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Pathophysiology of pneumonia

Pneumonia is an inflammatory disease of the lungs, characterized by inflammation of the parenchyma of the lungs, or alveoli, and abnormal alveolar filling with fluid. Pneumonia can be caused by a great number of factors, including infection with bacteria, viruses, fungi, or parasites, and chemical or physical injury to the lungs.


Pneumonia is common, but very serious, disease that may occur at any point of life. It is one of the leading causes of death among children, elderly people and chronically ill. This disease is the leading cause of children’s death in undeveloped countries. According to the World Health Organization, one in three newborn infant deaths is due to pneumonia. It is estimated that somewhere 150 million new cases of pneumonia occur worldwide each year among children younger than 5 years. This means approximately 10-20 million hospitalizations. Up to 1 million of these deaths are caused by the bacteria Streptococcus pneumoniae, and vaccine preventable.

Pathophysiology of pneumonia

As already mentioned, pneumonia develops as a result of inflammation of the alveolar space. The alveoli are tiny sacs in human lungs, filled with air. The alveoli are responsible for gas exchange, and any kind of inflammation in alveolar space compromises this process. Pneumonia commonly develops as a complication of other lower respiratory infections, such as bronchiolitis or laryngotracheobronchitis. However, pneumonia may occur as a result of invasion by bacteria, viruses, or fungi, chemical injury or direct lung injury.

Bacteria are the most common cause of pneumonia. Streptococcus pneumoniae is the most common but other pathogens such as anaerobic bacteria, Staphylococcus aureus, Haemophilus influenzae, Chlamydia pneumoniae, C. psittaci, C. trachomatis, Moraxella (Branhamella) catarrhalis, Legionella pneumophila, Klebsiella pneumoniae, and other gram-negative bacilli, may also cause pneumonia.

Patophysiology of pneumonia is characterized by four stages. The first stage of the disease occurs within 24 hours of infection and it is distinguished by vascular congestion and alveolar edema. The lungs are also invaded by bacteria and few neutrophils. Typical symptoms include cough, fever, and sputum production, usually developing over days and sometimes accompanied by pleurisy.

The second stage of the disease is called “red hepatization”. In this stage lungs are similar to the consistency of liver, with the presence of many erythrocytes, neutrophils, desquamated epithelial cells, and fibrin within the alveoli.

The third stage is the “gray hepatization” in which the lung is gray-brown to yellow because of fibrinopurulent exudate, disintegration of red cells, and hemosiderin.

The fourth, and the final, stage is called “resolution” and it is characterized by breakdown of the substances causing inflammation. In this stage, white blood cells are fighting off invading micro-organisms, and the remains are usually coughed up. This stage is characterized by restoration of the pulmonary architecture.

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