system
Given the following post, respond as if you're commenting on Reddit:
human
Does breathing in through the nose and out through the mouth during strenuous activity have any real effect?

I've always been told that during an exercise or exhausting activity that breathing in through the nose and out through the mouth is the best way to breath. Is there any merit to this technique, is it any better to do this than say the opposite or even in-mouth-out-mouth? Also I remember being told by a few trainers that I could over-come my asthma by doing an exercise where I would inhale to capacity, hold my breath and continue trying to inhale. I wasn't sure what this was supposed to do and felt it wasn't helpful but is there any science to back it up?
gpt
Both of the things they've mentioned have actual true physiological benefits, so get ready kids, we're doing anatomy!

Your lungs are awesome, seriously amazingly cool.  They *always* have air in them, even when you breath out as far as you can.  This is called your functional residual capactiy or FRC and is a % of total lung capacity or TLC.  It should be about ~45% of TLC in a normal healthy adult.

Now, your small airways are held open mostly by smooth muscle, thus, during strenous activity if you're asthmatic, ill, emphyesematous any number of things, they can, and do, collapse.  This can actually trap extra air in the lungs, it's called air trapping.  This increases the proportion of FRC to TLC, and that means you have less capacity available for gas exchange.


Breathing out through *pursed lips* allows you to maintain a positive airway pressure at end expiration.  This is called PEEP and helps to keep those airways from collapsing.  Breathing in through the nose smooths airflow, filters, warms and humidifies it, which protects the lungs from a number of pathogens and from drying out the mucus lining them.


Breathing in, holding, and breathing in more is a basic recruitment maneuver.  You're trying to literally pop open areas that have collapsed by holding increased pressure in the lungs, and then slightly more increasing it.  It's debatable how effective this is naturally as it's more commonly performed to ventilated patients, but there's no good evidence and it would be logistically extremely difficult to measure.