Airway resistance

Breathing means moving air through tubes, and those tubes push back. Airway resistance is, specifically, the pressure difference between the mouth and the alveoli .. the little sacs where air meets blood .. divided by flow, that is, by how much air is moving.

What changes that resistance most is the width of the tube. Inside the lung, air almost always travels in laminar fashion, in orderly layers rather than swirling, and that kind of flow follows Poiseuille's law, the formula that relates the flow of a liquid or a gas to the radius of the tube. The practical consequence is wildly out of proportion: a tube that narrows a little lets through far less air than that small narrowing would lead you to expect.

So resistance is not the same all the time. During inspiration, the lung expands and the airways expand with it, and resistance falls. During expiration, the lung deflates, the airways narrow and resistance rises.

And some things besides breathing itself narrow the tube: smoke and other inhaled irritants, arterial hypoxemia .. low oxygen in the blood .., cold air and certain medications. They all act through reflex bronchoconstriction, which is the muscle wrapped around the bronchus tightening without anyone telling it to.