Just don’t hold your breath: Life Under Water

Good morning all,

Now firstly I have to apologise. I believe it has been an unforgivable two weeks since I last published a blog entry. I am going to place the blame squarely on my on-call medical rota and two weekends worth of open water scuba diving courses! Of course, I am joking – the responsibility rests with me, doesn’t it, as with most of the actions we take in life. But I am back!

I have also genuinely missed writing this over the past two weeks – it’s a reminder how much I really enjoy sharing with you what I have been up to – as long as it’s interesting for you that is. So let’s begin shall we…

A patient’s ability to breathe is key to supporting life. This rather obviously fact is what I am going to focus on first. We are, in fact, not even going to talk about the heart or blood or once. For when you assess somebody in an emergency situation (or any patient for that matter really), you follow ‘ABC’, otherwise known fully as ‘airway, breathing, and circulation’. They are written that way as that is indeed the likely order in which they will kill you the fastest.

Without an airway i.e. the tube (your ‘trachea’) connecting your mouth to your lungs, the 21% of oxygen within the sea-level air we breathe cannot get in. You die. If that works, then comes the next possible problem are – the breathing part of the acronym. For without being able to breathe, because of say, a collapsed lung or muscular paralysis from a rare infection – OR perhaps being able to breathe but having nothing to actually breathe, will kill you next fastest. We see these situations often in patients not so much in the extremes of those (although within intensive care we do), but more along a spectrum of breathlessness from the mild in people with well-controlled asthma to those with very severe chronic obstructive pulmonary disease (COPD) with a superimposed chest infection (known as an ‘infective exacerbation of COPD’) that requires machines known as non-invasive ventilators (NIV) to force air in against their failing lungs. COPD is a result of smoking. Please don’t become one of the people I see who struggle for breath daily because of something you did in your earlier years. It makes me so sad. That’s the only plug you’ll ever see me write.

Now, we digress! Yes, so airway and breathing – both very important to life.

So I decided I wanted to try exploring an environment that was not, without assistance, compatible with an airway or breathing – under water! Now, you will see, from the photograph below, that from an early age, I have always been a bit of a water lover…..


….and so 5 years after this photograph (ok, ok, maybe more like 20 years!) I decided to take the plunge again and go scuba diving in the most romantic, picturesque, majestical diving mecca I could find – a swimming pool in Tooting Bec in London, and then a man-made silt-lined murky lake in Wraysbury, 40 minutes outside of London. The blue hole, Belize, The Great Barrier Reef…..I’ll be seeing you soon!


The course for achieving the qualification of Open Water Diver, was just absolutely brilliant! Although, true to my passion for health and medicine, I found myself very sadly becoming more excited and enthralled with the physiology of how people become survive under water and also fall ill under water when I really should have been listening about the ‘how not to drown under water’ part of the lectures! Plus the kit you get is fantastic – who doesn’t love gadgets! The bulk of my breathing apparatus while scuba diving comprised the following:

1. An Air tank (10-15L) of 200 bar pressure – the big tanks divers lug around

2. A Buoyancy Control Device (or BCD) – those flashy looking vests

3. A Regulator which has the 1st stage which attaches to the tank and the 2nd stages which are where you breathe from (I say second stage’s’ as you have one primary and one back up) – the bit you put in your mouth and all the tubes popping off the tank

4. Submersible pressure gauge (SPG) so you know how much you air you have – just a gauge!

5. All the other fashionable accessories like: mask and snorkel, dins, compass, and dry suit (looks like a ridiculous oversized boiler suit out of water, I know) when in open water (because diving in the UK can be very cold even just 8m underwater despite the surface water temperature deceptively that of a warm bath!

And when you are all kitted up you end up looking something like this…

Pool diving


dry suit

Breathing underwater is simply magical. If you have had the honour of doing it then you will know what I am describing. For those of you who haven’t, if you have the time and are willing to pay (I forfeited going on holiday to do this course) then I really encourage it – or at least try out some snorkelling on holiday or your local swimming pool. It was for me an incredibly relaxing place to be and you are just in your own little, mask tinted world, with the sound of your own breath as the melodic backdrop to this underworld existence. Unsurprisingly I didn’t see any tropical fish or sharks in Wraysbury – I saw one crayfish and a bus that they had deliberately sunk in the lake to give us SOMETHING to look at! The first breath you take in when you go underwater is one of trust by the way…and a VERY surreal experience.

What I want to finish is by telling you about something a bit ‘sciency’ about diving and how you all may be able to relate to it. It is called the mammalian diving reflex.

Now this mammalian diving reflex is a primitive reflex that many mammals have. This includes (a little more weakly) us humans. It is a reflex that aims to slow down your respiration rate or ‘breathing rate’ simply put, while under water.

In humans, the trigger is when our face hits cold water (that’s less than 21 degrees Celsius)! Interestingly it is only triggered by our face – if you were to put your hands into ice cold water, or your leg, the same effect would not occur. It happens specifically, you see, when the sensing sites in the nose called ‘receptors’ are hit with cold water and trigger the 5th cranial nerve known as the trigeminal nerve. This nerve then sends a cascade of signals to the brain and activates your autonomic nervous system. So, what actually happens when your face plunges into cold water and this is autonomic nervous system is activated? Well two key things:

1.  Your heart rate slows down by a fifth (called a ‘bradycardia’), and

2. Blood vessels on the outside of your body and muscles narrow (called ‘vasoconstriction’) to help keep the blood flowing to your important heart and brain

…and incidentally while we are on the subject, that is why people can survive for longer underwater – basically their need for oxygen by the body goes down. I love how the body can survive by conserving oxygen in this way – it is yet another marvel of our body. In medicine this understanding makes it one of the more novel ways in which you can slow down a very fast heart rate in a child (i.e. a supraventricular tachycardia) -that is, by plunging a child’s head first into ice cold water! Quite simply, the reflex we talked about is triggered and the heart rate slowed down as a response. The parents look horrified obviously….

I could go on to talk about nitrogen narcosis and decompression illness but I am sure that I can tell you about those another time! One thing I would like to end with is this question: do you know why, when you are underwater, you end up taking that breath or having to come up for air to stop you from taking that horrendous mouthful of chlorinated, urinated, pool water?! Is it:

a) Lack of oxygen?


b) Too much carbon dioxide?

Ok, so this is a bit of a loaded question but the answer is in fact a build up of carbon dioxide. You see our respiratory centre which lives in our brain and tells us how much and when to breath, is very sensitive to how much carbon dioxide there is in the blood.

We make carbon dioxide when our cells use oxygen and release energy. As we hold our breath therefore the oxygen is used in the cells and the carbon dioxide is made as a bi-product – and gradually builds up. When it hits a threshold of too much carbon dioxide the brain’s respiratory centre says “time to breathe” and so you breathe – this is not voluntary at that point!

That’s why before a ‘breath holding competition in the pool with your friends or family everyone automatically takes those big breaths – not only are you loading your blood with oxygen but you are also blowing off as much of the carbon dioxide in the blood so that it takes longer to build up – and hence take you to breath-holding competition glory!

A few fun facts to round this week’s blog off with:

1. World record for deepest ‘free dive’ (I.e. one breath hold) = 253.2m!!

2. World record for deepest ‘scuba dive’ (i.e. with breathing equipment) = 330m

(note that the reason we cannot go deeper currently is largely to do with the massive pressure that would be exerted on the body down in more depths)

3. World record for deepest dive of a mammal = 2,992m by the Cuvier’s beaked whale (and they were down there for 138 minutes!)

It is clear that human physiology, as amazing as it is, still has a lot to learn from mammalian physiology – when it comes to life under water.

Have a great week everyone!

Dr Nick




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