Freediving Wissen · 05
The diving response – what happens to your body underwater?
What your heart, circulation and oxygen do when your face meets the water.
You hold your breath and put your face in the water. Before you even reach depth, your body begins to respond.
Your heart can beat more slowly. The blood vessels in your arms and legs narrow. Your circulation changes so that the available oxygen is used as sparingly as possible.
And the fascinating part is this: you do not have to control these responses consciously.
Your body has an ancient programme for life underwater.
1. The diving response – a built-in protective mechanism
The so-called mammalian diving response is a natural reaction that occurs in mammals – including humans.
It is triggered by holding the breath and strengthened by contact of the face with water. Cold water in particular can intensify the response.
Nerves in the face play an important part. They send information to the brain, which then coordinates several circulatory responses.
The aim of these responses is above all to keep the oxygen you have available for as long as possible for the vital organs.
Worth knowing: you do not have to dive ten or twenty metres deep for this. The response can begin at the surface.
2. Bradycardia – why your heart beats more slowly
One of the best-known responses is bradycardia: heart rate falls.
During breath-holding, the parasympathetic system is activated, among other things – the part of the nervous system that can slow heart rate.
A heart that beats more slowly generally needs less oxygen for its own work.
How far heart rate falls, however, varies a great deal.
A scientific review describes a reduction of about 15 to 40 percent as common in humans. That is an observed range, not a fixed rule for every freediver.
And something else is striking: while the heart beats more slowly, the blood vessels can narrow at the same time and blood pressure can rise.
The diving response is therefore not simply a state of relaxation, but a complex circulatory reaction.
3. Peripheral vasoconstriction – oxygen for the most important organs
It sounds complicated, but the idea is simple.
Peripheral vasoconstriction means that certain blood vessels outside the vital organs narrow.
Blood flow is reduced, for example in the arms and legs.
The body prefers to send its oxygen-rich blood where it is needed most urgently: to the brain and the heart.
You could say your body sets priorities.
That is one reason the diving response is considered an oxygen-sparing mechanism.
But this redistribution has another side: as the vessels narrow, blood pressure can rise.
The diving response helps the body deal with a lack of oxygen – it does not make a lack of oxygen harmless.
4. Can the diving response be trained?
Why does the pulse of some experienced freedivers seem to fall more strongly during a dive than in beginners?
Scientific studies suggest that regular apnea training can be associated with a more pronounced diving response.
A research review from 2021 describes trained breath-hold divers as often showing a stronger diving response than non-divers.
People respond differently, however. Not every change can be clearly attributed to training, and a stronger response does not automatically mean a longer or safer apnea.
The long-term effects of intensive apnea training are not yet fully understood either.
The aim should therefore not be to lower the pulse as far as possible, but to understand your own physiology better and to train responsibly.
What does this mean for your freediving?
The diving response is fascinating, but it is not a free pass for longer or deeper dives.
What you can take from the research:
Your body already responds to breath-holding and to contact of the face with water.
Heart rate and blood flow can change in order to save oxygen.
These responses differ from person to person and can be influenced by training.
A stronger diving response is no guarantee against hypoxia or blackout.
Understand your body. Train deliberately.
Freediving is not only about holding your breath for as long as possible.
It is also about understanding your own body's responses, judging load correctly and planning enough recovery.
That is the philosophy we follow with ApneaReady: training should not be equally intense every day, but should follow your current capacity.
The Readiness Score helps you choose a suitable training intensity. It does not measure the strength of your diving response, and it can predict neither your apnea performance nor your safety.
Your body knows how it can respond to the water. Good training still means respecting its limits.
Safety note
The diving response does not prevent hypoxic blackout. Never do apnea training in the water alone, and never after hyperventilation. This article replaces neither qualified freediving training nor medical advice.
References
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The human diving response, its function, and its control
Scandinavian Journal of Medicine & Science in Sports, 15(1), 3–12
Review of bradycardia, vasoconstriction and the oxygen-conserving role of the human diving response for the brain and heart.
Read publication ↗ -
Mechanism of the human diving response
Integrative Physiological and Behavioral Science, 29(1), 6–16
Breath-holding plus facial contact with water triggers bradycardia, reduced limb blood flow and a rise in mean arterial pressure. Water temperature can modify the response.
Read publication ↗ -
Diving bradycardia: a mechanism of defence against hypoxic damage
Journal of Cardiovascular Medicine, 12(6), 422–427
In humans the fall in heart rate commonly ranges from about 15 to 40 percent. Cold-water face immersion accentuates the response, which redistributes blood flow toward the heart and brain.
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Physiology, pathophysiology and (mal)adaptations to chronic apnoeic training: a state-of-the-art review
European Journal of Applied Physiology, 121(6), 1543–1566
Trained breath-hold divers often show a more pronounced diving response than non-divers. Long-term effects of intensive apnea training are not fully established.
Read publication ↗ -
Hypoxia in breath-hold diving
Hyperventilation and ascent can lead to hypoxic blackout. A diving response does not remove that risk.
Read publication ↗
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