made of tiny
cells. Your heart's electrical system controls the timing of your heartbeat by
sending an electrical signal through these cells.
Two different types of cells in your heart enable the electrical
signal to control your heartbeat:
The electrical signal travels through the network of conducting
cell "pathways," which stimulates your upper chambers (atria) and lower
chambers (ventricles) to contract. The signal is able to travel along these
pathways by means of a complex reaction that allows each cell to activate one
next to it, stimulating it to "pass along" the electrical signal in an orderly
manner. As cell after cell rapidly transmits the electrical charge, the entire
heart contracts in one coordinated motion, creating a heartbeat.
The electrical signal starts in a group of cells at the top of your
heart called the sinoatrial (SA) node. The signal then travels down through
your heart, triggering first your two atria and then your two ventricles. In a
healthy heart, the signal travels very quickly through the heart, allowing the
chambers to contract in a smooth, orderly fashion.
The heartbeat happens as
This cycle of an electrical signal followed by a contraction is one
When the SA node sends an electrical impulse, it triggers the following process:
After the electrical signal has caused your atria to contract and
pump blood into your ventricles, the electrical signal arrives at a group of
cells at the bottom of the right atrium called the atrioventricular node, or AV
node. The AV node briefly slows down the electrical signal, giving the
ventricles time to receive the blood from the atria. The electrical signal then
moves on to trigger your ventricles.
When the electrical signal leaves the AV node, it triggers the
After your atria and ventricles contract, each part of the system
electrically resets itself.
The cells of the SA node at the top of the heart are known as the
pacemaker of the heart because the rate at which these cells send out
electrical signals determines the rate at which the entire heart beats (heart
The normal heart rate at rest ranges between 60 and 100 beats per
minute. Your heart rate can adjust higher or lower to meet your body's needs.
Your brain and other parts of your body send signals to stimulate
your heart to beat either at a faster or a slower rate. Although the way all of
the chemical signals interact to affect your heart rate is complex, the net
result is that these signals tell the SA node to fire charges at either a
faster or slower pace, resulting in a faster or a slower heart rate.
For example, during periods of exercise, when the body requires
more oxygen to function, signals from your body cause your heart rate to
increase significantly to deliver more blood (and therefore more oxygen) to the
body. Your heart rate can increase beyond 100 beats per minute to meet your
body's increased needs during physical exertion.
Similarly, during periods of rest or sleep, when the body needs
less oxygen, the heart rate decreases. Some athletes actually may have normal
heart rates well below 60 because their hearts are very efficient and don't
need to beat as fast. Changes in your heart rate, therefore, are a normal part
of your heart's effort to meet the needs of your body.
Your body controls your heart by:
The sympathetic and parasympathetic nervous systems are opposing
forces that affect your heart rate. Both systems are made up of very tiny
nerves that travel from the brain or spinal cord to your heart. The sympathetic
nervous system is triggered during stress or a need for increased cardiac
output and sends signals to your heart to increase its rate. The
parasympathetic system is active during periods of rest and sends signals to
your heart to decrease its rate.
During stress or a need for increased cardiac output, the adrenal
glands release a hormone called norepinephrine into the bloodstream at the same
time that the sympathetic nervous system is also triggered to increase your
heart rate. This hormone causes the heart to beat faster, and unlike the
sympathetic nervous system that sends an instantaneous and short-lived signal,
norepinephrine released into the bloodstream increases the heart rate for
several minutes or more.
March 7, 2012
Rakesh K. Pai, MD, FACC - Cardiology, Electrophysiology & George Philippides, MD - Cardiology
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