Pneumonia is an infection that inflames the air sacs within one or both lungs. These air sacs, known as alveoli, can fill with fluid or pus, which can make it difficult to breathe. The infection can be caused by bacteria, viruses, or fungi, leading to swelling and fluid accumulation in the lung tissue.
The Nature of Pneumonia
Pneumonia involves inflammation of the lung’s alveoli, the tiny air sacs responsible for gas exchange. When an infection takes hold, the body’s immune response triggers inflammation, causing fluid and immune cells to accumulate in these normally air-filled spaces. This accumulation, called consolidation, impedes the efficient transfer of oxygen into the bloodstream and the removal of carbon dioxide. The presence of fluid and pus in the alveoli reduces lung capacity, leading to breathing difficulties. This process drives the disease’s progression through distinct stages as the body combats the infection.
First Stage: Congestion
The initial phase of pneumonia is congestion, typically within the first 24 hours. The bacteria or virus establishes itself, causing blood vessels in affected lung areas to engorge and dilate, leading to increased blood flow. Serous fluid, rich in infectious organisms, accumulates within the alveolar spaces. The lung tissue appears heavy, boggy, and reddish due to vascular congestion. Few red blood cells or neutrophils are present.
Second Stage: Red Hepatization
Following congestion, the lung progresses to red hepatization, starting around 48 to 72 hours after infection and lasting two to four days. During this phase, a significant influx of red blood cells, fibrin, and neutrophils enters the alveolar spaces. This exudate causes the lung tissue to become firm, dry, and airless, resembling a liver, hence “hepatization.” Numerous red blood cells contribute to the reddish-brown color of the affected tissue. This consolidation severely impairs the lung’s ability to exchange gases.
Third Stage: Gray Hepatization
The third stage, gray hepatization, emerges around four to six days after infection and persists for four to eight days. In this phase, the red blood cells accumulated during red hepatization begin to break down. The lung tissue remains firm and liver-like, but its color shifts from reddish-brown to a grayish or yellowish hue due to the breakdown of red blood cells and continued presence of fibrin and pus. Macrophages, immune cells responsible for clearing cellular debris, become more prominent in the alveolar spaces. This stage reflects ongoing inflammation and the body’s attempt to manage the infection, even as consolidation persists.
Fourth Stage: Resolution
The final stage of pneumonia is resolution, which begins around eight to ten days after the initial infection. This healing phase sees the inflammatory process subside. Enzymes within the lung tissue start to digest accumulated exudates, including fibrin and white blood cells. Macrophages actively engulf and clear away cellular debris and fluid, cleaning the alveolar spaces. As exudates are reabsorbed and cleared, the normal structure of the alveoli is gradually restored, allowing for the return of proper lung function.