best freediving school Archives

In the last couple of months, I noticed that even some Freediving professionals are not completely understanding how breathing control in humans happens. And be honest, while I was writing this article I found out that I was not 100 % correct as well.

Hopefully, this article helps you to understand better what exactly happens with your respiratory system when you hold your breath. And if you find any mistake here – feel free to correct me – I am still learning as well!

As any other Freediving school, we are teaching about breathing control on our Freediving courses, but here I tried to put a little bit more details.

To start with – we have two different types of chemoreceptors which are detecting chemical changes in our body and sending signals to our respiratory center within Pons and Medulla Oblongata (both located in the brainstem), from where impulses send to our external intercostal muscles and diaphragm, to change the volume and frequency of our breathing (or cause “urge to breath” if you are holding your breath).

We can divide these receptors into 2 categories

Central chemoreceptors. Why “central”? Because these receptors are part of our central nervous system and literally part of our brain (located inside Pons and Medulla Oblongata). Since these receptors are not inside blood vessels, they are responding to high CO2/H+ not within the blood, but within cerebrospinal fluid (CSF), which separated from the blood vessels by the blood-brain barrier (BBB).

chemo receptors

Let’s make an example. You hold your breath for a few minutes. Amount of CO2 increases in your blood, the amount of H+ also increase creating low pH (respiratory acidosis). H+ doesn’t diffuse through BBB, but CO2 does. This CO2 bonds with water inside CSF and produce H+, an increased amount of which is going to be detected by central chemoreceptors.

CO2+H20↔H2CO3↔HCO3+H+

Recently I learned that lactate has an impact on this process as well. Lactate, which is produced during anaerobic energy production, in a form of lactic acid, can go through BBB where it brakes down to lactate and H+, which eventually lead to the activation of central chemoreceptors.

Eventually, central receptors can desensitize and this is why we have the potential to become less sensitive to high H+ over some period of training with exposure to a high CO2 (whether it is a breath hold training or some form of HIIT).

Peripheral chemoreceptors. They are not part of the central nervous system (instead, they are an extension of the peripheral nervous system) and located inside aorta (largest artery of the human body). More specifically – inside the aortic and carotid body. Interesting fact – here we have one of the highest blood flow in a human body.

regulation-of-respiration-14-728

Chemoreceptors inside aortic body sensitive to the change of partial pressure of CO2 and O2. If there is a change – they send the signal to Medulla Oblangata via Vagus nerve.

Chemoreceptors inside carotid body sensitive to change of partial pressure of CO2/O2 and change of pH (metabolic change, due to high lactate production for example). And if there is a significant change – send the signal to the respiratory center via Glossopharyngeal nerve.

The main function of peripheral chemoreceptors (glomus cells) is control of pO2 (in contrast with central chemoreceptors, where the main trigger is a change of pCO2/H+). As I said early, they also sensitive to the change of pCO2/H+ but secondary. It means that the sensitivity of these receptors to the low pO2 is greater when pCO2/H+ is high.

Activation of peripheral chemoreceptors are low when the partial pressure of O2 is close to the normal (100 mmHg), but when it is going below 60 mmHg the activity increases rapidly due to a decrease of hemoglobin-oxygen saturation.

Peripheral receptors are not desensitized over time.

Two common hypoxic ventilation responses (CO2/pH can stay at the normal level) – reaction to high altitude or high concentration of carbon monoxide in breathing air.

How all of this can be useful for us Freedivers? In the middle part of the breath hold, when your contractions start, it is a reaction to a high CO2/H+ sensed by central chemoreceptors. Peripheral chemoreceptors are not playing an important role at this moment since the partial pressure of O2 is close to normal. But close to the end of your MAX attempt, when pO2is going to be close to 60 mmHg and low, a reaction from them will contribute to your urge to breathe.

For further reading

Useful videos to watch

Is it possible to have DCS for Freediver? Unfortunately yes…But at the same time, it is quite easy to put the risk at the minimum. At our school, we normally discuss in details this topic at PADI Master Freediver course, so here are our thoughts on this subject…

In scuba diving, when you are breathing compressed air underwater, your body is saturated more and more with Nitrogen (79% of the air is Nitrogen). The chain looks like this – you breathe in the compressed air from a scuba tank and now the pressure of N(Nitrogen) in your lungs more than in your blood, so, it going to your blood. And now in your blood pressure of N is higher than in your tissues, so, it is going to your tissues. Until you have an equal pressure of N everywhere (this is what we call saturation). And when you start coming up, opposite happens. Now pressure of N is smaller in your lungs, so the reverse process is happening – from your tissues N going to the blood and then to your lungs and then you exhale it. But it doesn’t happen that quickly. In some tissues, this process is slower than in others.

So, this is why when scuba divers coming up, they need to do it slowly (in this case N slowly coming from your body with every exhale) and they also need to have a long surface interval between dives (to release as much N from their body as possible before the next dive). And why they can spend the only a certain amount of time on certain depth (not to get too much N to their tissues).

dcs2 If they violate these rules, they have a very good chance to have DCS! Without making it too complicated, DCS is when molecules of N combined in your blood (or tissues) in the form of a bubble and can cause very serious damage!

But why it can be an issue for Freediver?
Among the first, spearfishers who were shooting fish relatively deep (around 20 meters) noticed that after the long session they have symptoms which are commonly associated with “bends” (another word for DCS). So, imagine you dive to 20 meters. On the surface, you did big inhale and bring this air (and around 79% of it is nitrogen) to 20 meters, where the pressure is 3 times higher. So, the pressure of any gases inside your lungs (we call it partial pressure) going to be also 3 times higher. If the pressure is going to be higher in the lungs, then it will go to your blood and further to your tissues in the attempt to reach equilibrium. When you start your ascend, the reverse process will take place. But not 100% (some of our tissue need time to release N2). Some N2 will still be in your tissues. Very very small amount. Not a dangerous amount. But then you are going to do 50 dives more to this depth. And now you have much more N2 in your blood. And on your next ascent, it can forms bubbles and this is DCS….

There are also some predisposing factors which can increase the risk of DCS. Let’s name a few

Dehydration
Exercises
High level of CO2
Cold
Not enough surface time

As you can see, spearfisher who is doing a long session with relatively deep dives and small recovery time has a very good chance to get DCS….What to do, if you are spearfisher? Limit duration of your spearfishing session, have enough rest between dives, use appropriate thermal protection, stay hydrated!

Ok, what about DCS for Deep Freedivers?

Let’s say you are one of the most famous Freediver of all time, Herbert Nitsh, and diving not to 20 meters but to 214 meters!! You make a big inhale as possible (don’t forget, around 79% of inhaling air is N2) and put it under a huuuuuge pressure of 21 atm (this is what you have at 200 meters). During your descent, this N2 from your lungs will come into your bloodstream and from there to your tissues.
And now you will start your ascent. It would be No Limit discipline and you are not going to use your muscles for going up (which is very good to reduce the risk of DCS), you are probably also not dehydrate it nor cold (which is also very good)
But you are quite fast for this huge amount of N2, dissolved into your tissues. The bubbles will form. But not big in the beginning. But when you ascent with the constant speed, they are going to increase in size (because the pressure is decreasing). How to prevent it? You need to slow down…And this is what Herbert did – he dramatically decreased his ascent rate after 50 meters (from 3 m/s to about 1 m/s). He also did some safety stop at 10 meters to make it even safer. And this is how he became The Deepest Man on Earth by making a dive to 214 meters!!
But later, when he was trying to beat his own record he lost his conscious on his way up and failed to do all of the above (safety stops and slow down his speed). As a result, massive DCS….Luckily (actually his story of recovery is almost like a miracle) he survived and recovered from it!

But if you are diving not that deep, let’s say to 40 meters; is it possible to have DCS? And the answer is yes… Last year Freediving legend William Trubridge got DCS during his routine training. I mean routine for him but crazy for all of us! Let’s have a look at it

William did 5 dives to 40 meters with “hangs”. Time of each dive was about 1.50, the surface time between dives only 2.15. As a result, after his 5^th dive, William felt one of the symptoms of DCS – numbness in his leg. He went to a hospital and took special treatment for DCS there (pretty expensive!). This story has a lucky end as well, William completely recovered and competing again!

According to him, he done this type of training before (even harder) and never had any problem.

So, what is the problem here and how to avoid it? The dives were not too deep and not too long (especially if you are World Champion). But the recovery time was quite short.

So, if you are just starting freediving – this is a basic advice – DO have long surface intervals. How long? Here a couple of ways how to count it

The time between your dives should be 3-4 times longer than your dive time. For example, if your dive time 30 secs, then have a rest for at least 2 mins
Divide dive time by 5 and the result is a time in minutes. If you dive to 40 meters, have a rest for at least 8 mins.

And be even more conservative – have even longer rest!

A few things which I would you to remember

dcs3 First of all – symptoms of DCS. Type 1 DCS – skin rash on shoulders and upper chest, joint and limb pain. Type 2 DCS – peripheral tingling and numbness, unconsciousness, respiratory arrest and paralysis, coughing, feeling of air-starving.

Second – predisposing factors. Obesity, intense exercise less than 12 hrs before diving, age, fitness level, dehydration, injury and illness, alcohol, carbon dioxide, cold. Obviously, scuba diving before freediving is absolutely no-no!

Treatment for DCS – the patient should lay down and breath 100% O2. Transportation to a recompression chamber should be organized as quickly as possible. dcs4