The science

Why carbon dioxide runs your breathing

By GyroscopeUpdated November 15, 202617 sources

Your breathing is set mostly by carbon dioxide. Nerve cells in your brainstem track it, and the acid it makes, and keep it within a few points of the same level all day. There are oxygen sensors in your neck too, but they only push hard once oxygen has fallen a long way, which almost never happens at rest near sea level.

Running short of air feels like running short of oxygen. It’s why “take a deep breath” sounds like a refill, and why “oxygenating your body” is such a common line in breathwork. But the feeling of needing to breathe is mostly a carbon dioxide alarm, and you can see it in people at the extremes. Climbers near the top of Everest have had between a quarter and a third of normal oxygen pressure in their blood. And people who hold their breath after fast breathing can go well past a minute on empty lungs while the oxygen in their fingers drops to 60 percent.

What’s watching your carbon dioxide?

Your cells burn fuel all day and make carbon dioxide as waste. In the blood it turns into an acid, so more carbon dioxide means more acidic blood. A small cluster of nerve cells on the underside of the brainstem, the retrotrapezoid nucleus, senses that acid and drives the breathing muscles harder. Guyenet and Bayliss, at the University of Virginia, put the cluster at about 2,000 cells in rats and 800 in mice. Some serotonin-making cells in the brainstem respond to carbon dioxide as well.

The system is tight. In a healthy adult, carbon dioxide in arterial blood sits between 38 and 42 mmHg, and blood pH between 7.38 and 7.42, according to MedlinePlus. Researchers reviewing brain blood flow in 2026 noted that a change of just 1 or 2 mmHg is enough to shift the acid level around brain cells.

Why does a small change in carbon dioxide move your blood so much?

The link between carbon dioxide and acid follows a curve, worked out from the standard chemistry formula for blood (the Henderson-Hasselbalch equation). The other half of that formula is bicarbonate, which the kidneys control, and the kidneys take hours to days to respond. A few minutes of hard breathing can swing pH a long way before they do. Halve your carbon dioxide and pH climbs from 7.4 to about 7.7.

The fast end of the curve is where breathwork sensations come from. MedlinePlus says breathing too much lowers carbon dioxide and causes many of the symptoms of overbreathing, including numbness and tingling around the mouth and spasms in the hands and feet. What happens in a fast breathing session? follows one round step by step.

What do the oxygen sensors in your neck do?

Where each carotid artery splits in your neck there’s a tiny organ called the carotid body, and it’s the body’s main oxygen sensor. In 1970, Weil and colleagues held carbon dioxide steady while lowering the oxygen people breathed, and traced how breathing answered. As oxygen deep in the lungs fell from normal, breathing rose only a little until oxygen reached 50 to 60 mmHg. Below that, breathing shot up steeply.

The bend in that curve sits at the same oxygen level where your red blood cells start to give up their load. Above it, your blood stays nearly full whatever you do, which is why a pulse oximeter shows 95 to 100 percent for most people at rest, according to Cleveland Clinic. The carotid bodies feel acid too, and they help with small rises in carbon dioxide, but for oxygen they mostly wait for a real drop. oxygen.guide explains what the oximeter number means and why deep breaths can’t raise it.

What happens at altitude?

Altitude is where most people meet the oxygen sensor for real. Each breath carries less oxygen, blood oxygen slides past the bend, and the carotid bodies drive you to breathe harder. But breathing harder blows off carbon dioxide, and the brainstem pushes back against the very thing the neck sensors want. Martin Burtscher and colleagues at the University of Innsbruck lay it out in a 2026 review. Breathing can only rise all the way once the kidneys get rid of bicarbonate, which brings pH back toward normal and takes the brainstem’s foot off the brake.

How long that takes depends on how high you are. The CDC puts the breathing adjustment at 3 to 5 days. The Innsbruck review gives about 4 days at 9,800 feet, 5 to 10 days at 13,000 to 16,400 feet, and 4 to 6 weeks above 19,700 feet. Acetazolamide, the altitude medicine, works by making the kidneys dump bicarbonate on purpose, and the CDC says it cuts the adjustment to about a day. oxygen.guide’s altitude chapter has the oxygen side.

The blood they drew on Everest

The most extreme numbers come from a team led by Mike Grocott at University College London, published in 2009. They drew arterial blood from climbers on Everest. In four samples taken at 27,559 feet, oxygen pressure averaged 24.6 mmHg, against 75 to 100 at sea level. Carbon dioxide averaged 13.3 mmHg, a third of normal, from breathing so hard.

With bicarbonate at its normal level, 13 mmHg would put pH off the top of the curve above. Above 19,700 feet the kidneys need 4 to 6 weeks to catch up, shedding bicarbonate to pull pH back toward normal.

Why can breathing stop during sleep?

Awake, your breathing runs on two kinds of control, chemical feedback and the drive that comes with being awake and active. When you fall asleep the second one drops away, and breathing runs entirely on chemical feedback. In non-REM sleep there’s a carbon dioxide level below which the brainstem stops sending breaths at all. The physiologist Jerome Dempsey, in a 2019 review, puts it 2 to 5 mmHg below a healthy person’s normal awake level.

A big breath or a brief wake-up can push carbon dioxide under that line. Breathing pauses until carbon dioxide builds back up, then overshoots, and the cycle repeats. Doctors call this central sleep apnea, since the brain stops sending the signal to breathe, and it makes up about 2 to 3 percent of all sleep apnea. oxygen.guide covers sleep apnea, including the common kind where the airway gets blocked.

At altitude the gap between normal and that line shrinks to 1 to 2 mmHg, and the CDC says some periodic breathing becomes nearly universal above about 9,000 feet. In sleeping visitors to high altitude, Dempsey’s review says, a little extra carbon dioxide in the air they breathe stops it completely. Raising blood carbon dioxide by just 1 to 2 mmHg is enough, and extra oxygen works too.

A simple way to picture it

Think of a busy restaurant that decides how many cooks to schedule by counting dirty dishes in the sink. It only checks the walk-in fridge when the food is nearly gone. Dirty dishes pile up every minute, so they’re a fast and reliable sign of how busy things are, and the fridge is big and almost never runs low.

Carbon dioxide is the dirty dishes and oxygen is the fridge. Fast breathing is a dishwasher running flat out. The sink looks empty, the manager sends the cooks home, and nobody checks the fridge while it empties.

The paper bag got half of it right

The old advice to breathe into a paper bag came from knowing that overbreathing drains carbon dioxide. In 1989 Callaham, an emergency doctor in San Francisco, had 20 volunteers breathe hard until the carbon dioxide in their breath fell to 21.6 mmHg on average, then keep breathing into a bag. Carbon dioxide came back to about 39 within 30 seconds. But the oxygen in the bag fell the whole time, by 26 mmHg on average at its lowest. Breathe into a paper bag when you panic explains why that’s dangerous.

Where the explanation stops

The carotid bodies and the brainstem work together, and researchers still debate how much of the carbon dioxide response belongs to each. Most of the cell-by-cell work comes from rats and mice, and some of it hasn’t been confirmed in people. And the urge to breathe isn’t all chemistry. In a 2016 hospital study in Birmingham, England, 15 women held their breath for over 5 minutes, after extra oxygen and a ventilator to lower their carbon dioxide. When they finally had to breathe, their oxygen and carbon dioxide were both still normal. What happens when you hold your breath? picks up there.