The volume of blood circulated by a human heart over a lifetime would fill more than a trio of oil supertankers. The contraction of the heart muscle constantly regulates this flow in the form of a pulse, measured in heartbeats per minute. The right atrium, which is one of four heart chambers, contains the sinus node, which acts as the pacemaker for the heart. The body's nervous system, neurotransmitters and hormones regulate the sinus node. In addition, exercise, physical activity and emotional and physical stress also affect heart rate.
Two opposing mechanisms -- the sympathetic and parasympathetic nervous systems -- regulate the heart rate. Your heart keeps beating whether you are awake or asleep, and the parasympathetic nervous system carries out this constant background beating. The sympathetic nervous system, when activated, causes the heart rate to speed up. When the heart rate is high, the parasympathetic system brings the heart rate down again to the background level. In a part of the brain, called the medulla, a "cardiac center" receives information from different parts of the body and decides whether to activate the parasympathetic system to slow the heart rate down or to stimulate the sympathetic system to increase the heart rate.
Neurotransmitters and Hormones
Neurotransmitters are substances or chemicals that activate nerve cells and allow them to communicate with other nerve cells and muscle cells. Norepinephrine, or noradrenaline, and epinephrine, or adrenaline, activate the sympathetic nervous system and cause the heart rate to speed up. Acetylcholine stimulates the parasympathetic nervous system and lowers the heart rate. Thyroid hormones, which affect virtually all cells in the body, increase the heart rate. During hyperthyroidism, thyroid hormone levels are abnormally high and force the heart to beat at a high rate that can harm the heart muscle.
Exercise or other forms of physical activity stimulate the sympathetic nervous system pathway, causing the heart to beat faster, thereby increasing the blood supply to muscles. During physical activity, muscle activity delivers more blood to the right atrial chamber of the heart, and nerve cells communicate this information to the cardiac center in the medulla. Exercise can cause the heart rate to rise from a basal heart rate of 60 to 80 beats per minute to a maximum of about 200 beats per minute, depending on an individual's genes and age. When physical activity stops, loss of pressure in the arteries is communicated to the medulla, and the parasympathetic nervous system kicks in, lowering the heart rate.
Both emotional and physical stress, such as an injury or illness, can increase the heart rate. Even if your heart rate is at rest while watching a movie, a car chase in the movie will increase your heart rate. The increased heart rate is produced by the body's "fight and flight" response, which causes the secretion of epinephrine by the adrenal glands. Epinephrine stimulates the sympathetic nervous system, raising the heart rate. Fever or injury, accompanied by an increase in blood flow to peripheral tissues, such as the skin, will also increase the heart rate via the sympathetic nervous system.