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Science8 min read

How breathing acts on the nervous system

The vagus nerve, heart rate variability, CO2 tolerance, and why a longer exhale calms you down. A clear, unhurried look at the physiology of breathing.

Telling someone to take a deep breath has worked for centuries. What is interesting is that only in the last thirty years have we been able to describe why in detail. The mechanism is not mystical. It is a measurable, repeatable circuit running from the brainstem to the heart and on to the pressure sensors in the arteries.

This piece walks through that circuit. You do not need a medical background. But we are not going to say anything false in the name of simplicity.

Where breathing begins

The rhythm of breathing is generated in the brainstem, in a cluster of a few thousand neurons on each side called the preBötzinger complex. Remove it and inspiration stops. The comprehensive 2018 review by Del Negro, Funk and Feldman describes its job as far more than keeping time. The preBötzinger complex coordinates breathing with speech, swallowing and chewing, and it is strongly shaped by emotion and cognition — while also shaping them.

The implication is plain but consequential. Breath is not simply a downstream readout of mood. The circuit can be entered from either end.

A neighbouring group of neurons carries voluntary commands down from the cortex. When you hold or slow your breath, you temporarily override the automatic rhythm. The whole architecture of breathwork rests on this dual control.

The vagus nerve, and the heart listening to the breath

The distance between your heartbeats is not fixed. Heart rate rises slightly on the inhale and falls on the exhale. This is called respiratory sinus arrhythmia, and it is a sign of health, not disease.

The cause is how the vagus nerve works. The vagus runs from the brainstem to the heart, lungs and abdominal organs, and is the main cable of the parasympathetic system. It applies a continuous brake to the heart's sinus node. During inhalation that brake is partly lifted and the heart speeds up. During exhalation the brake reapplies and the heart slows.

Lengthen your exhale and you lengthen the time the vagal brake is dominant. Gerritsen and Band's 2018 respiratory vagal stimulation model argues that this is precisely the common denominator of practices built on slow, extended exhalation. The model is plausible and consistent with the physiology. It is worth naming it as a model: not a single proven mechanism, but the field's best current explanation.

What heart rate variability measures

The magnitude of that natural fluctuation between beats is called heart rate variability. Broadly, higher variability suggests the autonomic nervous system is flexible and can shift quickly with circumstance. Lower variability may suggest a system that is stuck.

Two cautions. First, heart rate variability is not a grade. It varies enormously with age, sleep, alcohol, illness and measurement method. Comparing across people is usually meaningless; comparing a person against their own trend is not. Second, an increase in variability during a single breathing session does not mean variability has risen durably across the day. In Balban and colleagues' one-month study of 108 participants, mood improved significantly while heart rate variability and resting heart rate did not change significantly.

Resonance: about five and a half breaths a minute

The cardiovascular system has its own oscillation frequency. When blood pressure drops, the baroreflex raises heart rate; when pressure rises, the reverse. Completing that feedback loop takes roughly ten seconds.

Vaschillo and colleagues showed that when breathing is brought close to the frequency of that loop, heart rate oscillations grow dramatically. Heart rate and blood pressure fall into a 180-degree phase relationship and the system approaches resonance. Lehrer and Gevirtz's 2014 review sets out the mechanism in detail.

In numbers, this lands at roughly 5.5 to 6 breaths per minute for most adults — ten to twelve seconds per breath. That is the origin of what is marketed as "coherent" or "resonance" breathing.

One detail matters. This frequency varies between individuals, and within an individual it is not perfectly stable over time. 5.5 is a good starting estimate, not a universal constant. The way to find yours is to test: spend a few minutes each at 4.5, 5, 5.5, 6 and 6.5 breaths per minute and notice which settles you most.

Carbon dioxide, and the question of tolerance

A common misconception is that breathing is mainly about oxygen. Under normal conditions your oxygen saturation is already above 95 percent, and deep breathing does not meaningfully raise it.

The variable that drives the urge to breathe is carbon dioxide. When CO2 rises in the blood, pH falls, chemoreceptors in the brainstem and great vessels detect it, and the sense that you need to breathe appears. That familiar air hunger is not a lack of oxygen. It is an accumulation of carbon dioxide.

This explains two things.

First, why fast deep breathing causes dizziness and tingling. Hyperventilation drops CO2 quickly, blood turns alkaline, and cerebral blood vessels constrict. Cramping in the hands and around the mouth — tetany — is part of the same chain. These effects are temporary and resolve on return to normal breathing.

Second, why carbon dioxide is useful. Haemoglobin releases oxygen more readily where CO2 is high and pH is low. This is the Bohr effect, and it describes how oxygen actually gets delivered to your tissues. Carbon dioxide is not waste to be flushed out. It is part of the delivery system.

The phrase "CO2 tolerance" should be used carefully here. People who practise slow breathing regularly do tend to hold their breath longer. How much of that is a genuine shift in chemoreceptor sensitivity, and how much is habituation to discomfort and reduced anxiety, has not been cleanly separated. It is too early to speak with confidence.

What sighing is for

A healthy person sighs several times an hour without noticing. A sigh is a second, shorter inhalation stacked on top of a normal one.

Li and colleagues, publishing in Nature in 2016, mapped the neural circuit for this behaviour. Two small peptidergic neuron populations in the brainstem signal the preBötzinger complex to convert an ordinary breath into a sigh. Block both pathways and sighing disappears entirely.

The function is mechanical: some alveoli collapse over time, and a sigh reopens them. The circuit is also thought to receive emotional input. The human practice known as the physiological sigh — a two-stage inhale followed by a long, complete exhale — is a deliberate repetition of that natural reflex. It was the best-performing method in Balban and colleagues' study.

What we do not know

Honestly, a fair amount.

The acute physiological effects of slow breathing are well documented. Long-term effects are far less studied. Zaccaro and colleagues state explicitly that longitudinal research is close to absent.

Whether short-term autonomic changes translate into a measurable difference in wellbeing months later is unclear. Effect sizes are generally small to moderate. And how far breathwork separates from placebo has not always been demonstrated in well-designed comparative trials.

None of this makes the practice worthless. It does place the expectation where it belongs.

A note on safety

If you are pregnant, or live with uncontrolled hypertension, arrhythmia or other cardiovascular disease, epilepsy, glaucoma, panic disorder or a significant psychiatric history, consult your physician before beginning intensive breathing practices. Never practise breath retention in or near water or while driving; always work seated or lying down. This article is for information and is not medical advice.

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