Why does holding my breath make me want to poo? | Freedive Earth

Why does holding my breath make me want to poo?

We’ve all been there. (Right? Guys…?!) You’re in the middle of a static breath-hold, everything’s going fine, you’re handling the contractions perfectly, feeling relaxed and then suddenly… oops! Maybe a little too relaxed. 

Floaters - Terrifying

I for one am not too proud to admit that I’ve had to wash my speedos a little more carefully than usual on occasion, and I know I’m not the only one. Sources tell me, for example, that at least one former CWT world record holder has been heard to claim that she shits herself “on the way up from every dive”. On balance I think it’s probably best if I don’t say who that is. So what’s going on? Why is it that, even if we don’t actually get the old “turtle’s head”, or worse, many of us feel the urge to empty our bowels quite strongly, especially towards the middle or the end of a long breath-hold. This was clearly a burning (geddit?) issue for newly crowned NZ record holder and FIM silver medallist Kate Middleton of Freedive Gili, who submitted this question to Dr Otter during the recent World Championships. Thanks Kate!

The Physiology of Defecation

Here are some bowels:

 

 As you probably already know, they’re divided up into Stomach, Small Bowel (or small intestine) Large Bowel (large intestine or Colon), Rectum (a holding place for poo) and Anus, a small space containing the sphincters (rings of muscle) that (ideally) keep everything inside until you’re ready. Food passes from the mouth, down the oesophagus to the stomach. It’s held there for a bit in a bath of enzymes and acid and smooshed around by smooth muscle in the stomach wall. This is where most of the digesting of protein is done. The partially digested food then passes into the small bowel where pancreatic juices, bile and more enzymes are added and most of the nutrients from the food are absorbed. From there, the “chyme” (as it’s called) passes into the Caecum (the first part of the large bowel) where it’s fermented to break down fibre and other roughage, and then on into the rest of the Colon where most of the water is absorbed. What arrives in the Rectum is recognisably well-formed poo, which is held there (hopefully) until you actively push it out. When the rectum gets full, you get a signal to your brain that you need to go. Wait too long, though, and the Rectum starts contracting on its own, leaving you “touching cloth”. The whole process of food’s movement through the bowel (known as Transit) is controlled by the rhythmic contraction of smooth muscle lining the intestines in a coordinated squeezing movement called “Peristalsis”. Check this out!

 

Peristalsis - Moving How frikin’ awesome is that?!

So Why Does Breath-Holding Affect the Bowels?

First of all, it’s important to understand that Gut Motility (that is, how much peristalsis is going on, and how quickly the food is moving through the gut) is controlled, at least in part, by the Autonomic Nervous System. This is a network of neurons that is involved in all those processes in the body over which we have almost no control. The rise in heart-rate associated with exercise is a good example of this, so, interestingly, is the bradycardia (slowing down of the heart) associated with the Mammalian Dive Reflex. We’ll come back to that in a minute. The Autonomic Nervous System is divided up into two parts: Sympathetic and Parasympathetic:

 

 

Notice that the Parasympathetic Nervous System tends to stimulate all those functions involved in slowing down, resting and digestion (the so-called “rest and digest” response), including increasing peristalsis, whereas the Sympathetic system readies the body for action (sometimes called the “fight or flight” response). For all you Yogis out there, there are some interesting (though unproven) parallels between the organisation of the autonomic nervous system, and the system of chakras:

 

 Here are the Chakras of a Dog(!) for comparison:

 So here’s the thing: One of the driving forces of the Mammalian Dive Refex, which kicks in in response to rising CO2 during a static breath-hold, is massive Parasympathetic activation. That’s also a major cause of increased motility in the gut. Surprisingly enough, there’s no actual research on the topic that I can find, but it makes sense that holding your breath, causing CO2 to rise, activating the parasympathetic nervous system and slowing down the heart would also dramatically increase the rate of gut peristalsis, decrease the transit time of food in the gut, and cause the rectum to quickly fill with faeces, stimulating the urge to defecate. Simples.

Anything Else?

Aside from this no-doubt startling piece of physiological deduction, there is also some evidence of a direct effect of CO2 on intestinal motility, and of blood flow in the gut from experiments done in dogs, cats, and rats. It’s reasonable to assume that these processes also happen when we hold our breath. WARNING! These links contain descriptions of flagrant cruelty to animals. If you’re against such things, have a look at this picture (which also involves research done in cats) instead: 

 

What About Coffee?

About 60% of regular coffee drinkers will have noticed an association between drinking coffee and needing a poo, and the short answer to the question “Why?” Is that no-one knows. Sorry. It’s probably something to do with the localised effect of caffeine on the smooth muscles of the intestine itself, which is the other part of the control of the rate of peristalsis: Even if you cut all the parasympathetic nerves to the gut (more tortured cats again no doubt) peristalsis still continues, implying that local processes in the gut itself, are just as important as central ones controlled by the autonomic nervous system. Just having food or fluid in the gut (without any autonomic activation) is enough to trigger peristalsis through the activation of local pressure and stretch receptors in the lining of the bowel. Coffee, we think, stimulates either these receptors, or the muscles directly. The effect is much smaller with decaffeinated coffee, implying that Caffeine is the culprit. Here is my wife's cousin Albert. He's an Asian Palm Civet. His poo is essential to the production of the highly prized "Kopi Luwak" from Indonesia, which neatly brings this article full circle.

The relationship between coffee and poop has been well established for centuries...

Now if you’ll excuse me, I’m going to drop the kids off at the big white nursery. See you next time! Dr Otter is the resident physiology whizz kid at Freedive-Earth. If you’ve got any freediving related physiology questions, get in touch by commenting below, or email us at info@freedive-earth.com!

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