Flow volume loop. The âflow volume loopâ shows how much air you can forcefully exhale (forced vital capacity, or FVC). It also shows how much air is exhaled in the first second of a forced exhalation (forced expired volume in one second, or FEV-1). You will be asked to take a deep breath in and blow out as hard and as fast as you can through a mouthpiece. You will blow out until you feel there is no more air left in your lungs.