Pneumonia

Respiratory Physiology

The respiratory system is composed of the lungs (each lung has several lobes), which exchange gas, the chest wall, and the respiratory muscles that ventilate or pump air in and out of the lungs. Air initially enters the respiratory system from the nasal passages and pharynx where it can be warmed and humidified. From there, it travels through the windpipe (trachea) into a network of continuously branching hollow passages consisting of bronchi, bronchioles, and ultimately ending in small pouches called alveoli. These alveoli are surrounded by tiny blood vessels (capillaries) that allow for exchange of gases in the blood.

The anatomy of the respiratory system contains several mechanisms to decrease the chance of infection. The upper airway consists of a sharp turn from the nose to the pharynx, forcing inhaled particles to become impacted on the mucosa and preventing transport to the trachea.

The trachea, bronchi, and terminal brochioles are lined with cells that have small cilia which can move a thin mucous layer which traps pathogens away from the lungs and into the back of the throat to be swallowed or coughed out. In addition, there are antibodies and other antimicrobial proteins secreted by the epithelial cell layer that lines the lower respiratory tract.

Definition

Pneumonia results from the infection of the airways of the lung. Pneumonia can be caused by bacteria, viral infections, or fungi. For this to occur, a pathogen must breech the defenses of the airway and establish infection.

This can occur after aspiration (inhaling particles of food, liquid, and so forth into the lungs), microaspiration of pathogens found in the mouth and throat, aerosolization of pathogens, or spread through the blood to the lungs. These open areas for air exchange are replaced by growing bacteria, fluid, white blood cells, red blood cells, and debris. This accumulation of materials leads to a decrease in the ability of the lung to expand and contract, decrease in total lung capacity and poor oxygenation leading to hypoxemia.

The location of the infection in the lung can be described in several ways. Lobar pneumonia involves an entire lobe of a lung. Bronchopneumonia involves several lobes in a patchwork fashion, with poorly defined borders between infected and noninfected lung tissue.

Interstitial pneumonia involves the actual tissue of the lungs and does not significantly affect the hollow airways. Last, military pneumonia consists of diffusely distributed discreet pockets of infection.

Pneumonia is more likely to occur in individuals who have an impaired immune system or have underlying disease. Anatomical defects in the airway that prevent removal of pathogens, dysfunction of cilia in the respiratory tract, and deficiencies in components of the immune system all predispose people to pneumonia. Vaccination with the influenza and pneumococcal vaccines can help prevent the development of certain types of pneumonia.

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