Heart rate regulation is primarily under the control of the cardiac control center located within the medulla oblongata of the brain. It receives sensory information about the level of blood pressure from baroreceptors located in the aortic arch and the carotid sinuses of the internal carotid arteries. It also receives sensory information from chemo- receptors in the aortic arch and the carotid bodies of the external carotid arteries. Baroreceptors are sensitive to changes in vessel wall stretching caused by both high and low blood pressure. Chemoreceptors are stimulated by low blood pH, high blood carbon dioxide levels, and very low blood oxygen levels. The cardiac control center is also affected by emotions, which are generated by the limbic system. The cardiac control center consists of both sympathetic and parasympathetic components. Nerve impulses transmitted to the heart via sympathetic axons cause an increase in heart rate and contraction strength, while nerve impulses transmitted by parasympathetic axons cause a decrease in heart rate. The cardiac control center constantly adjusts the frequency of sympathetic and parasympathetic nerve impulses to produce a heart rate and a contraction strength that meets the changing needs of tissue cells.
Neurons of the sympathetic division extend axons from the cardiac control center down the spinal cord to the thoracic region. There the sympathetic axons exit the spinal cord to innervate the SA node, AV node, and portions of the myocardium. The transmission of nerve impulses causes the sympathetic axons to secrete norepinephrine at synapses in the heart. Norepinephrine increases the heart rate and strengthens the force of myocardial contraction. Physical and emotional stresses, such as exercise, excitement, anxiety, and fear, stimulate the sympathetic division to increase heart rate and contraction strength.
Parasympathetic axons arise from the cardiac control center and exit in the vagus nerve (CN X) to innervate the SA and AV nodes. The transmission of nerve impulses causes the parasympathetic axons to secrete acetylcholine at the heart synapses, which decreases the heart rate. The greater the frequency of parasympathetic nerve impulses sent to the heart, the slower the heart rate. Excessive blood pressure and emotional factors, such as grief and depression, stimulate the parasympathetic division to decrease the heart rate.
When the heart is at rest, more parasympathetic nerve impulses than sympathetic nerve impulses are sent to the heart. As cellular needs for blood increase, a decrease in the frequency of parasympathetic nerve impulses and an increase in sympathetic nerve impulses cause heart rate to increase.
Age, sex, physical condition, temperature, epinephrine, thyroxine, and the blood levels of calcium and potassium ions also affect the heart rate and contraction strength.
The resting heart rate gradually declines with age, and it is slightly faster in females than in males. Average resting heart rates in females are 72 to 80 beats per minute, as opposed to 64 to 72 beats per minute in males. People who are in good physical condition have a slower resting heart rate than those in poor condition. Athletes may have a resting heart rate of only 40 to 60 beats per minute. An increase in body temperature, which occurs during exercise or when feverish, increases the heart rate.
Epinephrine, which is secreted by the adrenal glands during stress or excitement, affects the heart like norepinephrine-it increases the rate and strength of heart contractions. An excess of thyroxine produces a lesser, but longer-lasting, increase in heart rate.
Reduced levels of blood Ca2+ decrease the rate and strength of heart contraction, while increased levels of blood Ca2+ increase heart rate and contraction strength, and prolong contraction. In extreme cases, an excessively prolonged contraction may result in death. Excessive levels of blood K+ decrease both heart rate and contraction strength. A high dose of K+ is often used in lethal injections, in which the abnormally high levels of blood K+ cause the heart to stop contracting. Abnormally low levels of blood K+ may cause potentially life-threatening abnormal heart rhythms.