The master regulator of blood flow is the heart, which responds to the needs of the body by altering the rate at which it contracts and pumps blood throughout the circulatory system. Through a variety of signaling mechanisms, the heart can detect changes in movement and activities that require a change in the rate of its contraction.
Heart Physiology
Cardiac output is equal to heart rate multiplied by stroke volume, or the amount of blood pumped by the heart with a single contraction. By increasing either of these variables, the heart can increase cardiac output as needed. Therefore, situations that require the heart to pump more blood will commonly produce elevated heart rates to meet these demands. The autonomic nervous system, which involuntarily controls heart rate, is a major player in this process and exerts its effects through the release of hormones, such as adrenaline and cortisol, into the circulatory system.
Exercise on Beta Blockers
One popular activity that has significant effects on heart rate is exercise. In order to supply muscles with enough energy to support physical activity, heart rate can be adjusted to increase cardiac output and blood flow as needed. This adaptation is accomplished through the release of adrenaline and other hormones during exercise, which exert widespread effects throughout your body. Notably, adrenaline binds to protein receptors on heart cells that are responsible for controlling heart rate and rhythm. Once bound, intracellular signals are generated, resulting in accelerating heart rates. Through this mechanism, the heart is able to properly respond to the demands placed on the body during exercise by increasing heart rate and cardiac output to muscles.
Stress
When you become psychologically stressed, many of the same regulatory pathways for controlling heart rate during exercise are activated. When under stress, your body activates the sympathetic branch of the autonomic nervous system, just as it does during exercise. As a result, large amounts of the hormone cortisol are released into the blood, which acts to increase blood pressure and heart rate. Chronic stress will lead to long-term elevations in stress hormones and heart rate, which may wear down your system overtime. According to the article "The Science of Exercise," individuals may differ in the degree of heart rate variation during stress because of differences in their genetic makeup.
Changes in Posture
If you have ever felt lightheaded or dizzy when standing up after lying down for a long period, you have experienced the effect of postural changes on heart rate. When you are lying down, blood can easily circulate throughout the body. The effects of gravity are minimized when in this position, which facilitates the pumping of blood out of the heart. Over time your body becomes accustomed to these circumstances; the heart adapts to the decrease in resistance by slowing the rate of pumping. When you rapidly stand up, this postural change suddenly reintroduces the force of gravity into the equation. For that instant, your heart is pumping slower than it must in order to supply enough blood to the brain. This slight decrease in blood perfusion may be experienced as feelings of lightheadedness. Pressure sensors within the carotid artery, the major artery supplying blood to the brain, detect these changes in blood supply and respond by increasing heart rate. In some individuals, these pressure sensors do not function properly, leading to more severe symptoms with dramatic changes in posture; this condition is known as orthostatic hypotension.
Applications
While changes in heart rate may be completely innocuous, and even predictable based on principles of normal physiology, this is not always the case. If you have a history of cardiovascular disease, high blood pressure, or arrhythmias, changes in heart rate may indicate the presence of an underlying problem. Talk to your doctor if you experience changes in heart rate for no particular reason, or simultaneously experience chest pain.
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