There are actions and functions in the body that we are in control of, and these are part of the somatic nervous system. Then, there are functions that happen automatically, and these are part of the autonomic nervous system. Breathing is unique in that it is part of both systems.
We can choose to hold our breath or breathe in a particular pattern for a period of time, but when we are sleeping, breathing occurs without thought. So, we can be in control until we are not.
What do we do with the control we have?
It depends on what you want to accomplish. Do you need to get excited or mobilized for an intense effort? Or do you need to settle/calm the system, in other words, relax for better recovery or to handle a stressful situation? How you breathe and control your breath can work in both situations.
Breathing and CO2 levels influence the sympathetic and parasympathetic aspects of the autonomic nervous system. Hypocapnia reduces CO2 levels (over-breathing or hyperventilative breathing) and is the way to mobilize the sympathetic (fight or flight) nervous system.
Hypercapnia is an elevation in CO2 levels (breath hold or slow breathing) that allows us to move into the parasympathetic (rest and digest) nervous system.
In SECOND WIND express, Pavel dives deep into many aspects of breathing that affect performance, recovery, and how to influence the sympathetic/parasympathetic nervous systems.
One easily accessed breathing strategy for hypercapnia and parasympathetic activation is straw breathing, a form of dead space training. Straw breathing is as simple as it sounds. Breathe through a standard drinking straw for 2-3 minutes while keeping the nose pinched closed.
What you will notice is that you are forced to slow your breathing. The inhale will be slower, but the exhale, in particular, will have to be slowed. Since the opening of the straw is small, the volume of air released will be less, and you are required to extend the exhale. This is magnified as you add straws by taping them to the end. Two to three straws are a good goal.