Final answer:
The normal forced expiratory volume (FEV) is typically 75 - 85% of the vital capacity, which is crucial for diagnosing various lung diseases through spirometry such as restrictive and obstructive lung diseases.
Step-by-step explanation:
Under normal conditions, the forced expiratory volume (FEV) is normally 75 - 85% of the vital capacity. This measure reflects how much air a person can forcibly exhale from their lungs in a specific period, which is typically measured over one second (FEV1). FEV is a critical component of lung function testing, known as spirometry. The ratio of FEV1 to forced vital capacity (FVC) is crucial for diagnosing lung diseases such as asthma, emphysema, and fibrosis.
Spirometry tests can indicate whether someone has restrictive or obstructive lung disease based on the FEV1/FVC ratio. In restrictive lung disease, such as caused by lung fibrosis, the lungs become less pliable and decrease in size, so the ratio tends to be higher because the lungs can expel air quickly. In contrast, obstructive lung disease, which includes conditions like asthma, is characterized by a lower FEV1/FVC ratio due to slower expelling of air.