Spirometry is a breathing test, not a picture of the lungs. After a deep breath, you blow hard into a tube attached to a spirometer. The machine records the volume of air and how quickly it leaves.
The recorded blow
The shape of the recorded blow helps a healthcare team investigate airflow problems such as asthma or COPD. A technician may ask for several attempts, because a hesitant start, a leak around the mouthpiece, or stopping early changes the result. Sometimes the test is repeated after a medicine that opens the airways to see whether the measurements change.
What the numbers mean
The numbers need context: age, body size, symptoms, medical history, and the quality of the effort all affect interpretation. Spirometry does not directly measure how much oxygen crosses from the lungs into the blood; that is a different kind of test. Some people feel briefly lightheaded or tired from blowing forcefully.
Related: how a ventilator moves air into the lungs.