Pneumonia - Introduction

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Disease Process Leading to Pneumonia

Pneumonia-causing agents reach the lungs through different routes:

• In most cases, a person breathes in the infectious organism, which then travels through the airways to the lungs.
• Sometimes, the normally harmless bacteria in the mouth, or on items placed in the mouth, can enter the lungs. This usually happens if the body's "gag reflex," an extreme throat contraction that keeps substances out of the lungs, is not working properly.
• Infections can spread through the bloodstream from other organs to the lungs.

However, in normal situations, the airways protect the lungs from substances that can cause infection.

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• The nose filters out large particles.
• If smaller particles pass through, nerves along the airway prompt a cough or sneeze. This forces many particles back out of the body.
• Tiny particles that reach the small tubes in the lungs (bronchioles) are trapped in a thick, sticky substance called mucus. The mucus and particles are pushed up and out of the lungs by tiny hair-like cells called cilia, which beat like a drum. This action is called the "mucociliary escalator."
• If bacteria or other infectious organisms manage to avoid the airway's defenses, the body's immune system attacks them. Large white blood cells called macrophages destroy the foreign particles.

Click the icon to see an image of a macrophage.

The above-mentioned defense systems normally keep the lungs healthy. If these defenses are weakened or damaged, however, bacteria, viruses, fungi, and parasites can easily infect the lungs, producing pneumonia.

The Lungs

The lungs are two spongy organs in the chest surrounded by a thin, moist membrane called the pleura. Each lung is composed of smooth, shiny lobes; the right lung has three lobes and the left has two. About 90% of the lung is filled with air. Only 10% is solid tissue. There are several parts to each lung.

When a person takes a breath (inhales), air travels from the windpipe (trachea) into the lung through the main bronchus, which branches into tiny flexible tubes called bronchi.