How do lungs work?

When you breathe in (inhale), air containing oxygen enters your windpipe, passes through the bronchi and eventually reaches the air sacs. These air sacs, called alveoli, 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.

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.

In adults, the windpipe (trachea) is about ten centimeters long and branches into two main bronchi known as the right bronchus and the left bronchus. These main bronchi then divide into smaller secondary bronchi (lobar bronchi) – 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 bronchi then branch into a number of tertiary bronchi (segmental bronchi). 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 bronchi 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.