In man, heart is
a pulsatile, four-chambered, roughly triangular, highly muscular organ which
contracts rhythmically to push the blood through a network of blood vessels.
The contraction of the heart, which is commonly known as "heart beat", begins by an automatic, inbuilt excitation or stimulation of a region
located in the upper right chamber, or "right atrium". This region is
known as sinoatrial node (SA node).
(Please refer to the diagram). The heartbeat, which is initiated at SA node, is
an electrical signal and it spreads
from its origin to other regions of the heart, resulting in its
contraction as a whole. This is a rhythmical phenomenon and therefore, the
pulsatile rate of a normal adult heart beat is 72 beats per minute. The
sinoatrial node, where the heart beat begins is, also called as the normal "pacemaker" of the heart,
because it sets the rhythm of the heart beat. If the heart beat originates at a
site other than the normal pacemaker of the heart, it is known as ectopic focus.
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