How does Freediving Work?

You’ve heard of it (at least now you have), but you might still be wondering how freediving works. How can divers hold their breath for so long and dive so deep? This is a beginner’s guide to the 3 major bodily responses that freedivers need to work with, and how they make the most out of their biology to dive deeper and longer.

What is Freediving?

The concept of freediving is super simple. If you hold your breath and dive down under the water, you’re freediving. It’s different from SCUBA, because divers don’t take any breathing apparatus with them. They dive on a single big breath. Experienced freedivers can spend several minutes underwater without coming back up for air, and they might dive pretty deep.

Also known as apnea, breath-hold diving or skin-diving, freediving is used in a whole range of other water sports – spearfishing, underwater hockey, underwater rugby, synchronised swimming, and probably more.

Some people think of freediving as an extreme, dangerous sport. You might have read stories about freediving deaths or seen pictures of divers returning to the surface with blood dripping from their noses. Yay. It is true that in competitive freediving, divers push themselves to levels that us regular people can’t even imagine. But, the for the majority of those who enter the world of freediving, the sport is a safe, beautiful and pretty life-changing way to be outside and exploring.

For many new divers, the appeal of freediving is the freedom to explore the underwater world without the burden of SCUBA training and equipment. Sure, the amount of time you can spend underwater is much greater with an air supply. But freediving lets you interact with underwater life differently, tune into your body, and experience a unique kind of peace.

A beautiful TEDTalk by William Trubridge on what happens when you freedive

How does Freediving Work?

Holding your breath for fun and pushing yourself underwater might seem like a weird and unsafe thing to do. However, your body is pretty amazing, with a range of functions how your organs interact in different situations, and when we freedive we start to consciously and unconsciously play with these functions.

Quick note here to remind you to never, ever, try any breath-holding in water without
1. A trained buddy, and
2. Having done a course. Reading a blog doesn’t count.

I’m not going to spend the next 5000 words dragging you through the a detailed physiology lessons. There are plenty of people who will do a way better job than me, and I will link to some of their amazing videos and articles at the bottom of the post.

For now, here’s the Dummy’s guide on what happens to your body when you freedive.

How Freediving Works #1: Breathing

Under-Over photo of a freediver on the surface

Every action in your body needs oxygen, and it produces carbon dioxide (CO2) as a waste product.

When you breathe, you take air into your lungs, so your body can absorb nice, fresh oxygen into your blood. It can also ditch carbon dioxide back into your lungs so you can exhale and get rid of it.

When you stop breathing, your oxygen level starts to decrease, and your CO2 level increases as it build up in your blood. If your oxygen levels gets too low, you black out. On dry land, you’ll wake back up almost instantly. But, if you black out in water, you obviously need to be rescued. This is why you only ever dive with a trained buddy. Even in the bath. OK? Cool.

Your brain is crap at signalling low oxygen, but it does react to increasing CO2, and sends signals to your body to breathe – or breathe harder –  to get rid of it. We call this the ‘urge to breathe’ – inspired. Once the urge to breathe becomes strong, you’ll also experience something called a contraction, with is a jolt or squeeze of the muscles around your lungs, trying to get you to breathe. Your brain is trying to keep you safe and stop you from dying (thanks brain) but it’s also a drama queen. 

Here is how a breath-holding conversation between your conscious and unconscious brain goes:

Brain: Hey, your CO2 is higher than normal. Breathe pls thx.
Me: Nope, I am doing APNEA
Brain: OK
Brain: Actually nah fuck apnea, breathe pls
Me: Nope
Brain: OK, enjoy THIS! 
*contractions begin*
Me: That’s not enjoyable, but I am super relaxed and I’m going to embrace these feelings and remind myself that I’m ok
Brain: Cool
Brain: But how about THIS ONE
…and so on, until the urge to breathe becomes pretty unbearable.

It’s important to know all of this once you start breath-holding (on land please), so that you understand what your body is trying to tell you. Most importantly, the urge to breathe is NOT an indicator that you are running out of oxygen. In fact, your body is pretty useless at signalling low oxygen. The need to breathe is caused by CO2.

When I was a new freediver, simply knowing this information helped to improve my breath-hold. Once I start feeling that urge to breathe, I can slip into self-talk and remind myself that I’m all good. Now it’s just a case of relaxation and knowing your body.

How Freediving Works #2: Depth and Pressure

Atmospheric pressure increases like crazy once we enter water. At 10 metres below the surface, the pressure squeezing us is twice the regular pressure at surface level. It was once believed that a human would not be able to dive deeper than about 40 metres before their organs would collapse, but world-class freedivers regularly pass 100m and beyond. Your body has ways to protect you at depth which I will describe more in the next section.

Pressure is also important because it affects the air you are carrying. Pressure causes air to contract/squash on the way down, and expand as you come back up to the surface.  This goes for air in your lungs, your mask, and your ears. 

The first thing you will notice as you dive is the feeling of pressure in your ears, and mask. You can release the pressure by equalising your ears and mask (click here to find out how). If you don’t, it will start to really hurt and you’ll eventually damage your eardrum. Most divers need to start equalising in the first 1-5 metres of the dive.

Another change that starts to happen as you dive deeper is your buoyancy (buoyancy = tendency to float). The deeper you dive, the less buoyant (or floaty) you become. Once you hit a certain depth you will become neutrally buoyant (not floating, not sinking), and beyond that you will begin to sink. This is a pretty cool feeling, and deeper divers use it as a way to drop down without using energy to fin downwards. Clearly, it also means that you need to be diving safely because you won’t naturally bounce back up to the surface. Freedivers usually carry weights to counter their natural buoyancy.

Another quick note on safety: I don’t recommend wearing weight before you’ve had any training. It makes you harder to rescue and therefore adds some risk.

How Freediving Works #3: The Mammalian Dive Reflex

Your body has a pretty fascinating set of pre-programmed responses left over from the watery ocean days of our ancestors built specifically to protect your survival in water. It’s called the Mammalian Dive Reflex (MDR – it’s also knows as the Mammalian Dive Response). Eventually, as a freediver, you’ll learn how to trick these responses and use them to your advantage. Again, there are so many articles and videos on the detailed science behind the MDR. Here’s a basic intro.

The response is triggered once we submerge our faces in water (ideally cool water), and start to hold our breath. Our brain registers the water, and the growing level of CO2, and starts to engage the MDR. This is what happens:

  1. Peripheral Vasoconstriction and Blood Shift – This basically means that our body redirects blood from our limbs towards our brain and vital organs. This helps to supply oxygen to our most important organs, and also helps to protect our lungs and airspaces from collapsing under pressure as we dive. It’s also the reason that you pee more than usual during your dive session (yep). Your body interprets all that excess blood as excess fluid and spend the floodgates.
  2. Our Heart Rate Slows (Bradycardia) – Now we have all this high blood pressure in the centre of our bodies, and our heart rate slows down. This helps to conserve the oxygen we have.
  3. Our Spleen Contracts – weird, but amazing. By contracting, our spleen squeezes out a bunch of new red blood cells which will help to carry oxygen around in our blood vessels.

Ultimately, the point of the MDR is to help us survive underwater for longer by protecting our brain and other vital organs from hypoxia (lack of oxygen) and pressure damage. This is the reason that many people can hold their breath for longer in water than on land, but we are not going to test that theory until we are trained and with an experienced buddy :).

Many people take advantage of the MDR without even knowing it. You know how calming it is to jump into the ocean and get your head wet? Now you know why that is.

I love this video by Minute Earth, explaining the Mammalian Dive Reflex really simply


There you go! That’s a very basic intro to the major mechanics of freediving. As with all physiological topics, there is extensive and ever-changing science behind these concepts. Here are a few more links if you’re interested in exploring the next level of detail.

As always, please remember to be safe. Do not try any breath-holding in the water without formal training, and an experienced buddy who knows how to recognise and react to any problem.

If you enjoyed this post, please comment and share with your friends so more people know that this site exists 🙂 Let me know if you have questions on the topic (no matter how dumb), or if there’s anything else you’d like to see. 

Bella xx 

More Resources:


Depth and Pressure:

Mammalian Dive Reflex:

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