Tag Archives: peripheral vaso-constriction

Cold Shock Response and the Mammalian Diving Reflex in cold water swimming – Positive & Negative Feedback systems

I read a blog recently about cold immersion and cold baths, and cold swimming to a lesser extent. The author was speaking about the positive physical and mental benefits of regular ice baths. Similar benefits to what we as cold water swimmers regularly experience. All well and good. For aspirant Channel swimmers without access to regular cold water swimming, the recommendation for cold showers and baths is old and trusted.

However in his explanation of what was happening in the body the author focused exclusively on the Mammalian Diving Reflex as the primary response of the body when being immersed in cold water and completely ignored or didn’t understand the effect of Cold Shock Response and its place in the equation. (I didn’t save the blog link, sorry.)

That blog wasn’t the only place you see this. If you search on Mammalian Diving Reflex you will see it widely referred as the (only) process  in action when people are immersed or submerged in (cold) water. It’s a classic example of people taking all their knowledge from Wikipedia, because it seems the same Wikipedia core text is used all over the place.

I’ve covered both before and as a cold water swimmer, rather than someone sitting into a cool water bath, and I’ve focused as  much on Cold Shock Response, and the issue of Habituation, the process of getting used to getting into cold water, (not the process of staying in it).

The blog author just cut and pasted a Wikipedia article on Mammalian Diving Reflex, and while the Wikipedia article wasn’t wrong, both it and the blog were incomplete from the perspective of a  cold water swimmer.

So what are each of these and do they interact?

While swimming in Tramore Bay the other day, the water having risen by a degree in two weeks to about 7.5 Celsius, I got thinking about these two responses and how one, Cold Shock Response could be considered a positive feedback system while the Mammalian Diving Reflex could be considered a negative feedback system.

It’s not the first time I’ve wondered about Positive Feedback in a biological sense in open cold water swimming. Previously I considered that the Habituation/Acclimatisation process in cold water swimmers could also be a positive feedback system.

The simple process of improving cold ability
The simple process of improving cold ability

In Systems Theory (and elsewhere) a Positive Feedback System is where a small change causes a further bigger change. Therefore positive feedback is often considered a de-stabilising process. One example might be the international banking system that led to the 2008 collapse: Increased risks led to larger profits which led to larger risks until the system collapsed. However positive feedback can also be a process for change or improvement: If you swim more, you get fitter and able to swim even more, i.e. the training effect. Or the more you get in cold water, the better you get at getting into cold water.

Negative Feedback is often considered a stabilising process, the most common example is a thermostat which regulates heats by switching off when it gets too hot, switching on when it gets too cold: Negative Feedback acts in the opposite direction to the initial impulse.

Cold Shock Response is the bodies response to sudden cold water immersion. It results in varying degrees according to the person’s habituation experience, primarily in elevated heart rate, and elevated stress hormones. It is the elevated heart rate which is dangerous, to lesser extent in the increased chance of cardiac arrest, but more commonly in the chance of aspirating water due to shock and subsequently drowning. Less habituated or experienced swimmers will note an increased heart rate and nervousness even before immersion occurs if they are expecting the cold. I noted some years ago that the first time I ever swam during winter without a wet-suit, I was literally terrified beforehand. Then the initial cold shock drives the heart rate higher. This is a limited example of Positive Feedback, where the initial is destabilised by something (cold) that acts on the input.

The Mammalian Diving Reflex is another innate biological response to immersion. As the name implies all mammals exhibit this, human to weaker extent, but it exists to extend the time that animals can survive while submerged by reducing the need for respiration. This occurs in swimmers through two main biological reactions; decreasing heart rate (brachycardia) and therefore slowing the buildup of carbon dioxide in the body, (as excess carbon dioxide is what cause us to have to breathe); our constant companion, peripheral vaso-constriction, where the capillaries and blood flow in the extremities is restricted to allow more oxygenated blood to be available to the heart and brain.

The Mammalian Diving Reflex is initiated when the fact is submerged, and this is the reason I have previously written many times that you should splash your face before getting in the water, rather than the incorrect but widely cited slashing water down your back.

The Mammalian Diving Reflex is obviously a case of Negative Feedback, where the body reacts in opposition to submersion to protect itself.

So we can see that there is both positive and later negative feedback in operation in cold water swimming, where the negative feedback occurs to stabilise and protect a human through adaptive physiological response. But the initial negative feedback of Cold Shock is very significant and should not be ignores, as so many non-cold water writers seem to do, as it carries its own significant risk factor.

The bottom line though, is that this is another way of saying, that ability in cold water swimming improves with repetition. Habituation improves much more quickly than Acclimatization. In a little as four to five repeats, people become much more comfortable with getting into cold water.

Now get out there!

Infrared hands

What is cold immersion diuresis in swimmers?

That is: Why you pee more during and after swimming.

Diuresis is the medical term for increased urination. Cold immersion diuresis is common to cold water swimmers and is the strong desire to pee after (and sometimes during) swimming in cold water.

As we have now discussed many times, swimming in cold water leads to peripheral vasoconstriction, by the cold of the water leaching away the heat of the blood vessels closer to the surface. The blood vessels constrict resulting in reduction of blood flow in the body’s extremities to conserve heat in the cold.

This reduction in blood flow to the periphery therefore actually leads to a quick increase in blood pressure. The body attempts to compensate for this increase in arterial blood pressure by relieving itself of liquid elsewhere. The easiest, quickest and least costly expenditure from a metabolic (energetic) point of view is urine. So you will often find as cold swimmers do, that walking into cold water even before you are fully immersed, you will develop a sudden urge to urinate caused by this blood pressure increase. And during a swim liquid also builds up in the bladder but without practice, they are unable to urinate in really cold water while swimming, many swimmers, even very experienced cold water swimmers will have to stop or momentarily pause to urinate. Also when you exit the water the demand to urinate can reach quite powerful levels as the muscles finally relax.

A consequence of this increased urination often forgotten is mild dehydration. Marathon swimmers in cold water, such as the English Channel are taking most of their food as liquid carbohydrates. The volume of water needed or used is generally close to one litre an hour, and because of cold diuresis, more of the liquid processed by the kidney, instead of being absorbed back as is normal, goes to the bladder. Part of this mechanism is that the cold suppresses the production of ADH, aka vasopressin, the anti-diuretic hormone that suppresses diuresis (urination). The swimmer urinates more, so the swimmer needs more liquid to compensate for the mild dehydration. And you have a self-sustaining cycle as long as the swimmer is immersed in tolerable cold water.

Thus … the well-nourished high weeds behind the Sandycove Island parking spaces.

Related articles:

Hypothermia mortality rates in Ireland.

Mechanism of cold diuresis in rats.

Review: Amphibia Sports Ring

The simple ideas are often the best ones.

I’ve previously reviewed Irish Company Amphibia Sport‘s excellent Evo Sports Bag and I recently received in the post one of the company’s long-awaited (for me) silicon Sports Ring.

I forgot to test swim it the weekend after I received it, so it had to wait until this past week to be taken for a swim. And no better weekend to test it. How did it perform?

Before I answer, I’ll explain further what it is, and why it’s necessary.

Amphibia Sport ring protector
Amphibia Sport ring protector

Technically, it’s a ring itself, as it goes around your finger. But it could also accurately be described  as a ring-protector for sport, to wear over and protect a metal ring.

I used to wear a ring all the time, a thick silver band that I had custom-made many years ago. I learned early in open-water swimming to remove it (well most of the time I remembered), as shortly after immersion it would feel like it was going to slip off my finger and I would have to swim with my fingers tightly closed, something I don’t normally do. All my concentration would still be on whether the ring was still going to slip off, not a comfortable way to swim. The ring wasn’t oversized, so it wasn’t a case of being too loose. Metals, particularly precious metals, have very well-defined thermal properties, their expansion and contraction in heat and cold.  We don’t think of organic materials as have expansion or contraction properties to anywhere near the same degree. We don’t for example think that our pants might fall down if the weather is colder!

So why does your well-fitted ring feel like it is going to fall off? (And in fact there are many cases of this actually happening to swimmers).

As we’ve often discussed about cold, cold immersion leads to various physiological responses, amongst which the most important is peripheral vaso-constriction, the reduction of blood-flow in the extremities. Another that goes along with this it he constriction of external blood vessels. This constriction causes other responses, (on which I’ve written a post that I still need to publish), but the relevant one here is the reduction in diameter of the fingers. And that contraction occurs in relatively warm water (for us Irish skin-swimmers) such as 12 to 14 degrees Celsius. In fact the diameter of the finger seems to contract more than any ring diameter will contract, so the ring becomes loose. Wedding bands, engagement rings and more have been lost this way.

The alcove at the Guillamenes. Toasty.
The alcove at the Guillamenes. Room for one.

Test conditions at the Guillamenes were pretty ideal for experimentation. The air temperature had been dropping for a few days. The water was a quite acceptable 8.6 degrees Celsius, warm for the time of year, slightly up from where it had been for the previous weeks. But the air temperature was 4° C, with a biting northerly wind whose wind chill contributed to making it feel sub-zero. I put the ring on before I arrived and spent a short while taking some photographs (nothing much useful) so my hands were cold before I swam, something I usually try to avoid. I’ve also lost some weight recently so the ring hadn’t to be forced on, though I’d given up wearing it about two years ago.

And the water was rough, despite the off-shore wind, there was about two metres of swell incoming, with plenty of surface chop on top of it. 

Cold Guillamenes.resized.rotated

I swam for about 30 minutes, and with the wind it felt the coldest swim of this winter thus far. With the Amphibia Sport Ring protector in place, my hands could take their normal shape with fingers slightly parted (the optimum spacing for fingers while swimming is about half a centimetre). If I thought about it, I could feel the silicon protector comfortably touching the adjacent fingers. I never once felt like I was going to lose my ring, and in fact, and this is the important part, I very quickly forgot about the ring.




I’d hazard that maybe Amphibia Sport didn’t get to test the product in what many people would consider to be such extreme conditions, but not abnormal for Irish open water winter swimmers!

That’s a lot of words to say that the Amphibia Sport Sports Ring (protector) performs extremely well for open water swimming, a simple idea very well executed. Highly recommended.


Where did my Claw go?

Cold, my favourite subject. With so many ways to talk about it.

Every year I note changes in my adaptation and responses. The fun in this, is that I can treat myself like a long-term experiment and see what happens, it makes the cold swimming even more interesting, adds more personal value to it. My main criteria is that for the last five years I’ve never been out of the sea longer than two weeks. The hypothesis therefore (and import of that) is that I never lose my hardening, the ability to swim in cold water. (It wouldn’t even be that long if I lived a bit closer to the sea).  Last year I noticed a huge change in initial cold response to very cold water, where my cold shock largely disappeared with reduction in gasping and heart rate increase and pre-cold tension.

So this year, as we are within weeks of the normal coldest sea temperatures of the year, what I’ve observed is how one of the best recognised symptoms of cold for cold waters swimmers, the Claw, is now longer appearing during the times I am currently swimming. I have extended my cold swimming times from last year and at forty plus minutes in six degrees Celsius, my fingers are still closed and my hand is under control.

Above, we see again peripheral vaso-constriction in the hands, where there is no blood flow in the fingers. As often mentioned before, peripheral vaso-constriction is the body’s response to cold, where blood flow is concentrated in the core to retain essential body heat for survival. In more cold-adapted swimmers (and others) peripheral vaso-constriction seems to occur even more quickly.

Therefore I have to admit, I don’t really understand why my hand isn’t Clawing recently. I’ve certainly been getting cold. So for now I just put it down to another adaptation and if anyone has any thoughts on this I’d love to hear them. It does help to understand also that there no muscles in the fingers. Yes, no muscles, your fingers are operated by muscles running through the Carpal Canal  or Tunnel, which connect via tendons to the bones. It’s the flexion of the muscles in your wrist and forearm pulling on the tendons that moves the fingers, but there are no actual muscles in your fingers so the fingers get cold easily, as there is therefore less blood flow.

I am NOT saying my Claw is gone altogether, just for these shorter, colder times. I have no doubt that longer swims in warmer waters, doing two hours in ten or eleven degrees will see the return of the Claw, as deep winter is not the problem for us, but spring, when temperatures are slightly elevated but swim times must be much longer.

Edit: I should explain: As muscles get colder they contract. This is what pulls the tendons in the fingers apart. The swimmer’s ability to pull is compromised. It affects also the arms and legs so the whole stroke becomes shorter and less effective.

I should also add, the extent of the Claw is determined by your ability to close or touch fingers. A mini Claw will leave you unable to close your small and ring fingers, a full Claw will mean you are unable to touch your thumb and small finger. Unless you have a lot of cold water experience and safety cover, you should not be swimming with full Claw.

Peripheral vaso-constriction from cold water swimming – in pictures

I’ve often talked about vaso-constriction. As a reminder, upon immersion in cold water, in order to retain core warmth, blood flow to the extremities and skin slows. Upon exiting cold water a swimmer’s skin will feel really cold, even if the swimmer is experienced and may actually feel completely comfortable.

I watched the BBC’s Wild Swimming programme (synopsis, pretty poor). In talking about cold though they had a few useful images.

To set the scene, this was in an outdoor pool with temperature of 16 °C. (Really warm for many of us). But the presenter had no cold experience. At 16 °C she was gasping and going numb. After what seemed like two lengths, she had lost limb coordination and was having difficulty getting out of the pool. But that’s not why I bring it to your attention.

They had an infrared camera. Those images are useful for us, because regardless of your experience they apply at some stage. So I took some screen captures.

First was an image before immersion. You can see the higher heat radiative surfaces, particularly the arms and shoulders. Most of the body is at or well above 25 °C.

Before swimming.

Next we have the image immediately after swimming. Look how cold the body surface is. From watching the program this seemed like it was only a few minutes. Most of the body temperature is below 20 °C, much of it at 16 °C to 18 °C. (The image was tilted that way in the original programme with half the temperature scale missing). Notice particularly the legs and torso, hands, nose, all around 16 °C. You can see the indicator on the upper torso at 17.2 °C.

After swimming

Finally, we have an image taken a minute or so after exiting.

You can see blood flow and heat returning quickly. There are hot spots in the forearm and biceps for example.



Just as a matter of interest, I used my infrared thermometer (€18 inc. shipping on eBay) on myself the other day. I’d swam for about 75 minutes in 10 °C water. I measured my lower torso at 18.1 °C.