Exercise increases heart rate, which may be closely monitored under certain training regimens. Breathing also increases and the skin produces sweat. In fact, breathing and sweat production are processes that respond to and benefit from an increased heart rate. These three body responses are closely linked and work together during exercise.
Function
Breathing expels carbon dioxide from the body and takes in oxygen. The importance of this exchange is demonstrated in muscle cell metabolism. Muscle cells use oxygen to produce energy, a process that creates carbon dioxide. The blood removes the carbon dioxide from the muscles and carries it to the lungs to be exchanged for oxygen. Exercise increases the work done by muscles, so more oxygen is needed and more carbon dioxide is produced. Heart rate increases to more quickly move the oxygen- and carbon dioxide-carrying blood to the muscles and lungs.
Stress and Anticipation
Both stress and the anticipation of exercise can cause the heart rate to rise. According to the Illinois Wesleyan University presentation, "Cardiovascular Control During Exercise," the brain releases hormones in response to anxiety or anticipation to prepare the body for the "fight or flight" response. Together with increased breathing, the increased heart rate delivers more oxygen and nutrients to the muscles so they can quickly react when it is time. The stress response may also lead to heightened awareness.
Carbon Dioxide Receptors
Increased cellular respiration leads to increased levels of carbon dioxide in the blood. Receptors in the carotid arteries and aorta sense the increased levels of carbon dioxide and send an impulse to the brain's medulla oblongata, which sends an impulse to the heart, telling it to increase its rate. This increased heart rate delivers the carbon dioxide-rich blood to the lungs at a faster rate for quicker carbon dioxide removal.
Increased Blood Return
Exhalation pulls blood into the chest cavity and muscles push oxygen-free blood back to the heart when they contract. Breathing accelerates and muscles contract more often during exercise, so more blood is pushed toward the heart. The right atrium is the heart chamber that receives blood when it returns from the body. According to Harvard University's Dr. John Kimball, when blood return increases, the atrium stretches, causing it to send a signal to the brain telling it that blood flow is increasing from the body. The brain responds by sending a signal to make the heart rate accelerate.
Body Temperature Regulation
Springfield Technical Community College's "Skin Function" article describes the body's method of temperature regulation as "cooling down your blood." Blood vessels dilate to allow more blood to flow into the skin, causing "it to run very near the surface of your body, and [allowing] for heat to escape from your blood quite easily into the air." Increased body temperature causes a heart rate increase to push more blood into the skin.
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