The heart has a natural pacemaker that regulates
the pace or rate of the heart. It sits in the upper
portion of the right atrium (RA) and is a collection
of specializes electrical cells known as the SINUS
or SINO-ATRIAL (SA) node.
Like the spark-plug of an automobile
it generates a number of "sparks" per minute.
Each "spark" travels across a specialized
electrical pathway and stimulates the muscle wall
of the four chambers of the heart to contract (and
thus empty) in a certain sequence or pattern. The
upper chambers or atria are first stimulated. This
is followed by a slight delay to allow the two atria
(atria is plural for atrium and pronounced ay-tree-ya)
to empty. Finally, the two ventricles are electrically
In an automobile, the number of
sparks per minute generated by a spark plug is increased
when you press the gas pedal or accelerator. This
revs up the motor. In case of the heart, adrenaline
acts as a gas pedal and causes the sinus node to increase
the number of sparks per minute, which in turn increases
the heart rate. The release of adrenaline is controlled
by the nervous system. The heart normally beats at
around 72 times per minute and the sinus node speeds
up during exertion, emotional stress, fever, etc.,
or whenever our body needs an extra boost of blood
supply. In contrast, it and slows down during rest
or under the influence of certain medications. Well
trained athletes also tend to have a slower heart
The sequence of electrical activity
within the heart is displayed in the diagrams above
and occurs as follows:
As the SA node fires, each electrical impulse travels
through the right and left atrium. This electrical
activity causes the two upper chambers of the heart
to contract. This electrical activity and can be recorded
from the surface of the body as a "P" wave"
on the patient's EKG or ECG (electrocardiogram).
The electrical impulse then moves
to an area known as the AV (atrio-ventricular) node.
This node sits just above the ventricles. Here, the
electrical impulse is held up for a brief period.
This delay allows the right and left atrium to continue
emptying it's blood contents into the two ventricles.
This delay is recorded as a "PR interval."
The AV node thus acts as a "relay station"
delaying stimulation of the ventricles long enough
to allow the two atria to finish emptying.
Following the delay, the electrical
impulse travels through both ventricles (via special
electrical pathways known as the right and left bundle
branches). The electrically stimulated ventricles
contract and blood is pumped into the pulmonary artery
and aorta. This electrical activity is recorded from
the surface of the body as a "QRS complex".
The ventricles then recover from this electrical stimulation
and generates an "ST segment" and T wave
on the EKG.
In summary, the heart constantly
generates a sequence of electrical activity with every
single heart beat. This can be recorded on paper or
displayed on a monitor by attaching special electrodes
to a machine that can amplify and record an EKG or
ECG (electrocardiogram). The animation (above) shows
the sequence of electrical activity throughout the
heart. Note how the chambers of the heart contract
when they are electrically stimulated. This in turn
makes the heart valves open and shut.
Click on the NEXT button below to move to the EKG
section . .