What else is so essential, so continuous, and yet so unnoticed? When all goes well, you breathe in and out about 25,000 times each day. Every inward breath carries oxygen into your lungs, where it passes into your bloodstream and is pumped with each heartbeat throughout your body, powering the chemical energy that keeps you alive and operating. An exhalation always follows, ridding your body of a waste product: carbon dioxide.

The respiratory system is similar to a tree, branching from the huge trunk that is your windpipe into the tiniest of twiglets, the alveoli deep inside your lungs. Air arrives through your nose and mouth, passing through your pharynx (throat) and past your larynx (voice box) into the trachea (windpipe). A flap of cartilage (the epiglottis) covers the entrance to the larynx, keeping food and liquids out of your breathing passages.

The trachea branches into two main airways (the right and left mainstem bronchi) that deliver air into your lungs. (See Diagram 1, below)

Diagram 1-The Respiratory System

These branch into smaller bronchi and eventually into even smaller airways called bronchioles. (See Diagrams 1 and 2, below)

Diagram 2-The Bronchioles

At the end of each bronchiole are clusters of dozens of alveoli: small air-filled sacs surrounded by a thick layer of tiny blood vessels called capillaries. (See Diagram 2)

Here is where the gas exchange occurs: Oxygen from inhaled air travels through the thin walls of the alveoli into the capillaries, where it reaches the bloodstream. At the same time, carbon dioxide in the blood moves from the capillaries into the alveoli, where it proceeds up through the bronchi to be exhaled.

Each breath begins with a motion of the diaphragm, a muscle located in your upper abdomen, which helps to pull air into your lungs. These two organs, pinkish and spongelike, are located on either side of your chest cavity, protected from injury by the spine, the breastbone, the rib cage, and a slippery membrane (called the pleura) that lines them.

The lungs work together with the heart to provide oxygen to and to remove carbon dioxide from the blood. The pulmonary veins carry oxygen-rich blood from your lungs to the left side of your heart, which delivers it throughout your body. Returning deprived of oxygen and full of carbon dioxide, blood from the rest of your body passes through two veins called the venae cavae into the right side of the heart and is then pumped through the pulmonary artery into the lungs, where it is reoxygenated.

The lungs also defend against bacteria, viruses, and other foreign matter that enter when you breathe. Cells in the airway walls of the lungs secrete mucus that traps dust and germs. Lining the airways are small hairlike structures called cilia, which pass the mucus in a wavelike motion up through the bronchioles and into the throat, where germs are expelled when you cough or sneeze, or they are swallowed. Germs that reach all the way to the alveoli are ingested and killed by the immune system.