Pneumonia - Introduction

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

The bronchioles lead to a group of microscopic sacs called alveoli, which look like clusters 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 has a thin membrane that allows oxygen and carbon dioxide to pass in and out of the capillaries, the smallest of the blood vessels. When you take a deep breath, the membrane unfolds and expands. Fresh oxygen moves into the capillaries, and carbon dioxide passes from the capillaries into the bloodstream, where it is carried out of the body through the lungs.

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

Review Date: 04/13/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org).

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