Albany Internal Medicine


Lungs - How They Work

The lungs are organs in your chest that deliver oxygen into the blood and remove carbon dioxide (CO2). Carbon dioxide waste product of the body's metabolism.

Lung diseases can result in the body getting too little oxygen. Lung diseases include asthma, COPD, pneumonia, sarcoidosis, and pulmonary fibrosis.

  • There are two lungs, one on each side of the chest cavity. The lungs are divided into sections called lobes. The left lung has 2 lobes, and the right lung has 2 lobes. The left lung is slightly smaller than the right lung to allow room for the heart.
  • When you breathe in, your diaphragm and intercostal muscles contract to increase the space in your chest cavity, into which your lungs expand. As your lungs expand, air is sucked in through your nose or mouth. The air travels down your windpipe and into your lungs' air sacs.
  • The lungs provide a very large surface area (the size of a football field) for the exchange of oxygen and carbon dioxide between the body and the environment. Surrounding blood vessels carry the oxygen-rich air to the rest of the body.
  • When you breathe out, your diaphragm and intercostal muscles relax to make the size of the chest cavity smaller. As the chest cavity gets smaller, air rich in carbon dioxide is forced out of your lungs and windpipe, and then out of your nose or mouth.
  • Your breathing is controlled by the base of your brain and sensors located in the brain, blood vessels, muscles, and lungs. These sensors adjust your breathing to changing needs.
  • Some people need to have a diseased lung lobe removed. However, they can still breathe well using the rest of their lung lobes.
Lung Anatomy

Figure A shows the lungs and other parts of the respiratory system. Figure B shows the alveoli, and the capillaries where oxygen is absorbed into the blood. Figure C shows the location of gas exchange between the capillaries and alveoli.

The airways carry air into and out of the lungs. Air first enters the nose or mouth, which warms and humidifies the air. (Cold, dry air can irritate the lungs.) The air then travels through the voice box (larynx) and down the windpipe (trachea). The trachea splits into two bronchi that enter your lungs.

Within the lungs, the bronchi branch into thousands of smaller, thinner tubes called bronchioles. These tubes end in bunches of tiny round air sacs called alveoli. Each of these air sacs is covered in a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.

A thin flap of tissue called the epiglottis covers the trachea during swallowing. This prevents food or drink from entering the air passages. Food or liquid that enters the trachea can lead to coughing. Some people with neurological disorders may have difficulty swallowing properly and are at risk of inhaling food our liquid into the lungs, leading to a condition called "aspiration pneumonia".

The airways have small, thin hairs called cilia that are coated with mucus. These cilia trap germs and other foreign particles that enter the airways to help prevent infection.

Muscles used for breathing

Muscles near the lungs help expand and contract (tighten) the lungs to allow breathing. These muscles include the:

  • Diaphragm. The diaphragm is the main muscle used for breathing. It is a dome-shaped muscle located below the lungs that separates the chest cavity from the abdomen.
  • Intercostal muscles. The intercostal muscles, located between the ribs, also play a major role in breathing
  • Abdominal muscles. These stomach muscle can assist breathing during periods of increased respiration, such as during exercise.
  • Muscles in the neck and collarbone area. These muscles may be used to help breathing during when other breathing muscles need additional support, such as during a severe asthma attack.

What happens when you breathe?

Breathing in (inhalation)

When you breathe in, your diaphragm contracts (tightens) and moves downward. This increases the space in your chest cavity, into which your lungs expand. The intercostal muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.

As your lungs expand, air is sucked in through your nose or mouth. The air travels down your windpipe and into your lungs. After passing through your bronchial tubes, the air finally reaches and enters the alveoli (air sacs).

Breathing out (exhalation)

When you breathe out, your diaphragm relaxes and moves upward into the chest cavity. The intercostal muscles between the ribs also relax to make the chest cavity size smaller.

As the chest cavity gets smaller, air rich in carbon dioxide is forced out of your lungs and windpipe, and then out of your nose or mouth.

Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. When you're physically active, your abdominal muscles contract and push your diaphragm even more so against your lungs. This pushes the air in your lungs out rapidly.

What controls your breathing?

A respiratory control center at the base of your brain controls your breathing. This center sends ongoing signals down your spine and to the nerves of the muscles involved in breathing.

These signals ensure your breathing muscles contract (tighten) and relax regularly. This allows your breathing to happen automatically, without you being aware of it.

To a limited degree, you can change your breathing rate, such as by breathing faster or holding your breath. Your emotions also can change your breathing. For example, being scared or angry can affect your breathing pattern.

Your breathing will change depending on how active you are and the condition of the air around you. For example, you need to breathe more often when you do physical activity. In contrast, your body needs to restrict how much air you breathe if the air contains irritants or toxins.

To adjust your breathing to changing needs, your body has a number of sensors in your brain, blood vessels, muscles, and lungs.

Sensors in the brain and in two major blood vessels (the carotid artery and the aorta) detect carbon dioxide or oxygen levels in your blood and change your breathing rate as needed.

Sensors in the airways detect lung irritants. The sensors can trigger sneezing or coughing. In people who have asthma, the sensors may cause the muscles around the airways in the lungs to contract. This makes the airways smaller.

Sensors in the alveoli (air sacs) detect a buildup of fluid in the lung tissues. These sensors are thought to trigger rapid, shallow breathing.

Sensors in your joints and muscles detect movement of your arms or legs. These sensors may play a role in increasing your breathing rate when you're physically active.

Lung diseases and conditions

Many steps are involved in breathing. If injury, disease, or other factors affect any of the steps, you may have trouble breathing.

For example, the fine hairs (cilia) that line your upper airways may not trap all of the germs you breathe in. These germs can cause an infection in your bronchi (bronchitis) or deep in your lungs (pneumonia). These infections cause a buildup of mucus and/or fluid that narrows the airways and hinders airflow in and out of your lungs.

If you have asthma, breathing in certain substances that you're sensitive to can trigger your airways to narrow. This makes it hard for air to flow in and out of your lungs.

Over a long period, breathing in cigarette smoke or air pollutants can damage the airways and the air sacs. This can lead to a condition called chronic obstructive pulmonary disease (COPD). COPD prevents proper airflow in and out of your lungs and can hinder gas exchange in the air sacs.

The airways and air sacs in the lung are normally elastic—that is, they try to spring back to their original shape after being stretched or filled with air, just the way a new rubber band or balloon would. This elastic quality helps retain the normal structure of the lung and helps to move the air quickly in and out. In COPD, much of the elastic quality is gone, and the airways and air sacs no longer bounce back to their original shape. This means that the airways collapse, like a floppy hose, and the air sacs tend to stay inflated. The floppy airways obstruct the airflow out of the lungs, leading to an abnormal increase in the lungs' size. In addition, the airways may become inflamed and thickened, and mucus-producing cells produce more mucus, further contributing to the difficulty of getting air out of the lungs.

An important step to breathing is the movement of your diaphragm and other muscles in your chest, neck, and abdomen. This movement lets you inhale and exhale. Nerves that run from your brain to these muscles control their movement. Damage to these nerves in your upper spinal cord can cause breathing to stop, unless a machine is used to help you breathe. (This machine is called a ventilator or a respirator.)

A steady flow of blood in the small blood vessels that surround your air sacs is vital for gas exchange. Long periods of inactivity or surgery can cause a blood clot called a pulmonary embolism to block your lung artery. This reduces or stops the flow of blood in the small blood vessels and interferes with gas exchange.

Reference: The National Heart, Blood, and Lung Institute.

Last updated April 27, 2017