Why should we only breathe through nose

Because while breathing through our nose, thanks to the hair we have around our nostrill walls, air is purified which maintains our lungs cleaner and healthier. When we breathe through our mouth, air is not purified so dust goes in which can cause damage to our throat and lungs.

Hope it helped,

BioTeacher101

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.