Breathing is the only autonomic function you can switch to manual control on demand. Though, I am open to discussing whether there are any other such “functions”.
You cannot control your heart rate, your gut motility, your sweating, your hormone excretion, or your cell energy production on demand, they all happen automatically. So does breathing. But, you indeed can change your breathing to manual mode. 
Although I stopped thinking of any of the bodily functions as being the most important or end all be all, I do see breathing as something as close to that as it gets. Definitely though, depending on what your end all and be all are. If they are: sitting peacefully under a tree for 100 years, then breathing is definitely the thing you should be looking at. But also, if you just want to help your body have more time on this Earth, you should be looking at breathing.
I am only starting to grasp the power of it, after diligently doing some form of breathwork for the past year +, for the first time in my life. This text is not structured by any order of importance of the topics concerning breathing; what I’m writing about here is mostly personal philosophy (with bits of neurology) and it is not any more important than what is omitted from here (everything else).
The Switch
I am occasionally startled by the fact that we, humans, have something so easily attainable as – changing our breathing from autonomic to manual, literally in any given moment. And we really do not have to if we do not want to/need to, for our whole lives. Breathing will still be happening.
I’ll pose some questions which I will not answer. What is breathing when it’s happening autonomically, is it the subject or the object? Is it the mirror and reflection of the state of your body, happening as a result of the conditions it is in? Or is it the subject, autopilot driving our whole body to do the rest of its functions? That’s three questions, but they are the same.
And also, the other way around, but similarly. What is breathing when it’s happening manually? Is it becoming the central governor in that case? Is it successfully overriding all the autonomic processes and giving it directions of how to act?
Breathing manually is usually a manifestation of your wish to change something. There are many things you might want to try with breathing; putting yourself in hyperventilation mode on purpose and changing your state of consciousness; putting yourself in tough positions and teaching your system to be more stress resilient via breathing in those positions; or, in a great majority of cases I believe, you are aiming to calm yourself down.
If we focus on the calm yourself down part – it means upregulating your parasympathetic nervous system, which in turn should lower your heart rate, lower your breath rate, improve organ function and muscle reflex activity, because it is putting you out of the stress state. When the stress state is on (sympathetics) it is like someone put a brake on your recovery functions (the calm yourself down part), because hey – there ain’t no rest for the wicked.
Switch “ON”
So, why does it work? Why can I – just by changing how I breathe, affect literally everything else in my body – to the degree of being able to sit under a tree for 100 years (if I become really good at it)? 
You are unlocking the door to affecting autonomic functions of your body. You are, in a sense, hijacking whatever is currently happening in your system by default and making it more – the way you want it to be. The moment you take manual breath, everything can change in an instant. It is literally like you have peeked through the autonomic functions door and then it is up to you, to either continue peeking and opening them more and more, or not.
Humans have amazing ability to focus, though less and less utilized in modern societies. If you choose the breath to be your focus, that is like choosing to follow the ultimate granular atomic clock to the beat.
It is in a way, as if time loses the timing aspect of it.
Am I now the one directing the breath?
Or am I just letting it happen automatically, as it surely will?
It’s as if you are successfully tuning the strings of your nervous system, without ever being given the handbook of how it should be done, just being there for it.
Switch “OFF”?
Autonomic functions of your body are the underlying theme of how your body reacts, responds and thrives or does not thrive in any given conditions. They either do their job in concert with your support, or they try to do their job in spite of you putting obstacles in their way.
Breathing manually can indeed give your body rest. If we are unconsciously running around all the time without ever taking a pause to breathe, it’s as if we never stop at the pit stop to change tires, put gas in, fix broken parts. When it’s just happening autonomically, there’s a high chance that in today’s world of too much stimulus which does not care for the wellbeing of the human body, things will over time turn into havoc. Breathing manually is one of the low hanging fruits for unlocking the door of the repair mode.
I would propose that doing any kind of breathwork (as with anything there are good and less good options) will help, because it will help you stop for a second. Meet yourself again. There, in the moment of real breathwork effort, you are doing manual labor, but in the background you are re-tuning the autonomic functions to work with a little bit more ease. Finally, you do want your autopilot to work well. That way you have more time to live – both literally and figuratively.
It is all in the end, just a mere attempt of – slowing down the inevitable.
For private sessions of breathwork which will help you do that, DM at IG @matea_sed
[Feature Image from Pixabay.]