Autonomous or vegetative nervous system is an important part of the central and peripheral nervous system that ensures the function of internal organs in accordance with the variable conditions of the internal and external environment. It regulates human body reactions, by preparing the body for physical loads or stressful situations, mobilising organ systems (elevating the blood pressure, increasing the heart rate and respiration frequency, activating perspiration, etc.). Meanwhile at rest it relaxes the body enabling one to restore energy (the heart rate becomes slower, the blood pressure normalises, etc.). Thus, the autonomous nervous system regulates heart function, blood pressure, breathing, sweating, digestion, pupil reactions, urination/defecation, function of endocrine organs and sexual function.
Autonomous nervous system is divided into the sympathetic and parasympathetic nervous system. It functions autonomously and is not subject to human will, however, its functioning is partially affected by the central nervous system (the brain).