The pathophysiology of pneumonia involves the following steps:

• Inhalation of Infectious Agents: Pneumonia typically begins when pathogenic organisms (bacteria, viruses, fungi) are inhaled or aspirated into the lower respiratory tract.

• Overcoming Lung Defenses: Ideally, the respiratory tract has defense mechanisms like mucociliary clearance and cough reflex to prevent the establishment of pathogenic organisms. If these defenses are compromised, or the pathogen is highly virulent, infection can occur.

• Inflammation and Consolidation: Once the pathogens have invaded the alveoli, they multiply and cause an inflammatory response. White blood cells (neutrophils and macrophages) migrate to the infected alveoli. This influx of cells and the replication of the causative organism leads to the consolidation seen in pneumonia. This consolidation includes an exudate of cellular debris, microorganisms, and fluid filling the alveoli and bronchioles.

• Impaired Gas Exchange: The consolidation and inflammation in the alveoli hinder the exchange of oxygen and carbon dioxide, leading to the symptoms of dyspnea and hypoxia.

• Systemic Response: The body may respond to the lung infection with fever and fatigue. Severe infections can cause septic shock if the bacteria enter the bloodstream.

• Resolution or Complication: The immune system can clear the infection with proper treatment, and the alveolar exudate can be reabsorbed or coughed up. However, complications may occur in some cases, such as abscess formation, spread of infection to the pleural cavity (pleurisy or empyema), or bacteremia.

Clinical manifestations

Here are some common symptoms associated with pneumonia:

• Cough: Typically productive, with sputum that may be purulent or even bloody. The color and consistency of the sputum can provide insights into the causative organism.
• Fever and chills: These systemic symptoms indicate an immune response to the infection. The fever may be high and accompanied by chills or rigors.
• Dyspnea and tachypnea: Patients often experience shortness of breath, which may progress to rapid breathing (tachypnea) as the body compensates for impaired gas exchange in the lungs.
• Chest pain: Pleuritic chest pain, characterized by sharp, stabbing sensations worsened by deep breaths or coughing, is a common feature due to inflammation and irritation of the pleura.
• Crackles: When auscultating the chest, crackles may be heard, indicating the presence of fluid or exudate in the alveolar spaces.
• Decreased breath sounds: In some instances, breath sounds may be diminished over areas of consolidation.
• Egophony: While listening to the lungs, a change in the resonance of voice sounds may be observed. The patient's spoken "E" may sound like "A" to the listener.
• Fatigue and weakness: These symptoms result from the redirection of the body's resources to combat the infection.
• Confusion or altered mental status: Particularly prevalent in older adults, this can sometimes be the sole indication of pneumonia in this population.
• Nausea, vomiting, or diarrhea: Gastrointestinal symptoms may occasionally be present.