1. We collect lung tissues from patients with different ages and confirm that KLK8 expression is positively correlated with age.
2. We collect peripheral blood from patients with different ages and duration of mechanical ventilation to explore the correlation between the degree of endothelial/epithelial damage, age and duration of mechanical ventilation.

1. We collect lung tissues from patients with different ages and confirm that KLK8 expression is positively correlated with age. All clinical samples have been collected and tested uniformly. Correlation analysis between KLK8 and age will be performed by using Statistical Package for the Social Sciences (SPSS) 22.0 software, and P<0.05 indicates a statistically significant difference. The obtained clinical data should be recorded on survey forms, and researchers should verify patient information by recording basic information such as age and gender.
2. We collect peripheral blood from patients with different ages and duration of mechanical ventilation to explore the correlation between the degree of endothelial/epithelial damage, age and duration of mechanical ventilation. All clinical samples have been collected and tested uniformly. Correlation analysis between endothelial/epithelial damage index, age and duration of mechanical ventilation will be performed by using SPSS 22.0 software, and P<0.05 indicates a statistically significant difference. The obtained clinical data should be recorded on survey forms, and researchers should verify patient information by recording basic information such as age and gender.

Inclusion Criteria:

1. Lung tissues from patients with different ages:

   Patients undergoing elective general anesthesia surgery; Patients undergoing partial lung resection with mechanical ventilation; American Society of Anesthesiology (ASA) grade 2-3;

2. Peripheral blood from patients with different ages and duration of mechanical ventilation:

Patients undergoing elective general anesthesia surgery with mechanical ventilation; ASA grade 2-3;

Exclusion Criteria:

1. Lung tissues from patients with different ages:

   Patients with chronic lung diseases (asthma, chronic obstructive pulmonary disease (COPD), pulmonary heart disease, etc.); Patients with human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), or receiving immunotherapy; Patients who have recently suffered from pneumonia; Recent history of mechanical ventilation; Patients with thrombotic diseases; Children and pregnant women.

2. Peripheral blood from patients with different ages and duration of mechanical ventilation:

Patients with chronic lung diseases (asthma, COPD, pulmonary heart disease, etc.); Smoking patients; Patients with hypertension/diabetes/coronary heart disease; Patients with HIV, HBV, HCV, HDV, or receiving immunotherapy; Patients who have recently suffered from pneumonia; Recent history of mechanical ventilation; Patients with thrombotic diseases; Children and pregnant women.

We started collecting lung tissues and peripheral blood from patients with mechanical ventilation from June 2023. All clinical samples have been collected and tested uniformly. Correlation analysis between KLK8 and age using SPSS 22.0 software will be performed, and P<0.05 indicates a statistically significant difference. The obtained clinical data should be recorded on survey forms, and researchers should verify patient information by recording basic information such as age and gender.