The short answer
Every time you breathe, your lungs trade one gas for another. Air moves in, oxygen crosses into your blood, and carbon dioxide — a waste gas your cells produce — moves out. This exchange, called gas exchange, happens automatically thousands of times a day, controlled by muscles you rarely think about and a nervous system that adjusts your breathing rate to match what your body needs.
How air actually moves in and out
Your lungs can’t expand on their own — they’re more like sponges than balloons with their own muscle. A dome-shaped muscle below your lungs called the diaphragm does most of the work. When it contracts, it creates a slight vacuum in your chest that pulls air in. When it relaxes, your lungs deflate on their own, similar to an elastic balloon left open to the air.
Other muscles help depending on what you’re doing:
- The muscles between your ribs (intercostal muscles) assist during physical activity
- Abdominal muscles help push air out when you’re breathing hard, such as during exercise
- Muscles in your neck and collarbone area help you breathe in
The evidence ladder: what we know, in order
1. Confirmed by NHLBI: the basic exchange
When you inhale, oxygen from the air moves into your blood. At the same time, carbon dioxide moves from your blood into your lungs and is exhaled. Red blood cells carry that oxygen to cells throughout your body.
2. Confirmed by NHLBI: breathing runs on autopilot
You don’t have to think about breathing because it’s controlled by your autonomic (involuntary) nervous system, which has two branches that pull in opposite directions:
- The parasympathetic system slows your breathing rate and widens the blood vessels in your lungs
- The sympathetic system speeds your breathing rate and narrows those same blood vessels
Sensors in your airways, brain, blood vessels, joints, and muscles constantly report back to your brain’s breathing centers — detecting irritants, tracking carbon dioxide and oxygen levels in your blood, and picking up on physical movement — so your breathing rate adjusts to match your activity level.
3. Confirmed by NCCIH: stress changes your breathing, on purpose
When you’re under stress, your body releases hormones that trigger a “fight-or-flight” response: heart rate, breathing rate, and blood pressure all rise. This is a normal, short-term coping mechanism. NCCIH also describes a counter-response — the “relaxation response” — which slows heart rate, lowers blood pressure, and decreases oxygen consumption. This is the physiological basis for why slow breathing is sometimes framed as a stress management tool, not a treatment.
4. Preliminary evidence: can slow breathing influence stress markers?
NCCIH notes that studies on slow, deep (diaphragmatic) breathing have shown it may modestly lower blood pressure and reduce cortisol, a stress hormone. A 2019 review of three studies covering 880 participants found preliminary evidence that diaphragmatic breathing may help reduce stress, with some positive changes in cortisol levels and blood pressure. NCCIH is explicit that this is preliminary evidence, not a cure, and there is no drug or technique that eliminates stress outright.
Known versus unknown
What’s established: the mechanics of gas exchange, which muscles drive breathing, how the autonomic nervous system speeds or slows your breathing rate, and that stress reliably raises breathing and heart rate through the fight-or-flight response.
What’s still preliminary: exactly how much slow breathing lowers stress hormones or blood pressure for a given person, how long any effect lasts, and how it compares across different breathing techniques. NCCIH describes this evidence as encouraging but limited — not as an established treatment for any stress-related condition.
This article is general education about breathing physiology. It is not medical advice, and it does not diagnose, treat, or recommend a course of action for any condition. If you have a breathing problem, chest pain, or a diagnosed respiratory or cardiac condition, talk to a licensed healthcare provider. If you’re experiencing a medical emergency, contact local emergency services immediately.
A practical next-step checklist
Use this to build your own basic body-literacy notes on breathing — not as a treatment plan:
- Notice your resting breathing rate for one minute, without trying to change it
- Identify what raised or lowered your breathing rate today (physical activity, stress, rest)
- Read NHLBI’s plain-language breakdown of the muscles and nerves involved if you want the full picture
- If you’re curious about relaxation breathing, review NCCIH’s evidence summary before trying any technique, so you know what’s established versus preliminary
- Bring any persistent breathing changes — shortness of breath, wheezing, chest tightness — to a healthcare provider rather than trying to self-manage them
Sources: National Heart, Lung, and Blood Institute, “How the Lungs Work” (nhlbi.nih.gov/health/lungs); National Center for Complementary and Integrative Health, “Stress” (nccih.nih.gov/health/stress).
By Body Literacy Review Editorial Team. Last updated September 18, 2026.
Leave a Reply