What moves air out of the lings during normal relaxed exhalation?

During normal relaxed exhalation, air is moved out of the lungs primarily by the elastic recoil of the lung tissue and chest wall. The lungs and chest wall are naturally elastic and have a tendency to recoil or spring back to their resting state after being stretched. When we inhale, the muscles of inspiration (primarily the diaphragm and intercostal muscles) contract, expanding the chest cavity and lungs, and air is drawn in.

As we relax and exhale, these muscles relax, and the elastic recoil of the lungs and chest wall takes over. The lungs recoil back to their smaller resting volume, pushing the air out of the lungs through the airways. The recoil of the chest wall also contributes to exhalation by reducing the volume of the thoracic cavity and increasing the pressure inside, further aiding in the expulsion of air from the lungs.

Here's a step-by-step explanation of the process:

1. Relaxation of inspiratory muscles: During inhalation, the diaphragm and intercostal muscles contract to expand the chest cavity and lungs. As we begin to exhale, these muscles relax, allowing the chest cavity and lungs to recoil.

2. Elastic recoil of lung tissue: The lung tissue itself is elastic and has a natural tendency to recoil. As the inspiratory muscles relax, the elastic recoil of the lungs pulls them back to their smaller resting volume.

3. Elastic recoil of chest wall: The chest wall, composed of the ribs, sternum, and muscles, is also elastic and recoils back to its resting position. This movement further reduces the volume of the thoracic cavity.

4. Increased intrathoracic pressure: As the chest cavity recoils, the air pressure inside the lungs increases slightly compared to the atmospheric pressure outside. This pressure difference creates a force that pushes the air out of the lungs.

5. Expiratory airflow: The increased intrathoracic pressure generated by the elastic recoil of the lungs and chest wall propels the air out of the lungs through the airways and out of the body.

During relaxed exhalation, the force generated by the elastic recoil of the lungs and chest wall is usually sufficient to move the air out of the lungs. However, during more forceful exhalations, such as when coughing or blowing, additional muscles, called the expiratory muscles, may be recruited to assist in expelling the air.