The autorhythmicity inherent in cardiac cells keeps the heart beating at a regular pace; however, the heart is regulated by and responds to outside influences as well. Neural and endocrine controls are vital to the regulation of cardiac function. In addition, the heart is sensitive to several environmental factors, including electrolytes.
Heart Rates
Heart rates (HRs) vary considerably, not only with exercise and fitness levels, but also with age. Newborn resting HRs may be 120 bpm. HR gradually decreases until young adulthood and then gradually increases again with age.
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Maximum HRs are normally in the range of 200-220 bpm, although there are some extreme cases in which they may reach higher levels. As one ages, the ability to generate maximum rates decreases. This may be estimated by taking the maximal value of 220 bpm and subtracting the individual’s age. So a 40-year-old individual would be expected to hit a maximum rate of approximately 180, and a 60-year-old person would achieve a HR of 160.
Cardiovascular Centers
Nervous control over HR is centralized within the cardiovascular centers of the medulla oblongata (Figure 1). The cardioaccelerator regions stimulate activity via sympathetic stimulation of the cardioaccelerator nerves, and the cardioinhibitory centers decrease heart activity via parasympathetic stimulation. During rest, both centers provide slight stimulation to the heart, contributing to autonomic tone. This is a similar concept to tone in skeletal muscles. Normally, vagal stimulation predominates as, left unregulated, the SA node would initiate a sinus rhythm of approximately 100 bpm.
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The cardiovascular center receives input from a series of visceral receptors. Among these receptors are various proprioreceptors, baroreceptors, and chemoreceptors. Collectively, these inputs normally enable the cardiovascular centers to regulate heart function precisely, a process known as cardiac reflexes. Increased physical activity results in increased rates of firing by various proprioreceptors located in muscles, joint capsules, and tendons. Any such increase in physical activity would logically warrant increased blood flow. The cardiac centers monitor these increased rates of firing, and suppress parasympathetic stimulation and increase sympathetic stimulation as needed in order to increase blood flow.
Other Factors Influencing Heart Rate
Table 1. Major Factors Increasing Heart Rate and Force of Contraction Factor Effect Cardioaccelerator nerves Release of norepinephrine Proprioreceptors Increased rates of firing during exercise Chemoreceptors Decreased levels of O2; increased levels of H+, CO2, and lactic acid Baroreceptors Decreased rates of firing, indicating falling blood volume/pressure Limbic system Anticipation of physical exercise or strong emotions Catecholamines Increased epinephrine and norepinephrine Thyroid hormones Increased T3 and T4 Calcium Increased Ca2+ Potassium Decreased K+ Sodium Decreased Na+ Body temperature Increased body temperature Nicotine and caffeine Stimulants, increasing heart rate Table 2. Factors Decreasing Heart Rate and Force of Contraction Factor Effect Cardioinhibitor nerves (vagus) Release of acetylcholine Proprioreceptors Decreased rates of firing following exercise Chemoreceptors Increased levels of O2; decreased levels of H+ and CO2 Baroreceptors Increased rates of firing, indicating higher blood volume/pressure Limbic system Anticipation of relaxation Catecholamines Decreased epinephrine and norepinephrine Thyroid hormones Decreased T3 and T4 Calcium Decreased Ca2+ Potassium Increased K+ Sodium Increased Na+ Body temperature Decrease in body temperature
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