Definition of heart rate
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• Heart rate is determined by the rate of depolarizations of the sinoatrial (SA) node.
• Cardiac output is directly proportional to heart rate, the greater the heart rate the greater the cardiac output.
• Changes in heart rate are associated with exercise, stress or injury.
Nervous control of heart rate
▪ Heart rate is influenced by body's needs and regulated by autonomic nervous system and hormones.
▪ Situations that decrease blood pressure promote sympathetic stimulation and the release of norepinephrine, which increases heart rate and cardiac output.
▪ Situations that increase blood pressure promote parasympathetic stimulation and the release of acetylcholine, which decreases heart rate and cardiac output.
Chemical control of heart rate
• Heart rate is influenced by body's needs and regulated by autonomic nervous system and hormones.
• The hormones epinephrine and norepinephrine as well as thyroxine, accelerate heart rate.
• Low blood O2 and pH as well as high blood CO2 trigger an increased heart rate. • Moderate increases in cytosolic levels of ionic Ca+ also increase heart rate.