Several factors cause the heart rate to increase during exercise. The heart pumps blood throughout the body to deliver oxygen and nutrients to the cells, which use the oxygen and nutrients to produce energy. Muscle cells use energy to contract the muscles. Together with muscle relaxation, this facilitates movement.
Cell metabolism is the process of using energy for cell functions. This process creates wastes that are carried away by the blood. During exercise, muscle energy needs and waste production increase, so the heart must pump faster to deliver more blood to the working muscles.
Muscle Pump Mechanism and Increased Heart Rate
The muscle pump mechanism is the ability of the muscles to push blood towards the heart when they contract. This is especially important because veins have low blood pressure and require the aid of other organs for blood return.
The right atrium is the heart chamber that receives blood from the body. In the May 16, 2006 Kimball's Biology Pages article, " The Heartbeat," by Dr. John W. Kimball, the right atrium contains stretch receptors that send an impulse to the brain when increased amounts of blood stretch the chamber. Working muscles return a greater amount of blood to the heart stimulating the stretch receptors. The brain then signals the heart to beat faster to accommodate for the increased blood return.
Breathing Rate and Heart Rate Variability
Breathing affects the flow of blood from the veins into the chest cavity where the heart resides. Inhalation slows the flow of blood in the veins, and often stops it. Exhalation draws the blood through the veins. This lower pressure in the chest cavity pulls the blood toward the heart.
Increased breathing during exercise accelerates this process and works to fill the heart's right atrium more quickly causing the heart to beat faster. Decreased breathing decreases the rate of blood return to the heart which allows the heart rate to decrease.
Blood pH Levels
In their September 5, 2008 article, " pH Buffers in the Blood," Rachel Casiday and Regina Frey of the Washington University Department of Chemistry explain how exercise affects blood pH levels and heart rate. Increased muscle activity leads to increased carbon dioxide and hydrogen levels in the blood. These chemicals are the products of cell metabolism. They lower blood pH, or increase acidity, beyond optimal levels.
The major artery of the body, called the aorta, and the carotid arteries in the neck contain chemoreceptors that signal the brain when carbon dioxide levels are too high in the blood. The brain stimulates the heart rate to increase in order to accelerate processes that remove excess carbon dioxide, including blood filtering in the lungs and kidneys.
Thermoregulation
The working muscles produce heat that raises the body temperature. This stimulates the brain to trigger the skin to produce sweat. The blood vessels in the skin dilate to allow a greater volume of blood to flow into the skin to facilitate heat transfer to the sweat. The heart rate increases to push more blood into the skin where the heat will be lifted away through sweat evaporation.
Muscle activity, breathing, blood pH, and body temperature are all closely related. A change in one will lead to changes in the others. Together, they work to maintain normal chemical and temperature levels within the body.
Reference for Heart Rate and Breathing Rate in Response to Exercise
Shier, D., Butler, J., Lewis, Ricki. Hole's Human Anatomy & Physiology. New York: McGraw-Hill, 2002.
Diane Ursu - Diane Ursu joined Suite101 as a contributing writer in August 2009 and became a Feature Writer in January 2010. She is a freelance writer ...
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