inspiration
Inspiration is the part of a breath where you pull air in. Think of the chest as a bellows: when you enlarge the space inside, air rushes in to fill it. Unlike a balloon you blow up by pushing air, the lungs are filled by being stretched open from the outside.
It is the active phase of quiet breathing, meaning it costs muscular energy. The main worker is the diaphragm, a dome-shaped sheet of muscle under the lungs; when it contracts it flattens downward and lengthens the chest cavity. The external intercostal muscles between the ribs also contract, swinging the ribs up and out to widen the cavity.
Enlarging the chest lowers the pressure inside the lungs below the pressure of the outside air, and air flows down that pressure gradient until pressures equalize. During heavy breathing, accessory muscles in the neck and shoulders join in to enlarge the chest further and faster.
Inspiration is normally effortless and silent. When it becomes visibly hard — flaring nostrils, straining neck muscles, a harsh noise on breathing in (stridor) — it signals an obstruction or weakness that deserves attention.
At rest, an adult inhales about half a litre of air with each breath, drawn in over roughly one to one-and-a-half seconds, almost entirely by the quiet downward sweep of the diaphragm.
Quiet inspiration: a small, diaphragm-driven volume.
Inspiration is the energy-using phase; quiet expiration is mostly passive. This asymmetry matters in disease: when airways are obstructed, getting air out becomes the hard part, which is why expiration, not inspiration, is often prolonged in conditions like asthma and COPD.