In normal gas exchange, carbon dioxide diffuses across the endothelium of the capillaries, the interstitial space, and across the alveolar epithelium; oxygen diffuses in the other direction. Fluid accumulation interferes with gas exchange, and can cause the alveoli to fill with proteins and collapse due to edema and bleeding. The larger the area of the injury, the more severe respiratory compromise will be. Consolidation and collapse
Pulmonary contusion can cause parts of the lung to consolidate, alveoli to collapse, and atelectasis (partial or total lung collapse) to occur. Consolidation occurs when the parts of the lung that are normally filled with air fill with material from the pathological condition, such as blood. Over a period of hours after the injury, the alveoli in the injured area thicken and may become consolidated. A decrease in the amount of surfactant produced also contributes to the collapse and consolidation of alveoli; inactivation of surfactant increases their surface tension. Reduced production of surfactant can also occur in surrounding tissue that was not originally injured.Inflammation of the lungs, which can result when components of blood enter the tissue due to contusion, can also cause parts of the lung to collapse. Macrophages, neutrophils, and other inflammatory cells and blood components can enter the lung tissue and release factors that lead to inflammation, increasing the likelihood of respiratory failure. In response to inflammation, excess mucus is produced, potentially plugging parts of the lung and leading to their collapse.
Unlike other mechanisms of injury in which pulmonary contusion is often found alongside other injuries, explosions can cause pulmonary contusion without damage to the chest wall.In addition to blunt trauma, penetrating trauma can cause pulmonary contusion. Contusion resulting from penetration by a rapidly moving projectile usually surrounds the path along which the projectile traveled through the tissue. The pressure wave forces tissue out of the way, creating a temporary cavity; the tissue readily moves back into place, but it is damaged. Pulmonary contusions that accompany gun and knife wounds are not usually severe enough to have a major effect on outcome;  penetrating trauma causes less widespread lung damage than does blunt trauma. An exception is shotgun wounds, which can seriously damage large areas of lung tissue through a blast injury mechanism. Mechanism
The physical processes behind pulmonary contusion are poorly understood. However, it is known that lung tissue can be crushed when the chest wall bends inward on impact. Three other possible mechanisms have been suggested: the inertial effect, the spalling effect, and the implosion effect. In the inertial effect, the lighter alveolar tissue is sheared from the heavier hilar structures, an effect similar to diffuse axonal injury in head injury. It results from the fact that different tissues have different densities, and therefore different rates of acceleration or deceleration. In the spalling effect, lung tissue bursts or is sheared where a shock wave meets the lung tissue, at interfaces between gas and liquid.