If you take a bottle partially filled with liquid to high altitude, open it to use the contents, close the lid, and come back down, you will see the compression - the air is different up there. I've come back home to find my Nalgene (hard sided) with the sides pulled in because I sealed it shut on top of Mt Dana. Breathing is harder going up a couple thousand feet at 9 to 12,000 feet - hike on the coast from 500 feet to 5,000 feet and see how much easier it is to climb the mountain. Your physical condition doesn't change the amount of oxygen in the air. Your body can adjust over time to it, tho.

I've had a few symptoms related to altitude but don't get sick. Sometimes I have less of an appetite, once in a while I get lightheaded. But I have to be at 12,000 feet or more to get that way. Very happy about that - sulfa allergy means I can't take diamox.