What Happens in Your Body When You Hold Your Breath

Freediver in a black wetsuit underwater — A freediver underwater. Photo: Fluid Imagery on Unsplash

Take a breath, put your face in the water, and wait. In a few seconds, something happens that you did not decide: your heart begins to slow down. So what happens in your body when you hold your breath underwater? In short, an old reflex slows the heart, blood moves toward the centre of the body, pressure squeezes every air space, and rising carbon dioxide, not falling oxygen, creates the urge to breathe. Understanding this chain is the base of safe freediving, and it is surprisingly calming to know.

The body is more prepared for this than you think

We are not ocean animals, but the body still carries a set of old, automatic tools for the moments when breathing pauses underwater. Physiologists call this the mammalian dive reflex: a chain of involuntary reactions that switches on when the face is in the water, especially cool water.

It is not something you learn. You are born with it, in different degrees, like most mammals. Medical references describe how cool, wet receptors on the face send signals through the trigeminal nerve, and that putting only the face in the water is enough to trigger it. Putting your arms or legs in cold water does not.

From the first moment of a breath-hold, the body starts to adjust the heart rate, the circulation and where oxygen goes, all without you deciding any of it. None of this makes breath-holding free of risk. It simply explains why short, well-prepared breath-holds are possible for the body, and why the training around freediving and snorkelling exists in the first place.

The dive reflex and why the heart slows down

The clearest part of this response is bradycardia, a drop in heart rate that starts soon after the face touches the water. The trigeminal nerve carries the signal to the brainstem, and the vagus nerve, part of the parasympathetic nervous system, slows the heart. The effect is strongest when the breath is held with the face underwater, and it grows as the water gets colder.

A slower heart uses less oxygen, so in a way the body is buying itself a little more time. In adults the response is usually mild, and medical summaries report that it varies a lot between people, typically lowering the heart rate by something like 15 to 40 percent. How relaxed someone is also matters. This is exactly why so much of freediving training is about calming the nervous system, not about building huge lungs.

Blood shift: protecting the lungs under pressure

At the same time, blood vessels in the arms and legs narrow. This is called peripheral vasoconstriction, and it pushes blood toward the organs that need it most: the brain, the heart and the lungs.

In deeper freediving, a related adaptation takes over, the blood shift. As pressure increases with depth, blood moves into the vessels of the chest and lungs and helps fill the space that air leaves as it compresses. Reviews of breath-hold physiology describe this as one of the reasons trained freedivers can go deeper than old textbooks said was possible. It has limits, though. If the pressure in the airways drops too far below the water pressure, a "lung squeeze" can happen, with coughing, or even blood, after the dive.

The spleen helps too. Ultrasound studies have shown that during repeated breath-holds the spleen contracts and releases extra red blood cells into the blood, so it can carry a little more oxygen. A beginner does not need to think about any of this. It is one more reason why depth in freediving is something to build slowly, with people who know what they are doing.

Why does the urge to breathe come from CO2, not oxygen?

This is the part that surprises most people. During a breath-hold, the urge to breathe comes mainly from carbon dioxide building up in the blood, not from a lack of oxygen. CO2 is a natural by-product of using energy, and it rises steadily for as long as breathing is paused. Sensors in the body notice this rise and send a stronger and stronger signal, often felt as contractions of the diaphragm.

Here is why this matters, and it is the most important thing to understand about breath-hold safety: oxygen can keep dropping quietly in the background. Some people hyperventilate before a breath-hold to "clear" the CO2 and delay the urge. It works, and that is the danger, because it switches off the alarm. A 2023 study of 18 volunteers with little experience found that breath-holds lasted longer after hyperventilation, and that oxygen saturation fell lower and lower across a series of breath-holds. Oxygen can then fall to low levels without any warning. This is how hypoxic blackout, sometimes called shallow-water blackout, happens, usually in the last few metres before the surface.

That is why every serious freediving course teaches the same two rules, with no exceptions: never hyperventilate before a breath-hold, and never hold your breath in the water alone. What really keeps this activity safe is a trained buddy who can recognise the early signs and respond calmly, not willpower or big lungs.

Pressure, equalising, and why the descent is slow

Underwater, pressure increases with the weight of the water above you, and it affects every air space in the body: the lungs, the sinuses and the middle ear. Equalising is the technique of letting a little air move into those spaces to match the pressure outside. This is why divers learn to go down slowly and to equalise early and often, instead of rushing.

A mask is an air space too, and it needs a little air from the nose to equalise. A low-volume mask holds less air, so it takes less of your breath to equalise, and that is why freedivers prefer them. Our guide to freediving equalisation techniques goes deeper.

Relaxation changes how much oxygen you use

Tense muscles use more oxygen than relaxed ones. That is why so much of freediving preparation is about breathing calmly and moving gently before you go under, not about one huge last breath and pure force. A calm state, a slower heart and smooth movement work together to make a breath-hold more comfortable, without needing any more air at the start. Staying warm helps too, because shivering is work. A rash guard or a thin wetsuit makes a long session in the water easier.

Everyone’s limits are different, and that is fine

Breath-hold time depends on lung capacity, fitness, how relaxed someone is, and simply how familiar the sensations feel. Training can improve your tolerance to the rising CO2 signal and help you notice the body’s cues sooner. But good training is built on slow progress, not on chasing a personal best on the first day. Learning to read your own limits, not someone else’s, is really the whole point.

The ethical layer: calm divers, calm animals

There is a quiet link between this physiology and the way we behave around wildlife. A relaxed freediver moves slowly, makes no bubbles and does not need to grab anything to stay still. That is the kind of presence that lets a turtle keep eating or a fish stay in its hole. A diver who is fighting the urge to breathe kicks hard and touches things. So good breath-hold practice is also good manners: keep your distance, never chase an animal to make a dive longer, and never use the reef to hold yourself down.

Wildlife encounters are unpredictable. Freediving near turtles, fish or any marine animal should always follow local regulations and experienced professional guidance.

Keeping it safe

Freediving and snorkelling are wonderful, calm ways to spend time in the ocean, and they deserve the same respect as any activity that changes how your body works. Learn with qualified instructors and vetted local operators instead of alone, never hold your breath without a buddy watching, and never hyperventilate to hold on longer. Progress slowly, rest properly between attempts, and treat dizziness, tingling or any strange sensation as your sign to stop for the day, not to push through.

None of this is meant to make breath-holding feel scary. It is the opposite: understanding what is really happening in your body is usually what makes people feel calmer in the water. It is also why we build our freediving retreats around small groups, with a maximum of four guests per instructor, and slow progress instead of depth records.

Questions people ask

What is the mammalian dive reflex?

It is a set of automatic responses, including a slower heart rate and narrower blood vessels in the arms and legs, that switches on when the face is in water, especially cool water. In adults it is usually mild and changes from person to person.

Why does the urge to breathe get stronger during a breath-hold?

Mainly because carbon dioxide builds up in the blood, not because oxygen runs low. Hyperventilating delays this signal without protecting you, which is why it is dangerous.

What is shallow-water blackout?

It is a loss of consciousness from low oxygen, often with no warning, usually in the last few metres of an ascent or at the surface. A trained buddy watching you is the main protection.

What is blood shift?

At depth, blood moves into the vessels of the chest and lungs as the air compresses, which helps protect the lungs. It has limits, and lung squeeze can still happen, which is why depth is built slowly.

Does the spleen really help freedivers?

Studies of repeated breath-holds found that the spleen contracts and releases extra red blood cells into the blood. The size and timing of the effect are different from study to study.

Is breath-hold diving safe for beginners?

It can be, when you learn slowly with a qualified instructor and always with a buddy. It should never be practised alone in the water.

If this made you curious about meeting the ocean this way, you can explore our upcoming retreats.

Wildlife encounters are never guaranteed and never forced. Ocean Calling works only with vetted, ethical local operators, and every animal is free to approach, or not, entirely on its own terms.

Sources & further reading

Further reading on Ocean Calling: More from the journal · Wildlife & the law · See the retreats · Ocean Atlas

What Happens in Your Body When You Hold Your Breath Underwater