A normal heart rate-about 60 to 100 beats per
minute. Your heart rate can increase beyond 100 beats per minute if your body
needs more blood, such as during strenuous exercise. Some athletes may have
normal heart rates below 60 beats per minute because their hearts are very
efficient and don't need to beat as fast.
Normal sinus rhythm,
which is the regular normal contraction of your atria and ventricles.
Why is a normal sinus rhythm important?
When your heart's electrical system works correctly, your atria and
ventricles contract in a synchronized fashion. In a normal sinus rhythm, this
pattern repeats itself exactly the same way with each heartbeat. If one of the
heart chambers contracts out of synchrony, it can interfere with your heart's
ability to pump blood efficiently.
There is an orderly transmission of electrical signals through the
heart. This orderly electrical transmission ensures a synchronized regular
beating of the heart, so that the atria contract first, pumping blood into the
ventricles, and the ventricles contract next, pumping blood to the rest of the
body and lungs.
How is my heart rate controlled?
Heart rate is controlled very precisely by two mechanisms:
The sympathetic and parasympathetic nervous
systems, which have nerve endings in the heart
Hormones, such as
epinephrine and norepinephrine (catecholamines), which circulate in the
How do the sympathetic and parasympathetic nervous systems control my heart rate?
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.
How do catecholamines control my heart 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. In people with heart failure, the level of
norepinephrine in the blood is chronically elevated in an effort to increase
cardiac output by increasing heart rate.
How does the electrical system regulate the rhythm of my heartbeat?
Conducting cells (cells that carry your heart's
Muscle cells (cells that enable your heart's
chambers to contract, an action triggered by your heart's electrical
The electrical signal starts in a group of electrical cells at the
top of your heart called the sinoatrial (SA) node. The signal then travels down
through your heart and first triggers your two atria (upper chambers). It then
passes through the atrioventricular (AV) node and then to the two ventricles
How does the signal trigger my atria?
The SA node is the heart's natural pacemaker. It generates the
electrical signal and triggers the following process:
The signal travels from your SA node through
muscle cells in your right and left atria.
The signal causes the
muscle cells that make up your atria to contract and then goes on to the AV
node. The AV node is the relay point between the upper and lower chambers.
The atria contract, pumping blood into your left and right
The signal travels from the AV node down a structure called the
bundle of His, which divides into two branches; one branch goes to the left
ventricle, another to the right ventricle. These two branches divide further
into a system of conducting fibers that carry the signal rapidly into your left
and right ventricles, causing the ventricles to contract. When the ventricles
contract, your right ventricle pumps blood to your lungs and the left pumps
blood to the rest of your body. After the ventricles contract, the pattern
repeats itself with a new electrical impulse that starts in the sinus
Primary Medical Reviewer
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer
John M. Miller, MD - Electrophysiology
November 2, 2010
WebMD Medical Reference from Healthwise
November 02, 2010
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