Why Does Your Heart Beat Faster When You Run?

When you run, your heart rate increases for a variety of reasons, including an increase in oxygen consumption by exercising muscle, an increase in heat and an increase in volume loss from your circulatory system by way of sweating. However, the overarching mechanism is an increase in the activity of your body's sympathetic nervous system.

Normal Control of Heart Rate

Your heartbeat is driven by electrical impulses originating from a specialized group of cells located in the right atrium known as the sinoatrial, or SA, node. Without additional input, the SA node would normally fire between 100 and 115 times a minute. However, the SA node receives input from the vagus nerve in the form of acetylcholine molecules, which slow the rate of SA firing to between 60 and 100 beats per minute.

Running and the Nervous System

Exercise promotes the release of epinephrine and norepinephrine, the so-called catecholamines of the sympathetic nervous system. These molecules bind to receptors on the heart that increase the rate of firing at the SA node. At the same time, inhibitory input to the SA node from the vagus nerve decreases. Together, the increase in sympathetic output and the decrease in vagus nerve output causes an increase in heart rate.

Running and Oxygen Consumption

Exercising muscle requires more oxygen than resting muscle. Exercising muscles produce a set of metabolites, including lactic acid and adenosine, which cause arteries in the vicinity of those muscles to dilate. The goal is to bring more blood, and therefore more oxygen, to the exercising muscle. As a result of dilating arteries in the vicinity of exercising muscle, the average pressure in your cardiovascular system drops. This drop is sensed by pressure sensors in arteries close to your heart and signals in the form of sympathetic output are sent to the SA node to increase heart rate.

Running and Heat

Exercising also causes people to heat up. Part of the thermoregulatory response to increasing body temperature is to dilate arteries near the surface of the skin to facilitate heat exchange between your blood and the external environment. This is also known as sweating. When these vessels dilate, average blood pressure drops, just like it does when vessels in active muscle dilate, and heart rate increases in response.

Running and Dehydration

A final explanation for increased heart rate during exercise has to do with dehydration. Specifically, when you exercise and sweat, you are losing volume from your circulatory system. A volume loss decreases the average pressure in your arteries, and, as above, results in increased release of sympathetic molecules and an increase in heart rate.