Breathe

I read Breath by James Nestor a few summers ago, not expecting it to shift anything fundamental. I assumed it would be interesting enough: a collection of case studies, a tour of forgotten techniques, a small rearrangement of facts. What I didn’t expect was the jolt of recognition — not intellectual, but bodily — at the simple, almost impolite truth sitting at the centre of the book: that our breathing is governed not by oxygen, but by carbon dioxide. That the urge to draw breath does not come from a lack of oxygen but from the rising levels of CO₂ in the blood, sensed by chemoreceptors deep in the brainstem.

It was unsettling to realise how deeply I had misunderstood something so intimate and constant. Breathing had always felt automatic, background — an unexamined constant. Yet it was also precise, inheriting the logic of chemistry, pressure gradients, and ancient evolutionary reflex.

While discovering the insights of the book, I didn’t sit on a yoga mat or close my eyes to meditate. I went outside, to clear the fields and to muck out. It was my daily routine being close to nature and the work was simple. A wheelbarrow, a fork, a steady path from stable to pile. I began breathing through my nose, slower than was comfortable. I exhaled longer than felt natural. I practiced small breath holds.  At first it felt strange.  I had long practiced diaphragmatic breathing but this was different – expanding my ribs into the back, moving without rush or strain.  Counting the breaths, holding for slightly longer than was natural and the longer, slow exhalation.

Yet, it was there — a rhythm I didn’t invent so much as uncover. The body found its own cadence. The breath became less of a technique and more of a remembering. Something instinctive returning after years of being ignored.

I did not know, then, that I was training CO₂ tolerance. I did not know that I was teaching my chemoreceptors not to panic at the mild rise in carbon dioxide that accompanies slow, deep breathing. I simply knew that something in me felt steadier. The breath wasn’t making life calmer — life was still life, untidy and relentless — but it was giving me a way to stay inside myself while moving through it.

Later, competing in distance paddling races,  I used the same technique to maintain strength and performance without blowing up. I understood what I had practiced. A race does not care how emotionally stable or spiritually composed you feel about your breathing. It is physiology, not narrative. The working muscles produce CO₂ as a natural consequence of metabolism. The harder the effort, the more CO₂accumulates. The brainstem senses this rise in acidity and increases breathing drive. This is the ventilatory reflex. And yet — the subtle, critical, almost counterintuitive truth is that carbon dioxide is not something to be discarded as waste. Carbon dioxide is the signal that triggers the Bohr effect, the shift in haemoglobin’s affinity for oxygen. When CO₂ rises in active tissues, the pH drops just slightly, and haemoglobin releases oxygen more readily. Without that CO₂ signal, oxygen remains bound in the blood and passes by the very cells that need it.

This is why over-breathing — the reflexive response to panic, heat, effort, stress — is so costly. In breathing too fast, we exhale too much CO₂. The pH rises. The oxygen binds tighter. Muscles suffocate in the presence of abundance. This is not metaphor — it is chemistry.

When the breathing muscles begin to fatigue, the body responds with a reflex that has nothing to do with willpower and everything to do with survival. It’s called the respiratory metaboreflex. When the diaphragm starts to struggle—when the breath becomes frantic, shallow, urgent—the body diverts blood away from the limbs and sends it to the lungs to keep breathing alive. You feel it instantly: the legs turn heavy, the stride or stroke dulls, the world narrows to the effort of simply pulling air. It feels like failure. It isn’t. It is the body protecting its own continuity. The mistake is to panic and breathe harder. The work is to breathe deeper. To stay with the breath before the reflex is triggered. To hold the line where effort is fierce but the diaphragm is still steady and leading the rhythm.

It is also familiar. I know what it is to live with chronic stress. My home life has been shaped by complexity, pressure, emotional load that does not clock off at night. There have been evenings when the body was exhausted but the nervous system remained braced, coiled, unwilling to yield. On those nights, I would lie on my back and breathe slowly, through my nose, exhaling longer than I inhaled. Not performing calm. Not constructing peace. Just working with the reflex arc of the vagus nerve, letting the diaphragm's movement shift the balance of the autonomic nervous system. You do not need a device to know when the body softens — but I often check my HRV anyway as data to inform my training with my stress, hormonal and recovery state.  Mine sits consistently above 100. Not because I am naturally serene, but because I have learned — through necessity — to remain present inside rising CO₂ without reacting to it.

I learned that skill long before I understood it. Years ago, when I did my BSAC qualification and began deep diving, I noticed something that puzzled me at the time. After a dive, while others’ tanks came back nearly empty, mine returned still mostly full. I was not fitter than them, nor more experienced. What I lacked in skill, I compensated for in something unintentionally trained: I did not fight the water. I did not breathe to chase oxygen. I tolerated the subtle rise in CO₂ without panic. Underwater, urgency speaks loudly. The instinct is to gasp, to pull air in quickly. But the ocean rewards slowness. When I breathed with patience, my lungs increased in volume, my buoyancy shifted, and I rose just enough to float above the precious coral so as not to touch it. I learned to hover through breath alone. I learned to trust stillness and let the body remain in dialogue with pressure rather than conflict with it.

Years later, endurance training made the same conversation visible on land and above the water. When intensity rises and carbohydrate becomes the dominant fuel source, CO₂ generation increases sharply — carbohydrate metabolism produces more CO₂ per unit oxygen consumed than fat oxidation does. That is why we breathe harder when we shift into higher gears. But the mistake — especially among competitive athletes — is to believe that harder breathing equals better breathing. Often, harder breathing becomes inefficient ventilation, where the respiratory rate rises faster than is useful, blowing off too much carbon dioxide and choking oxygen delivery at the muscle level. The athlete feels this not as lack of will, but as the sudden heaviness, the narrowing of perception, the sense of running inside a tightening corridor. The solution is rarely to pull more air. It is to breathe more slowly, more deeply, with more authority, keeping CO₂available so that oxygen can do its work.

And yet effort is only one layer. Heat introduces another. When the body begins to overheat, blood is redistributed toward the skin to facilitate cooling. Plasma volume shifts. Stroke volume falls. Heart rate rises. Less blood is available to the working muscles, not because the muscles are failing but because survival takes precedence. In humidity, sweat does not evaporate; it accumulates. But evaporation — not sweating — is what cools us. When evaporation is limited, the body must choose between cooling and performance. The brain will always choose cooling. No one “pushes through” thermoregulation. The body protects its core.

I thought of this while watching Kona this year. Both Taylor Knibb and Lucy Charles-Barclay raced incredibly, pushing each other to the edge of human capacity. People questioned how they could give up with just km’s left.  But they did not give up. They did not mentally fail. They simply met the boundary where physiology enforces its laws. Heat, humidity, electrolyte balance, blood volume, muscle perfusion, respiratory drive — when one fails, the others cascade.

For women, the picture shifts further. Women generally have lower maximal sweat rates than men and rely more on cutaneous vasodilation for cooling. Oestrogen enhances nitric oxide-mediated vasodilation; progesterone raises core temperature. In menopause, oestrogen declines, and the thermoneutral zone narrows — the space between fine and overheating becomes tight. This does not make women fragile. It means that pacing, hydration, sodium concentration, and breath control must be attuned rather than assumed. Physiology demands respect, not apology.

Altitude overlays another layer of adaptation. At elevation, the reduced partial pressure of oxygen forces the body to increase ventilation. CO₂ drops. The blood becomes more alkaline. Over several days, the kidneys excrete bicarbonate to restore pH. Erythropoietin increases red cell mass. But the chemoreceptors also reset. When returning to sea level, one may have more oxygen-carrying capacity — and simultaneously a greater sensitivity to CO₂, making over-breathing more likely during high intensity efforts. Without retraining CO₂ tolerance, the benefit of altitude training is partially lost. The body must be reminded how to keep CO₂ long enough for oxygen to be useful.

Breath sits at the centre of all of this. It is not an accessory to training. It is not a technique reserved for yoga studios or stressful nights. Breath is the interface between physiology and perception — the mechanism by which effort becomes tolerable, heat becomes survivable, and emotional stress becomes inhabitable. Breath is how the body stays in conversation with itself rather than splitting into conflict.

The breath gives us space. And in space, there is choice.

References

Benner, A. (2023). Bohr Effect. StatPearls.Brinkman, J.E., et al. (2023). Respiratory Drive. StatPearls.Nestor, J. (2020). Breath: The New Science of a Lost Art. Riverhead Books.Gagnon, D., et al. (2012). Sex differences in sweating and vasodilation. Experimental Physiology.Yanovich, R., et al. (2020). Sex differences in thermoregulation. Physiology.Gomberg-Labedens, M., et al. (2025). Menopause and thermoregulation. Frontiers in Endocrinology.Periard, J.D., et al. (2021). Exercise under heat stress. Physiological Reviews.Sheel, A.W., & Romer, L.M. Respiratory muscle metaboreflex.Patel, H. (2023). Respiratory Quotient. StatPearls.