Electrical System of the Heart - Topic Overview
What makes your heart rate speed up or slow down? continued...
For example, during periods of exercise, when the body requires
more oxygen to function, signals from your body cause your heart rate to
increase significantly to deliver more blood (and therefore more oxygen) to the
body. Your heart rate can increase beyond 100 beats per minute to meet your
body's increased needs during physical exertion.
Similarly, during periods of rest or sleep, when the body needs
less oxygen, the heart rate decreases. Some athletes actually may have normal
heart rates well below 60 because their hearts are very efficient and don't
need to beat as fast. Changes in your heart rate, therefore, are a normal part
of your heart's effort to meet the needs of your body.
How does your body control your heart rate?
Your body controls your heart by:
- The sympathetic and parasympathetic nervous
systems, which have nerve endings in the heart.
- Hormones, such as
epinephrine and norepinephrine (catecholamines), which circulate in the
Sympathetic and parasympathetic nervous systems
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.
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.