Breathing Physiology Glossary: CO2, HRV, Vagal Tone and More

About this glossary

These are the body-science words you meet in breathing articles and research. Knowing them helps you judge claims, such as "this breath activates your vagus nerve". Understanding the physiology does not make forced or held breathing safe, and none of these terms is a way to diagnose yourself. If breathing is difficult, new or getting worse, contact a health professional; MedlinePlus advises contacting your provider for any breathing difficulty. [1] For warning signs, see when breathlessness needs medical care.

The breathing machinery

• Diaphragm. A dome-shaped muscle below your lungs that separates the chest cavity from the abdominal cavity. [2] It is the main muscle used for breathing. [2]
• Inhalation. NHLBI explains that when you breathe in, the diaphragm contracts and moves downward. [3]
• Exhalation. Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. [3]
• Alveoli. The air sacs that air reaches after passing through the bronchial tubes. [3]
• Gas exchange. Oxygen passes through the thin walls of the alveoli into surrounding capillaries, while carbon dioxide trades places with it and is breathed out. [3] See how breathing works.

Gases and the blood

• Carbon dioxide (CO2). The waste gas carried back to the lungs from the cells of the body. [3] Sensors in the brain and near blood vessels detect carbon dioxide and oxygen levels in your blood, which helps adjust your breathing. [2]
• Low CO2 from overbreathing. MedlinePlus says excessive breathing creates a low level of carbon dioxide in your blood, which causes many of the symptoms of hyperventilation. [4] Those include feeling lightheaded or dizzy, and numbness and tingling in the arms or around the mouth. [4] This is why we never teach overbreathing.
• The urge to breathe. A children's hospital explains that rising carbon dioxide in the bloodstream triggers an urge to breathe, and ignoring that urge in breath-holding challenges can lead to fainting underwater. [5] See CO2, oxygen and blood pH in breathing.
• End-tidal CO2. A CO2 measure used in a study of holotropic and conscious-connected breathwork, where a drop linked to deliberate hyperventilation was associated with altered states of consciousness. [6] The abstract we read does not define the term in plain language, so we do not go further.
• Oxygen, low oxygen. Sleep apnea can prevent your body from getting enough oxygen. [7] Home oximeters and "CO2 tolerance" claims are covered on pulse oximeters and SpO2 estimates and CO2 tolerance claims.

Nerves, heart and blood pressure

• Autonomic nervous system. The involuntary nervous system that usually controls breathing without thought. [2] See the autonomic nervous system and breathing.
• Parasympathetic and sympathetic systems. NHLBI says the parasympathetic system slows your breathing rate, and the sympathetic system increases it. [2]
• Heart rate variability (HRV). The fluctuation in the time intervals between adjacent heartbeats. [8] The same overview cautions that higher HRV is not always better, because some pathological conditions can raise it. [8]
• Respiratory sinus arrhythmia (RSA). Rhythmic fluctuations in heart rate synchronized to breathing in and out. [9] A review found slow breathing techniques increased HRV and RSA in healthy people. [10] That is a physiological finding, not proof of a health benefit.
• Vagal tone. A term used for activity of the vagus nerve, which plays a major role in the parasympathetic nervous system. [9] Experts warn that RSA is neither an invariably reliable index of cardiac vagal tone nor of central vagal outflow to the heart. [9] Another overview states that HF power and RSA do not represent vagal tone. [8] See the claim check on "activating the vagus nerve".
• Baroreflex. A control loop that links heart rate, blood pressure and vascular tone. [8] Heart-rate-variability biofeedback is described as breathing at an individual's resonance frequency, which stimulates RSA and the baroreflex. [11]
• Proposed mechanisms. One review calls strengthening of homeostasis in the baroreceptor the most supported possible mechanism for HRV biofeedback, which makes it a proposed explanation. [12] See vagal tone, HRV and RSA in breathing research.

Symptoms and conditions

• Hyperventilation. Rapid and deep breathing, also called overbreathing. [4] It may come from an emotional cause such as a panic attack, or from a medical problem such as bleeding or infection. [4] See overbreathing: symptoms, causes and emergencies.
• Hyperventilation syndrome. MedlinePlus says people who frequently overbreathe may have this medical problem. [4]
• Dyspnea (breathlessness). MedlinePlus lists shortness of breath, breathlessness, difficulty breathing and dyspnea as alternative names, and calls it a subjective feeling. [1]
• Wheezing. A high-pitched sound made when you breathe out. [1]
• Syncope (fainting). A brief loss of consciousness due to a drop in blood flow to the brain. [13]
• Sleep apnea. A common condition in which breathing stops and restarts many times during sleep. [7]

Normal breathing rate

We have not listed a "normal" rate here. Ranges depend on age and health, and the sources we read for this page did not give one. See breathing rate: normal ranges and what changes it, and for word meanings outside physiology, the breathwork glossary.