How is air moved out of the lungs?

The movement of air out of the lungs is known as expiration or exhalation. It occurs when the pressure inside the lungs becomes higher than the pressure outside. This change in pressure difference is achieved through the coordinated actions of the respiratory muscles, primarily the diaphragm and the intercostal muscles. Here's an explanation of how air is moved out of the lungs:

1. Diaphragm Relaxation: The diaphragm, the main muscle of respiration, contracts during inhalation to pull the lungs downward and expand the chest cavity. During expiration, the diaphragm relaxes and moves upward.

2. Intercostal Muscle Relaxation: The intercostal muscles, located between the ribs, also relax during expiration. As these muscles relax, the rib cage descends and moves inward.

3. Decrease in Lung Volume: As the diaphragm and intercostal muscles relax, the volume of the chest cavity decreases. This reduction in volume creates a negative pressure within the lungs.

4. Pressure Difference: The decrease in lung volume causes the air pressure inside the lungs to increase, becoming higher than the atmospheric pressure outside. As a result, a pressure gradient is established between the lungs and the surrounding environment.

5. Expiratory Airflow: The higher pressure inside the lungs creates a force that pushes the air out through the airways. The air flows from the alveoli, through the bronchi and bronchioles, and finally out through the nose and mouth.

6. Passive Process: Compared to inhalation, expiration is generally a passive process. It does not require significant muscle effort because the elastic recoil of the lungs, along with the relaxation of the diaphragm and intercostal muscles, is sufficient to expel the air.

7. Control by Respiratory Muscles: While expiration is primarily passive, the respiratory muscles can actively control the rate and force of exhalation. For instance, during forced exhalation, such as coughing or blowing, the abdominal muscles contract and further increase the pressure within the lungs, leading to a more forceful expulsion of air.

In summary, the movement of air out of the lungs is primarily achieved through the relaxation of the diaphragm and intercostal muscles, resulting in a decrease in lung volume and a higher pressure difference between the lungs and the outside environment. This pressure gradient causes the air to flow passively from the lungs through the airways and out of the body.