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The sympathetic nervous system triggers numerous physiological changes to prepare the body for the fight-or-flight response.

During rigorous physical activity, the sympathetic nervous system redirects blood flow from the skin and abdominal organs to the skeletal muscles, reducing the activity of the digestive and urinary systems.

It also regulates the heart rate by constricting and dilating blood vessels in response to changes in blood pressure. It further promotes the release of renin from the kidneys, an enzyme that controls blood pressure.

Sympathetic activation increases the metabolic rate and glycogen breakdown in the liver and muscles to meet the energy needs of the body during emergencies.

In stressful situations, the sympathetic nervous system increases mental alertness by stimulating the reticular activating system in the brain.

The sympathetic nervous system also maintains a stable body temperature.

When body temperature rises, sympathetic nerves dilate skin blood vessels and activate sweat production to release the excess heat. Conversely, when body temperature drops, skin blood vessels constrict to retain heat.