What Do the Alveoli Do?

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illustration of alveoli (tiny air sacs) in lungs and blood oxygen exchange
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You already know the lungs are the primary organ of the respiratory system, and that they play an essential role in delivering oxygen to the rest of the body. The lungs also help remove gaseous waste products from the body. The process of bringing air into the body, extracting oxygen, delivering it to the tissues of the body, and removing carbon dioxide—a potentially harmful gas that’s eliminated during respiration—is complex. The alveoli are part of the lungs; they are tiny air sacs that are the primary location of gas exchange—carbon dioxide for oxygen.

Learn more about alveoli function in gas exchange and how your alveoli contribute to your body’s overall health.

Alveoli Function

You’re probably seen multiple pictures and images of the lungs. You know you have an airway, called the trachea (“windpipe”), that runs from your nose and mouth down to the lungs. At the lungs, this airway branches off into two tubes—the bronchi. One bronchus supplies the left lung; one supplies the right.

The bronchi continue to branch off into smaller and smaller tubes, called bronchioles. Picture tree roots: The main bronchi are like the large roots you see at the surface of the ground. Under the earth, each of those big roots subdivides into smaller and smaller roots. The bronchi and bronchioles of the lungs are much smaller—in fact, a bronchiole is only about as thick as one strand of hair, but they look remarkably like the roots of a tree.

At the end of each bronchiole (and there are about 30,000 of them in each lung!) is a tiny cluster of alveoli, or air sacs. They are super tiny and inflatable; the alveoli expand and contract with each breath. Each alveolus (one alveoli) is covered with a mesh of tiny blood vessels called capillaries. Oxygen molecules enter the bloodstream and carbon dioxide leaves the blood at the alveoli.

Alveoli Gas Exchange

The alveoli and capillaries share a membrane, so gases like oxygen and carbon dioxide can move freely from the alveoli into the capillaries and from the capillaries into the alveoli.

When you breathe in, air travels through your bronchi into the lungs. At the alveoli, oxygen in the inhaled air passes through that shared membrane into the capillaries. There, oxygen molecules connect to red blood cells. As the blood circulates through the body, the cells throughout all tissues and organs access this oxygen, which is necessary for proper cell function.

When cells use oxygen, they produce carbon dioxide, which can be harmful to the body in large amounts. The carbon dioxide produced by cells is transferred to the blood (picture oxygen getting off a train and carbon dioxide stepping in). When the blood returns to the lungs, the carbon dioxide crosses the membrane from the capillaries into the alveoli—and when you exhale, you breathe out the carbon dioxide.

According to Merck Manuals, about three tenths of a liter of oxygen is transferred from the alveoli to the blood each minute, and a similar amount of carbon dioxide moves from the blood to the alveoli and is exhaled. One gas is exchanged for the other.

If the alveoli don’t work well, a person may have difficulty breathing. Emphysema, for instance, is characterized by irreversibly damaged and lost alveoli. Some of the air sacs rupture, so the body cannot effectively exchange oxygen and carbon dioxide. Some people with emphysema need to use supplemental oxygen to supply cells with enough oxygen to survive.

Adult lungs normally contain between 300-500 million alveoli. Healthy alveoli are essential to a properly functioning body. Take good care of them!

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Oct 28
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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