Pneumonia is a lung condition wherein the parenchyma of the lung becomes inflamed. The inflammation could occur due to a bacterial, viral, or fungal infection. At times, one may develop this infection after being admitted to a hospital for the treatment of another condition. Under such circumstances, one is diagnosed with hospital-acquired pneumonia. When the infection is acquired outside a hospital, due to contact with an infected individual, one is diagnosed with community-acquired pneumonia. Aspiration pneumonia occurs due to the aspiration of a foreign object or the contents of the stomach into the lower respiratory tract. On the basis of anatomy, pneumonia is classified into lobar, lobular, interstitial, and millary pneumonia.

Causal Organisms for Pneumonia

Pneumonia is a common condition that affects about 1 out of 100 people every year. The causative organism in more than half of the cases is a bacterium called Streptococcus pneumoniae. Other bacteria that might be responsible for causing this lung condition include Hemophylus influenza, Moraxella catarrhalis, Mycoplasma pneumoniae, Legionella, Chlamydia, Klebsiella, Staphylococcus aureus, and Pseudomonas aeruginosa.
Pathophysiology of Lobar Pneumonia

Bacterial pneumonia is mainly classified into lobar and lobular penumonia. Lobar pneumonia starts in the alveoli and spreads through the pores of Kohn. On the other hand, lobular pneumonia (bronchopneumonia) starts in the terminal and respiratory bronchioles, and spreads through the bronchial walls into the alveoli.

In case of lobar pneumonia, there could be homogeneous consolidation of one or more lung lobes. On the other hand, bronchopneumonia is characterized by patchy consolidation of alveolar and bronchial inflammation, often involving both the lower lobes.
The stages of lobar pneumonia include:

➠ 24-hour congestion stage
➠ Red hepatization stage
➠ Gray hepatization stage
➠ Resolution stage

24 Hour Congestion Stage
This is the first stage that occurs within 24 hours of infection. The lung is affected by vascular congestion and alveolar edema. Microscopic examination shows the presence of many bacteria and a few neutrophils.

Red Hepatization
The red hepatization stage is observed when the red blood cells and fibrin enter the alveoli. The lung tissue becomes red and firm. This leads to difficulty in breathing or rapid breathing.

Gray Hepatization Stage
In this stage, fibrin and the dying red and white blood cells collect in the alveolar spaces. The sputum contains a tinge of blood or purulent discharge. Atelectasis, which refers to the reduction of available area within the lung for gas exchange, could also occur.

Resolution Stage
In this stage, the enzymes in the lungs digest the exudate. The white blood cells fight off the causative organisms and the remains may be coughed up.