What happens when you breathe?

When you breathe in (inhale), your chest and lungs expand and air flows into your lungs. When you breathe out (exhale), your lungs get smaller again and air flows out of them. These breathing movements are caused by muscles – particularly the diaphragm and the muscles between the ribs (intercostal muscles). Breathing is controlled by the nervous system, but we normally breathe without thinking 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.

How is oxygen absorbed?

When you breathe in, air containing oxygen enters your windpipe, passes through the bronchi and then reaches the air sacs. These air sacs, called alveoli, look a bit like tiny 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 from the air sacs, carbon dioxide leaves the blood and passes into the air sacs. Carbon dioxide (CO2) is a waste product of reactions in the cells. You get rid of it when you breathe out. Because these two gases pass between the bloodstream and air sacs in opposite directions, this is referred to as "gas exchange" in the lungs.

Air sacs and gas exchange

Structure of the lower airways

The lower airways are made up of the windpipe (trachea) and the bronchi. In adults, the windpipe is about ten centimeters long. It branches into two main (primary) bronchi, each of which leads to a lung (one on the right side of the body, one on the left). In the lungs, these main bronchi then divide into increasingly smaller bronchi. The smaller ones are known as bronchioles.

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: The foreign particles get caught in the mucus, and the movements of the cilia carry it towards your throat like on a conveyor belt. Once it reaches your throat, you either swallow it or cough it out. As a result, the potentially harmful germs and substances are no longer harmful to you. If larger foreign objects enter the windpipe, a cough reflex is triggered.

Sources

• Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.
• Lippert H. Lehrbuch Anatomie. München: Urban und Fischer; 2020.
• Pschyrembel Online. 2023.

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