How Healthy Lungs Function

Reviewed by: HU Medical Review Board | Last reviewed: July 2021.

Our lungs are a pair of organs that sit in our chest on either side of the heart. Both lungs work the same way, but the left lung is naturally slightly smaller than the right lung. Our lungs have the job of supplying the body with oxygen and removing carbon dioxide. The lungs are part of the respiratory system, which is the name for the entire system that allows us to breathe.1,2

The air we breathe in, or inhale, contains oxygen (O2). Every cell and system in our bodies needs oxygen to function. Carbon dioxide gas (CO2) is a waste product of different body processes. It is expelled from the body with exhalation, or breathing out. For the body to function in a healthy way, there must be both a regular intake of oxygen and output of carbon dioxide. This happens through the breathing process.2,3

Breathing in

When we breathe in through the nose or mouth, the air first travels down the windpipe. This windpipe is also called the trachea. This airflow continues down the windpipe until it divides into 2 different tubes that lead separately into each of the lungs. These tubes are called airways or bronchial tubes.1,2

Within the right and left lungs, these airways branch into smaller and smaller thinner tubes called bronchioles. At the ends of our bronchioles are bunches of tiny round air sacs called alveoli. Each of our lungs have millions of these air sacs.2

The walls of the air sacs are thin and stretchy. Small blood vessels, called capillaries, run along the outside. When we breathe in, the air sacs fill up like a balloon, and oxygen can pass through their walls into the surrounding blood vessels. At the same time, carbon dioxide moves from the blood vessels into the air sacs. This process is called gas exchange.2

Breathing out

After our air sacs fill up with carbon dioxide, it needs to be removed from the body. This carbon dioxide retraces the same path our breath took into the body. The carbon dioxide moves through the smaller tubes and makes it to the airways. It then goes back up the windpipe, and we exhale it through our nose or mouth.2


The diaphragm is the main muscle that controls the breathing process. It is a large dome-shaped muscle that sits below the lungs. It separates the lungs from other organs in the torso like the stomach, kidneys, or liver.2

To inhale, the diaphragm and other muscles in the torso tighten, or contract downward. This creates a slight vacuum in the chest, and our lungs expand to fill the space. To exhale, these muscles relax, and our lungs deflate to their original size, just like letting the air out of a balloon.2

How healthy lungs protect themselves

Our respiratory systems have built-in features to protect us from airborne irritants or germs. The first line of defense is the nose. It acts as a filter to prevent large particles from being inhaled into the airways.3

Smaller particles might make it past the nose. They should then get trapped in the thin layer of mucus that coats the inside of the main bronchial tubes, or airways. This mucus is sometimes called sputum or phlegm.3

The airways are lined with tiny hairs called cilia. The cilia move like waves to carry the mucus and trapped particles up toward our throats. Once it is high enough, it can be either coughed up or swallowed.3

By providing your email address, you are agreeing to our privacy policy.