Breathing Techniques and the Nervous System

Ever been told to take a breath? Maybe it was before a difficult conversation, a high-stakes presentation, or a competition. The advice might feel intuitive but sometimes a bit irritating too, like something people fall back on when they don't know what else to say. Despite being cliche, it’s actually an incredibly powerful tool

Breathing is one of the few physiological processes we can consciously control, and that control turns out to be a direct line into systems that govern how we respond to stress, how efficiently our muscles receive fuel, and how effectively our nervous system recovers from load. The mechanism behind all of this isn’t simple relaxation. It’s a set of interacting biological systems that respond in measurable ways to the rate, depth, and rhythm of how we breathe.

How Carbon Dioxide Works in the Body

Carbon dioxide is widely thought of as a waste product and something we exhale and eliminate, but this framing greatly undersells a key piece of how our bodies function. The element actually plays a central role in how the body delivers oxygen to its tissues. The phenomenon, known as the Bohr effect, describes how hemoglobin, which is the protein in red blood cells responsible for carrying oxygen, releases oxygen more readily when CO2 levels in surrounding tissue are higher. In practical terms, regions under metabolic demand (i.e. working muscles or an active brain during high-stress periods) signal their need for oxygen through the CO2 they produce. More CO2 in a tissue means more oxygen gets released there.

The implication is counterintuitive. When we breathe rapidly and shallowly, which is usually the default when we’re under stress, we exhale CO2 faster than the body produces it. The concentration in our blood drops, and when CO2 drops, hemoglobin holds onto oxygen rather than releasing it to tissues. We inhale more air but deliver less oxygen where it's actually needed. The felt urge to breathe is not a signal of low oxygen, rather it’s a signal of rising CO2. When our body learns to tolerate slightly higher CO2 concentrations, the urge to take the next breath is delayed, breathing slows, and the gas exchange system works more efficiently.

The Nervous System Connection

Breathing directly modulates the balance between the two branches of the autonomic nervous system (ANS), which regulates our internal state without conscious input. The sympathetic branch, often called the fight-or-flight system, mobilizes resources in response to perceived threat. The parasympathetic branch restores equilibrium and allows us to recover. These systems exist in dynamic tension, and that balance shapes everything from heart rate and digestion to cognitive clarity and emotional reactivity.

The vagus nerve, which is the primary communication pathway of the parasympathetic system, is directly influenced by how we’re breathing. Each exhale produces a brief increase in vagal activity, slowing the heart slightly. This rhythmic fluctuation in heart rate functions as one of the most reliable non-invasive measures of our autonomic health. When our breathing is slow and relatively deep, vagal activity increases, heart rate variability (HRV) rises, and the nervous system shows markers of greater adaptive capacity. When our breathing is fast and shallow, that fluctuation diminishes and the system trends toward sympathetic dominance and stress modes.

A relatively small shift in CO2 tolerance, which is the body's capacity to remain functional at elevated CO2 concentrations, can produce meaningful changes in breathing rate, the timing of the breath urge, and the reactivity of the stress response. Lots of science here, but it goes to show that our breath provides a direct connection to stress management.

The Link Between CO2 Tolerance and Anxiety

The connection between chronic overbreathing (i.e. taking more short, shallow breaths than we need to both consciously and unconsciously) and anxiety is massive and measurable. Rapid shallow breathing produces the same chemical environment as acute anxiety, including elevated heart rate, reduced CO2, vasoconstriction (narrowing of blood vessels), and heightened chemoreceptor sensitivity. 

For people who habitually overbreathe, this isn’t a state they enter under pressure…it’s their baseline. The nervous system runs in a lower-grade threat response most of the time, leading to a cascade of unfavorable health effects. Research directly links our sensitivity to CO2 to elevated anxiety, and this is shown across general populations and athletic cohorts alike as well as every group in between.

To make this more tangible, we’ve likely all seen the “breathing into a paper bag during a panic attack” scene in a show, video, movie, etc. While a bit impractical, it’s actually on the right track from a physiological perspective. Breathing in our exhaled air raises CO2 concentration in the blood, which blunts the alarm signal and brings breathing back toward baseline. When CO2 returns to normal levels, the feedback loop driving the panic cycle quiets.

How This Appears in Athletic and Cognitive Performance

In athletic contexts, CO2 tolerance largely shapes how we perceive effort, or our RPE (rate of perceived exertion). As intensity rises, CO2 production increases, and people with higher tolerance are better positioned to delay the breathing threshold, which is the point at which our breathing rate accelerates rapidly and begins to feel distressing. 

This is distinct from our cardiovascular fitness. Two people with similar aerobic capacity can actually differ substantially in how they experience high-intensity effort based on their tolerance for CO2 accumulation.

When it comes to our brain and how we think, the implications are equally concrete. CO2 acts as a vasodilator, expanding blood vessels and increasing blood flow to active tissue. Chronic overbreathing reduces blood flow modestly, including blood to the brain, which can contribute to the “flat” feeling we might experience when we’ve been under stress for a prolonged period. It’s been shown that breathing techniques, even short ones, can improve attention, working memory, and stress reactivity, reinforcing that breathing pattern has measurable cognitive impact.

Using Breathing Techniques

Seen as a lever for regulation, how we breathe is less a habit and more a readout of how our nervous system is currently doing. Fast, shallow, mouth-dominant breathing is not just a response to stress, it actively sustains it by keeping CO2 low, vagal tone diminished, and carbon dioxide sensitivity elevated. Slow, nasal, diaphragm-driven breathing allows CO2 to rise to functional levels, increases HRV, and signals the brainstem that the immediate perceived threat has passed.

Performance at the highest levels, whether that’s personally, in sport, in high-stakes decision-making, or in sustained complex work, requires nervous system flexibility. The ability to ramp up under acute demand and return efficiently to baseline is a complex combination of mind and body in the long run, but in the moment, it can be shaped by how we utilize our breath. When that reset mechanism is chronically underused or degraded by habitual overbreathing, the baseline shifts upward, and the cost of sustained performance rises with it.

References

1. Bohr, C., Hasselbalch, K., & Krogh, A. (1904). On a biologically important influence exercised by the carbon dioxide tension of the blood on its oxygen binding. Scandinavian Archive of Physiology, 16, 402–412.

2. Kaufman, D. P., & Dhamoon, A. S. (2023). Physiology, Bohr Effect. In StatPearls. StatPearls Publishing.

3. Nattie, E., & Li, A. (2012). Central chemoreceptors: Locations and functions. Comprehensive Physiology, 2(1), 221–254.

4. Lehrer, P. M., & Gevirtz, R. (2014). Heart rate variability biofeedback: How and why does it work? Frontiers in Psychology, 5, 756.

5. Yasuma, F., & Hayano, J. (2004). Respiratory sinus arrhythmia: Why does the heartbeat synchronize with respiratory rhythm? Chest, 125(2), 683–690. 

6. Griez, E., & van den Hout, M. A. (1987). CO2 inhalation in the treatment of panic attacks. Behaviour Research and Therapy, 25(2), 141–145.