The AV node (AV
stands for
atrioventricular) is an electrical relay station between the atria
(the upper) and the ventricles (the lower chambers of the heart).
Electrical signals from the atria must pass through the
AV node to reach the ventricles.
The AV node 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
(SA stands for sinoatrial) 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
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 (an "electrical relay station") 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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