Image an open water bottle. If you squeeze the bottle is lets air out, and if you let it expand, it draws air in.
Now, lets put two straws in the bottle. When we squeeze the bottle it pushes air out of the two straws, and when we expand the bottle it sucks air in through the two straws.
The nose and mouth are just holes, or “orifices”, similar to the straws. All the breathing happens at your lungs via your diaphragm (which is a muscle under the lungs) and your intercostal muscles (which are around the ribs), and all they do is “expand the bottle” and relax.
When you breath only through your nose, you actually just close your mouth hole with your tongue so only air comes through the nose. (Imagine putting a thumb over one of the staws). You can’t really breath through your mouth without breathing through your nose too.
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