Hi Diana,
I can’t speak to the science, but I always breathe through my nose unless I’m really pushing hard — which I rarely do because I’ve still got a long way to go to build up my aerobic base. I feel like if I breathe through my mouth I get more oxygen, but it doesn’t go to the right places. Not sure if I can explain it better than that. As soon as I can, I switch back to breathing through my nose. I am much more comfortable breathing through my nose when running, power hiking uphill, etc.
Haley, as far as learning to breathe through your nose, you could find yoga helpful, if you’re not already a regular yoga practitioner. I fully credit yoga with ‘teaching me how to breathe’. I started doing yoga at least 3x/ week (I now do a 10-min routine to start every day to keep myself limber — I turned 50 in August) when I was 37 and it has had a profound effect on my freedom of movement — muscular, respiratory and every other way. Occasionally I think about how my body would be serving me if I hadn’t started doing yoga 13 years ago and continued to practice regularly, and I shudder to think of the alternative.
I wish you the best in figuring out what’s going on with your heart rate. My zone two top is around 135, so you can count yourself lucky! 🙂
Jane