What causes air to flow out of the lung during expiration?

Air flows out of the lungs during expiration due to the recoil of the respiratory muscles and the elastic properties of the lungs. Here's a detailed explanation of the mechanisms involved:

1. Relaxation of Respiratory Muscles:

During inhalation, the diaphragm and intercostal muscles contract, causing the chest cavity to expand and the lungs to inflate. During expiration, these muscles relax, allowing the chest cavity to recoil to its resting position.

2. Elastic Recoil of the Lungs:

The lungs have an inherent elasticity due to the presence of elastin fibers in their connective tissue. When the lungs are inflated during inhalation, the elastin fibers are stretched. As the respiratory muscles relax during expiration, the elastin fibers recoil, causing the lungs to passively deflate and expel air.

3. Pressure Gradients:

During inhalation, the contraction of respiratory muscles creates a negative pressure (lower pressure) in the chest cavity compared to the atmospheric pressure outside. This negative pressure draws air into the lungs. During expiration, the relaxation of the muscles allows the pressure in the chest cavity to become slightly positive compared to the atmospheric pressure. This positive pressure gradient causes air to flow out of the lungs.

4. Abdominal Muscles:

The contraction of abdominal muscles during expiration can further increase the pressure in the chest cavity, contributing to the expulsion of air from the lungs.

In summary, air flows out of the lungs during expiration primarily due to the recoil of the respiratory muscles and the elastic properties of the lungs. The relaxation of the respiratory muscles, the recoil of elastin fibers, the pressure gradients, and the contraction of abdominal muscles all contribute to the process of expiration.