Final answer:
In obstructive lung diseases such as asthma, FEV1 decreases due to narrowed airways, resulting in a lower FEV1/FVC ratio compared to restrictive lung diseases.
Step-by-step explanation:
In obstructive lung diseases such as chronic bronchitis and asthma, the radius of the airways is decreased due to inflammation, mucus secretion, and smooth muscle spasms. This leads to increased airway resistance and decreased airflow. Affected individuals will show a decrease in the Forced Expiratory Volume in one second (FEV1)—which is the amount of air expelled in the first second of a forceful exhalation, after taking a deep breath. Subsequently, the FEV1/FVC ratio, where FVC stands for the Forced Vital Capacity or the total air expelled after a forceful exhalation, is lower in obstructive diseases than in restrictive diseases.
In the condition of asthma, for example, the FEV1/FVC ratio typically falls below 69 percent, indicating a more pronounced difficulty in expelling air due to narrowed airways compared to restrictive lung diseases where the ratio is higher (88 to 90 percent), as these diseases mainly reduce lung volume rather than airway patency. Consequently, in the context of obstructive lung disorders, the FEV1 will decrease, signifying diminished airway function.