expiration
Expiration is the part of a breath where air leaves the lungs. Picture a stretched rubber band let go: the lungs, having been pulled open during inspiration, simply spring back toward their smaller resting size, squeezing air out as they go.
Quiet expiration is normally passive — it uses no muscular effort. When the inspiratory muscles relax, the elastic recoil of the lung tissue and the rebound of the stretched chest wall raise the pressure inside the lungs above atmospheric pressure, and air flows out down that gradient.
Forced or active expiration is different: to push air out faster or further, the abdominal muscles and internal intercostals contract, pressing the diaphragm upward and pulling the ribs down to shrink the chest more aggressively. This is what happens during exercise, coughing, blowing out candles, or speaking long phrases.
In healthy lungs, expiration is quicker and quieter than you might expect because the airways stay open. In obstructive disease, airways narrow or collapse during expiration, so emptying slows, becomes audible (wheeze), and may trap air behind the obstruction.
Blowing as hard and fast as possible into a spirometer measures forced expiration; a healthy adult can empty most of their vital capacity in the first second, a value clinicians call FEV1.
Forced expiration underlies the FEV1 measurement.
Because quiet expiration is passive, anything that weakens elastic recoil (as emphysema does by destroying lung tissue) makes emptying the lungs harder and slower, even though breathing in may still feel easy.