Final answer:
Forced vital capacity is decreased in restrictive diseases like respiratory distress syndrome and pulmonary fibrosis, where the lungs are less compliant and the air gets trapped upon exhalation.
Step-by-step explanation:
The forced vital capacity (FVC) is decreased in restrictive diseases such as respiratory distress syndrome and pulmonary fibrosis. These conditions make the airways less compliant, resulting in lungs that are stiff or fibrotic and cannot expand or contract normally. In restrictive diseases, the airways collapse upon exhalation, trapping air in the lungs and significantly reducing the FVC. Additionally, the time to forcibly exhale air is prolonged. On the other hand, obstructive diseases like emphysema and asthma lead to trapped air after exhalation due to different mechanisms, such as the destruction of alveolar walls or inflammation that obstructs the airways.