The SA node (SA stands for sinoatrial) is
one of the major elements in the cardiac conduction system, the
system that
controls the heart rate. This stunningly designed system generates
electrical impulses and conducts them throughout the muscle of the
heart, stimulating the heart to contract and pump blood.
The
SA node
is the heart's natural pacemaker. The SA
node consists of a cluster of cells that are situated in the upper
part of the wall of the right atrium (the right upper chamber of the
heart). The electrical impulses are generated there. The SA node is
also called the sinus node.
The electrical signal generated by the SA node moves from
cell to cell down through the heart until it reaches the
atrioventricular node
(AV node), a cluster of cells
situated in the center of the heart between the atria and ventricles.
The AV node serves as a gate that slows the electrical current
before the signal is permitted to pass down through to the
ventricles. This delay ensures that the atria have a chance to fully
contract before the ventricles are stimulated. After passing the AV
node, the electrical current travels to the ventricles along special
fibers embedded in the walls of the lower part of the heart.
The
autonomic nervous system
, the same part of the nervous
system
as controls the blood pressure, controls the firing of the SA node
to trigger the start of the cardiac cycle. The autonomic nervous
system can transmit a message quickly to the SA node so it in turn
can increase the heart rate to twice normal within only 3 to 5
seconds. This quick response is important during exercise when the
heart has to increase its beating speed to keep up with the body's
increased demand for oxygen.
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