Increased movement requires that the muscles use more oxygen and glucose from the blood. This increased use produces wastes that cause an imbalance in blood pH by decreasing it. The heart rate increases so that the blood can deliver the wastes to the lungs and kidneys to remove those wastes. The faster the muscles use energy, the faster the heart must pump blood.

Sustained Energy for Muscles

The heart pumps blood to the body tissues to deliver oxygen and nutrients and to carry away wastes. Exercise increases the energy needs of the muscles, so the heart must quickly deliver blood to the appropriate organs to replenish oxygen and nutrient stores as they are used up by the muscles. The increased use of energy also increases the production of wastes that must be carried away in the blood more quickly.


Muscle Pump Increases Heart Rate

Each time one exhales, pressure in the chest cavity decreases and blood is drawn from the extremities toward the heart. The pumping action of muscles is another way the body returns blood to the heart. When breathing and muscle activity increase, such as during exercise, more blood returns to the heart. The right atrium is the part of the heart that receives the blood from the body. It contains stretch receptors that send an impulse to the brain when they are stretched by the increased amount of blood returning from the body.

Carbon Dioxide – Blood pH

Active muscles produce carbon dioxide when they create energy. Carbon dioxide lowers the blood pH making it more acidic. The carbon dioxide is picked up by the blood and carried to the lungs for removal. When carbon dioxide levels in the blood increase, the brain is signaled by chemoreceptors in the aorta, the artery that carries oxygen-rich blood from the heart to the body, and the carotid arteries, the neck arteries that carry blood to the brain. The heart rate accelerates in response to a signal from the brain.

Heart Rate Variability and the Stress Response

Stress and anxiety trigger the “fight or flight” response. This may affect competitive athletes because they feel more pressure to be stronger and faster. When threatened by a competitor, such as at the start of a race, the body releases chemicals that accelerate the heart and breathing rates. In addition to stress and competition, the brain senses when one is preparing to exercise. It releases chemicals that stimulate the heart rate to accelerate in preparation for physical activity.

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