Bronchoalveolar breath sounds can be heard in the region of normal alveolar breath sounds. When lower airway resistance increases, inspiration and expiration are both difficult. Since the alveoli in the inspiratory phase are dilated and the airway diameter is relatively large, the airway resistance in the inspiratory phase is smaller than that in the expiratory phase, so the increased airway resistance is mainly manifested by prolonged expiration time. The strength of alveolar breath sounds in normal subjects is related to the depth of breathing, the elasticity of the lung tissue, the thickness of the chest wall, and the age and gender of the subject. What are the common causative factors for prolonged expiratory sounds? It is seen during chronic bronchitis and bronchial asthma attacks. In addition, prolonged expiratory sounds can also be caused by a weakening of the driving force of exhalation due to decreased elasticity of lung tissue, as seen in emphysema. Chronic bronchitis: It is a chronic non-specific inflammation of the trachea, bronchial mucosa and surrounding tissues due to infectious or non-infectious factors. Its pathology is characterized by bronchial glandular hyperplasia and increased mucus secretion. Clinical symptoms such as cough, sputum or shortness of breath are present for more than two consecutive years and last for more than three months each. Early symptoms are mild, mostly occurring in winter and remitting in spring; late inflammation increases and symptoms persist for years, regardless of season. The disease progresses and can be complicated by obstructive emphysema and pulmonary heart disease, which seriously affects labor and health. Bronchial asthma is a chronic inflammatory disease of the airways involving a variety of cells, especially mast cells, eosinophils and T-lymphocytes. In susceptible individuals, this inflammation can cause recurrent episodes of wheezing, shortness of breath, chest tightness and/or coughing, mostly at night and/or in the early morning, with increased airway responsiveness to a variety of irritants. However, symptoms may resolve on their own or with treatment. Emphysema: It is a pathological state in which the airways of the distal end fine bronchi (respiratory fine bronchi, alveolar ducts, alveolar sacs and alveoli) are hypoelastic, hyperinflated, inflated and have increased lung volume or are accompanied by destruction of the airway walls. There are several types of emphysema according to their pathogenesis: senile emphysema, compensatory emphysema, interstitial emphysema, focal emphysema, paracentral emphysema, and obstructive emphysema. The strength of alveolar breath sounds in normal subjects is related to the depth of breathing, the size of the elasticity of the lung tissue, the thickness of the chest wall, and the age and gender of the subject. Alveolar breath sounds are stronger in areas with more alveolar tissue and a thinner chest wall, the lower breast, lower scapula and lower axilla, and weaker at the lung apices and lower lung margins. Alveolar breath sounds were weaker in short and fat individuals than in long and thin individuals. Alveolar breath sounds are stronger in men than in women. Alveolar breath sounds are stronger in children than in older adults. This is because the chest wall is thinner and the alveoli are more elastic in children, whereas the alveoli are less elastic in the elderly.