The Mayo Clinic defines pneumonia as the inflammation of the air sacs in one or both lungs as a result of infection. Physiopedia categorizes the progression of pneumonia infections into four distinct stages: consolidation, red hepatization, gray hepatization and resolution.
The consolidation stage occurs within the first 24 hours of contracting the infection. Physiopedia explains that this stage is characterized by fluid displacing some of the air in the lungs and swelling of the pleurae. These conditions lead to coughing and difficulty breathing.
Physiopedia places red hepatization two to three days after the consolidation stage on the pneumonia timeline. Small blood vessels surrounding the air sacs in the lungs engorge with blood that leaks into the air sacs. This stage derives its name from the liver-like appearance of the blood-engorged lungs. Gray hepatization occurs up to three days after red hepatization and is named for the gray-brown color of the lungs, caused by the breakdown of the accumulated red blood cells.
The final stage is referred to as resolution. The lung's structures start to restore themselves once the body's immune defenses begin to fight the infection. Physiopedia explains that during the last stages, the patient may cough discolored or bloody mucus, known as sputum, as a mechanism to clear the lungs and allow re-aeration of air sacs.