Pneumonia - Introduction

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

The nose filters out large particles.
If smaller particles pass through,sensors along the airway prompt you to cough or sneeze. Thisforces 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 arepushed up and out of the lungs by tiny hair-like cells called cilia thatbeat like a drum.This action is calledthe "mucociliary escalator."

Click the icon to see an image of respiratory cilia.

If bacteria or other infectious organisms manage to avoid the airway's defenses, the body's immune systemwill attack them.Large white blood cellscalled macrophages destroy the foreign particles.

Click the icon to see an image of a macrophage.

The above-mentioneddefense systems normally keep the lung healthy. If these defenses are weakened or damaged, however, bacteria, viruses, fungi, and parasitescan easilyinfect the lung,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. Approximately 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), the air travels from the trachea (windpipe) into the lung through the main bronchus, which branches into tiny flexible tubes called bronchi.

The bronchi divide, like the branches of a tree, smaller airways called bronchioles.

The bronchioles lead to a group of miscroscopic sacs called alveoli, which look like a cluster of grapes.Each healthy adult lung contains millions of tiny alveoli. (Note: The singular of alveoli is alveolus.)

Click the icon to see an image of the lungs.

Each alveolus hasa thin membrane that allows oxygen and carbon dioxide to passin and out ofthe capillaries, the smallest of the blood vessels. When you take a deep breath,it unfolds and expands. Fresh oxygen moves into the capillaries andcarbon dioxide passes out the capillaries,then out of the body through the lungs.

Blood vessels carry the oxygen-rich blood to the heart, where it is pumped through the body.