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 bloodstream.
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.