How Healthy Lungs Function

Overview of the breathing process1

The lungs have two critical jobs:

  • Breathing in to deliver oxygen into the body, which is called inhalation
  • Breathing out to remove carbon dioxide from the body, which is called exhalation

The air we inhale contains oxygen (O2 for short), a gas that provides the fuel that the body’s systems require in order to function. Carbon dioxide gas (CO2 for short) is a waste product of various body processes. It needs to be expelled from the body via exhalation. For the body to function in a healthy way, there must be both a regular intake of oxygen and the output of carbon dioxide that happens through the breathing process.


After breathing in through the nose and/or mouth, the air travels down the windpipe (called the trachea). This airflow continues down the windpipe until it divides into two different breathing tubes that lead separately into each of the two lungs. These tubes are also called “airways” or “bronchial tubes.”

Inside of the right and left lungs, the right and left airways continue to sub-divide into smaller tubes called bronchioles [BRONG-key-ols]. There are thousands of these bronchioles in each lung, each of which continue to branch off into even tinier tubes. Each of the smallest tubes eventually ends in a very tiny air sac.

These air sacs are called alveoli [al-VEE-uhl-eye]. Healthy lungs contain more than 300 million of these tiny air sacs. In fact, normal lungs actually contain many more of these alveoli than we need to breathe well. These “back-up” air sacs help us to keep breathing well, even when the lung is temporarily weakened by illness or injury.

Alveoli are the final stopping point for air in the process of inhalation. These air sacs are the place where oxygen is transferred from the air into the blood system. A web of tiny blood vessels called capillaries [KAP­ih­lare­ees] surrounds each air sac. The walls of the air sac absorb the oxygen from the air in the sac and transfer that oxygen into the blood that flows through the capillaries.


As oxygen enters the blood in the capillaries, carbon dioxide exits the bloodstream by moving through the air sac walls into the air sacs. This is called gas exchange, and is a necessary part of a healthy breathing process. During exhalation, the carbon dioxide gas then moves out of the air sacs, through the bronchioles, and finally out through the airways and windpipe.

Diaphragm: the breathing muscle1

A large, flat muscle called the diaphragm also plays an important role in the breathing process. The diaphragm sits beneath the lungs. It separates the lungs from the organs in the lower part of the torso, such as the stomach, liver, kidneys, and intestines.

During inhalation, the diaphragm muscle contracts (or tightens) by flattening downward and causing the lungs to inflate with air. During exhalation, the diaphragm muscle relaxes allowing air to flow out of the lungs.

We use energy in the diaphragm muscle when we inhale, but exhaling does not require energy. This is because the walls of the air sacs are similar to a balloon – flexible and elastic. Inhaling requires effort to fill up the air sacs. But when exhaling, the air sacs naturally deflate and bounce back to their original shape without requiring any extra effort.

How do healthy lungs defend themselves from irritants in the air?1

The first line of defense against airborne irritants is the nose. It acts as a filter to screen out large particles from being inhaled into the airways. Smaller particles of irritants might make it through, but they then get trapped in the thin layer of mucus (also known as sputum or phlegm) that coats the inside of those two main bronchial tubes.

Tiny hairs called cilia line the walls of the airways. The cilia work together to move the mucus – along with any particles that are trapped in it - up the tubes, out of the windpipe, and to the entrance of the mouth. Healthy people swallow that small amount of mucus every day without even noticing it. If the bronchial tube is particularly irritated by some particles, or an infection or chest cold, then a cough works quickly to remove the mucus and irritants.

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Written by: Anna Nicholson | Last reviewed: July 2015.