Why does the lungs not collapse?

The lungs do not collapse due to several factors that maintain their structural integrity and prevent them from collapsing. Here are the key reasons why the lungs do not collapse:

1. Pleural Pressure: The lungs are enclosed within the thoracic cavity, which is lined by two pleural membranes (visceral and parietal pleura). The pleural space between these membranes contains a thin layer of pleural fluid. The pleural pressure is the pressure within this pleural space, and it is slightly negative (lower than atmospheric pressure). This negative pressure helps maintain lung expansion by creating a force that pulls the lungs outward against the thoracic cavity walls.

2. Elastic Recoil of Lung Tissue: The lung tissue itself possesses inherent elasticity due to the presence of elastin fibers. These fibers allow the lungs to recoil and snap back to their original shape after stretching. This elastic recoil helps maintain lung expansion by pulling the lung tissue outward and counteracting the collapsing forces.

3. Surface Tension of Alveoli: The alveoli, the tiny air sacs in the lungs, have a delicate lining of surfactant, a mixture of lipids and proteins. Surfactant reduces the surface tension at the air-liquid interface within the alveoli. Lower surface tension prevents the alveoli from collapsing by counteracting the cohesive forces between water molecules that tend to cause the alveoli to stick together.

4. Intercostal Muscles and Diaphragm: The respiratory muscles, including the intercostal muscles between the ribs and the diaphragm, play a crucial role in lung expansion and preventing collapse. During inspiration, these muscles contract, expanding the thoracic cavity and creating negative pleural pressure that draws the lungs outward.

5. Atmospheric Pressure: The atmospheric pressure outside the body is higher than the pleural pressure inside the chest cavity. This pressure difference creates a force that pushes the lungs outward, helping maintain their expansion.

By working together, these factors ensure that the lungs remain expanded, allowing for efficient gas exchange during respiration without collapsing.