How your lungs and diaphragm work, in plain English

The airways

Air comes in through your nose or mouth, down your windpipe (trachea) and into two main airways called bronchi, one for each lung. These branch again and again into smaller tubes called bronchioles, which end in tiny air sacs called alveoli. You have hundreds of millions of them. Their walls are so thin that oxygen passes straight through into the blood, and carbon dioxide passes back out to be breathed away.

Your lungs have no muscle of their own to pull air in. They are soft and spongy, and they fill because the muscles around them make the chest bigger.

Breathing in: the diaphragm

The diaphragm is a dome-shaped sheet of muscle that separates your chest from your abdomen. When it contracts, it flattens and moves down, like a piston. That makes the space in your chest bigger, and air flows into your lungs to fill it. At the same time, the muscles between your ribs (the intercostals) lift the ribs up and out.

In quiet breathing, the diaphragm does most of the work. As it moves down, it pushes the contents of your abdomen down and forward, which is why your tummy rises when you breathe in. That is all “belly breathing” or diaphragmatic breathing means: letting the diaphragm do its job, rather than lifting the upper chest.

Breathing out

At rest, breathing out is mostly passive. The diaphragm and rib muscles relax, and your lungs and chest wall spring back to their resting size, pushing the air out. You only use muscles to breathe out when you need to move more air, such as during exercise, coughing, blowing or singing.

In COPD, the airways narrow and the lungs lose some of their springiness, so breathing out takes longer and some air gets trapped. That is why pursed-lip breathing, which slows the out-breath, helps.

The extra muscles

When you are breathless, other muscles in your neck, shoulders and upper chest join in to lift the ribs higher. They are useful in a pinch but tire quickly and make breathing feel effortful. If you lean forward and rest your arms on something, these muscles can work on the ribs instead of holding up your arms, and your tummy helps push the diaphragm back into a better shape. A 2024 study in people with COPD found that sitting leaning forward needed the least effort from the breathing muscles. See positions to ease breathlessness.

Who sets the rhythm?

Your breathing is controlled by your brainstem, which adjusts it mainly according to the carbon dioxide in your blood. See carbon dioxide and over-breathing. A normal, quiet breath for an adult moves about half a litre of air, 12 to 20 times a minute. See normal breathing rate.