Pathophysiology

Pathophysiology of pneumonia can be explained by the following four stages:

Congestion, the the first stage occurring within the first 24 hours characterized by vascular engorgement which results into accumulation of fluid in the alveoli.

Red hepatization /Consolidation, a second stage occurring in 2 to 3 days after congestion in which the lungs become hyperemic due to extravasation of red blood cells into the alveolar spaces. This makes the lung parenchyma more solid like that of the liver and hence the term “hepatization”. This is when death typically occurs in severe cases.

Grey hepatization, a third stage in which red blood cells are broken down making the consolidated lung appear paler and drier. 

Resolution, a final stage, when the exudate is cleared by macrophages or coughed up.