Spirometry is the total amount of gas that is exhaled from the lungs after taking a deep breath. The normal lung capacity is about 3500-4000ml for men and 2500-3000ml for women, and it varies depending on gender, age, and current physical condition of the body. Generally, lung capacity is at its peak during young adulthood, and gradually decreases with age. For young children and the elderly, the lung capacity is relatively small. For patients with lung diseases, such as interstitial lung fibrosis, chronic obstructive pulmonary emphysema, pleurisy, atelectasis, pneumothorax, etc., the lung capacity of these patients is relatively smaller than that of normal healthy people.