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Breathing involves two phases: breathing in and breathing out. Your lungs deliver oxygen and remove carbon dioxide from your blood in a process called gas exchange. Gas exchange happens in the capillaries surrounding the alveoli, where the oxygen that is breathed in enters the circulatory system and carbon dioxide in the blood is released to the lungs and then breathed out. If you have problems breathing, gas exchange may be impaired, increasing the risk of serious health problems.

The body’s muscles and nervous system help control your breathing. The lungs are like sponges; they cannot move on their own. Muscles in your chest and abdomen contract, or tighten, to create space in your lungs for air to flow in. The muscles then relax, causing the space in the chest to get smaller and squeeze the air back out. Your breathing usually does not require any thought, because it is controlled by the autonomic nervous system, also called the involuntary nervous system.

The respiratory system helps you breathe. The main parts of the respiratory system are the lungs, the airways, and the muscles that enable breathing. The circulatory system, which is made up of the heart, veins, arteries, and capillaries, brings blood to and from the lungs and delivers nutrients and oxygen to tissues of the body while removing carbon dioxide and waste products. Other body systems that work with the respiratory system include the nervous system, lymph system, and immune system.