How do lungs work?

Our lungs are among our largest vital organs. The oxygen you breathe in goes into your lungs and passes into your blood from there. It is then transported to all the cells in your body through your bloodstream.

The lungs are located in the chest region, protected by the ribs in the rib cage. Their structure can be compared to that of an upside-down tree: The windpipe branches into two airways called , which lead to the lungs. Inside the lungs, the airways keep branching into narrower airways until the air sacs are reached.

A person’s general fitness greatly depends on how well their lungs and heart work. Your lung function can be measured using various breathing tests.

gas exchange in the lungs

What is pulmonary circulation?

When you breathe in (inhale), air containing oxygen enters your windpipe, passes through the and eventually reaches the air sacs. These air sacs, called , are responsible for gas exchange. They look a bit like grapes at the end of the bronchial branches. Healthy lungs have about 300 million air sacs in them. Each air sac is surrounded by a network of fine blood vessels (capillaries).

The oxygen in inhaled air passes across the thin lining of the air sacs and into the blood vessels. This is known as diffusion. The oxygen in the blood is then carried around the body in the bloodstream, reaching every cell. When oxygen passes into the bloodstream, carbon dioxide leaves it. Carbon dioxide (CO2) is a waste product of cellular metabolism. You get rid of it when you breathe out (exhale). This gas is transported in the opposite direction to oxygen: It passes from the bloodstream – across the lining of the air sacs – into the lungs and out into the open.

What happens when you breathe?

When you breathe in, your chest and lungs expand. When you breathe out, your lungs get smaller again. Both of these movements are caused by the diaphragm and muscles that run between the ribs (intercostal muscles). We breathe without having to think about it.

When at rest, adults breathe 14 to 16 times per minute. About half a liter of air is inhaled during one normal breath. When you are more active, your breathing becomes faster and deeper in order to get more oxygen into your blood.

The structure of the lungs

In adults, the windpipe (trachea) is about ten centimeters long and branches into two main known as the right bronchus and the left bronchus. These main then divide into smaller secondary (lobar ) – three in the right lung and two in the left lung. There is less room in the left lung because it shares space with the heart.

The secondary then branch into a number of tertiary (segmental ). The right lung is made up of ten areas known as bronchopulmonary segments. The left lung is made up of nine of these segments. Each segment is supplied by its own tertiary bronchus and its own branch of the pulmonary (lung) artery. This means that individual segments can be removed if necessary, for instance due to a serious lung disease or injury.

The windpipe and are lined with mucus-producing cells and millions of tiny hair-like projections called cilia. If you breathe in harmful substances like dust or other particles, the mucus and cilia ensure that they don’t stay in your lungs: Foreign matter gets caught in the mucus, and the cilia constantly move back and forth, carrying the mucus out of your lungs into your throat, where you either swallow it or cough it out. If larger foreign objects enter the windpipe, a cough reflex is triggered.

Andreae S. Lexikon der Krankheiten und Untersuchungen. Stuttgart: Thieme; 2008.

Menche N. (Hg.) Biologie Anatomie Physiologie. München: Urban & Fischer/ Elsevier; 2012.

Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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Updated on November 3, 2016
Next planned update: 2021


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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