What Is the Autonomic Nervous System and How Does It Affect Your Health?

Author: Lisette Portes, MD, Board-Certified Family Medicine Physician

Published / Last Reviewed: November 3, 2026 / Medically reviewed by Lisette Portes, MD

The autonomic nervous system (ANS) is the part of your nervous system that runs automatic body functions, including heart rate, blood pressure, breathing, digestion, sweating, bladder control, and pupil size. Its sympathetic branch prepares you for action, its parasympathetic branch supports rest and repair, and its enteric branch manages the gut. Because it links every organ, ANS balance shapes sleep, stress, heart rate variability, and chronic disease.

What Is the Autonomic Nervous System?

The autonomic nervous system is the network of nerves that controls the body functions you do not consciously think about, such as heart rate, blood pressure, breathing rate, digestion, and body temperature. The word autonomic means self-governing. According to Cleveland Clinic, the ANS has three divisions: the sympathetic, parasympathetic, and enteric nervous systems. Control centers in the brainstem and hypothalamus receive a constant stream of sensory reports from organs and blood vessels, then send adjusting signals back out. Cleveland Clinic notes that autonomic fibers travel in four of the 12 cranial nerves and in most of the 31 spinal nerves. This wiring lets the ANS respond within seconds when you stand up, eat a meal, exercise, or feel afraid. You can think of it as the body's background operating system. It keeps working while you sleep, and it constantly adjusts to keep your internal environment stable.

What Is the Difference Between the Sympathetic and Parasympathetic Nervous Systems?

The sympathetic nervous system prepares the body for action, while the parasympathetic nervous system supports rest, digestion, and recovery. Cleveland Clinic describes these as the fight-or-flight and rest-and-digest responses. Sympathetic activity speeds the heart, narrows many blood vessels, raises blood pressure, opens the airways, widens the pupils, and releases stored glucose for fuel. Parasympathetic activity slows the heart, stimulates saliva and digestive secretions, and helps the bowel and bladder empty at the right time. These branches are not simple on and off switches. Both are active all day, and healthy regulation means shifting smoothly between them as your needs change. A 2017 review by Shaffer and Ginsberg in Frontiers in Public Health explains that parasympathetic control predominates at rest, resulting in an average resting heart rate of about 75 beats per minute. Problems tend to appear when one branch stays stuck on high or the shifts become slow.

What Do the Vagus Nerve and the Enteric Nervous System Do?

The vagus nerve is the main nerve of the parasympathetic nervous system, and the enteric nervous system is a semi-independent web of nerves inside the wall of the gut. A 2018 review by Breit and colleagues in Frontiers in Psychiatry describes the vagus as the main component of the parasympathetic system. It is the 10th cranial nerve, and it travels from the brainstem to the heart, lungs, and digestive tract. It carries signals in both directions: commands from the brain to the organs, and sensory information from the organs back to the brain. The enteric nervous system is often called the second brain. A 2012 review by Furness in Nature Reviews Gastroenterology and Hepatology estimates that the human gut contains 400 to 600 million enteric neurons. These neurons coordinate gut muscle contractions, secretions, and local blood flow, while the vagus and sympathetic nerves fine-tune their work. This gut-brain connection helps explain why stress can upset digestion.

What Body Functions Does the Autonomic Nervous System Control?

The autonomic nervous system controls heart rate, blood pressure, breathing patterns, digestion, sweating, body temperature, bladder and bowel function, pupil size, and sexual function. Cleveland Clinic also lists tear and saliva production, airway width, immune reactions, insulin release from the pancreas, and glucose release from the liver. Most of these functions run through reflexes. For example, the baroreflex senses blood pressure in the large arteries of the neck and chest. When you stand and blood pools in your legs, this reflex tightens blood vessels and raises heart rate within seconds so your brain keeps its blood supply. Similar loops adjust sweating when you overheat and pupil size when light changes. Because these reflexes are fast and automatic, most people notice the ANS only when something goes wrong, such as lightheadedness on standing, a racing heart, or sudden digestive changes. The main areas it manages include:

Heart and blood vessels: heart rate, blood pressure, and vessel width
Digestion: saliva, movement of the stomach and intestines, and bowel control
Metabolism: insulin release and glucose output from the liver
Temperature: sweating, skin blood flow, and goosebumps
Bladder and sexual function: storage, emptying, and arousal
Eyes and glands: pupil size, focusing, tears, and nasal mucus

How Does the Autonomic Nervous System Affect Your Overall Health?

The autonomic nervous system affects nearly every part of health because it coordinates how your organs respond to stress, sleep, food, movement, and illness. That is why an integrative view treats the ANS as a common language linking symptoms that seem unrelated. Writing in the New England Journal of Medicine in 1998, Bruce McEwen called healthy adaptation allostasis, meaning the ability to achieve stability through change. He described allostatic load as the wear and tear from chronic overactivity or underactivity of these stress systems, which he linked to problems such as high blood pressure and insulin resistance. A 2002 paper by Kevin Tracey in Nature described an inflammatory reflex, in which vagus nerve signals help limit inflammation. When regulation falters, people may notice signs of autonomic dysfunction. Many chronic diseases also have an autonomic piece, and heart rate variability offers one window into this balance.

What Should Clinicians Know About Autonomic Physiology?

Clinicians should view autonomic function as integrated reflex control rather than a single sympathetic versus parasympathetic balance score. Afferent input from baroreceptors, chemoreceptors, and visceral receptors converges on the nucleus tractus solitarius, which shapes efferent output through the nucleus ambiguus, the dorsal motor nucleus of the vagus, and sympathetic premotor neurons. The neurovisceral integration model, proposed by Thayer and Lane in the Journal of Affective Disorders in 2000, links a central autonomic network, including prefrontal and limbic regions, to cardiac vagal control and heart rate variability. In practice, cardiovagal function is often estimated with RMSSD or high frequency power, as framed by the 1996 Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Shaffer and Ginsberg place the high frequency band at 0.15 to 0.40 Hz and note that 5 minutes is the conventional short-term recording. HRV alone is not diagnostic and needs clinical context.

How Can an Integrative, Physiology-Focused Doctor Help You Understand Your Autonomic Nervous System?

An integrative, physiology-focused doctor can help by connecting your symptoms, sleep, stress, activity, and chronic conditions through the lens of autonomic regulation. Dr. Lisette Portes is a board-certified family medicine physician who sees patients in Miami and offers telehealth for patients located in Florida, in English and Spanish. She uses an autonomic, physiology-focused perspective when evaluating patients. You can bring questions about lightheadedness, palpitations, digestive changes, fatigue, wearable HRV data, or how stress affects high blood pressure and diabetes. Her primary care visits focus on how your body systems interact, not just one symptom at a time. Learn more about Dr. Portes or call 305-328-5079 to schedule. If you faint and are injured, have chest pain, or notice stroke signs such as face drooping, arm weakness, or slurred speech, call 911. This article is general education, not medical advice.

Frequently Asked Questions About the Autonomic Nervous System

What are the three divisions of the autonomic nervous system?

The three divisions are the sympathetic, parasympathetic, and enteric nervous systems. The sympathetic division prepares the body for action by raising heart rate and blood pressure. The parasympathetic division supports rest, digestion, and recovery, mainly through the vagus nerve. The enteric division is a network of hundreds of millions of neurons in the gut wall that manages digestion and communicates with the brain.

Can you control your autonomic nervous system?

You cannot directly command your autonomic nervous system the way you move a muscle, but your daily habits influence it. Slow, relaxed breathing, regular physical activity, consistent sleep, limiting alcohol, and managing stress can all shift the system toward parasympathetic recovery. Breathing is a special case because it is both automatic and voluntary, which makes it a useful entry point for regulation.

What are the signs of autonomic nervous system problems?

Common signs include lightheadedness or fainting when standing, a racing heart, digestive problems such as nausea or constipation, sweating too much or too little, bladder changes, temperature intolerance, and fatigue or brain fog. These symptoms have many possible causes, so they deserve a medical evaluation. Our guide to autonomic dysfunction symptoms explains the signs, conditions, and tests in more detail.

Is the vagus nerve part of the autonomic nervous system?

Yes. The vagus nerve is the 10th cranial nerve and the main component of the parasympathetic nervous system, which is one branch of the autonomic nervous system. It connects the brainstem with the heart, lungs, and digestive tract. It sends calming signals that slow the heart and support digestion, and it carries sensory information from the organs back to the brain.

What conditions affect the autonomic nervous system?

Many conditions can affect autonomic function. Cleveland Clinic lists diabetes, Parkinson's disease, multiple system atrophy, amyloidosis, lupus, Sjogren's syndrome, vitamin B12 deficiency, postural orthostatic tachycardia syndrome, and long COVID among possible causes of dysautonomia. Medications, dehydration, and prolonged bed rest can also affect autonomic reflexes such as blood pressure control when you stand up.

How do you calm the sympathetic nervous system?

Calming the sympathetic nervous system usually means supporting parasympathetic activity. Practical steps include slow breathing with longer exhales, regular aerobic exercise, a steady sleep schedule, time outdoors, social connection, and limiting caffeine and alcohol. Therapy can help when stress or anxiety is persistent. If you feel stuck in a stressed state, talk with a clinician, because medical issues such as thyroid disease can play a role.

What Is the Autonomic Nervous System, and How Does It Shape Your Health?