Expiration, also known as the second phase, is breathing. The diaphragm relaxes as the lungs exhale, causing the thoracic cavity's volume to shrink and its pressure to rise. As a result, the lungs constrict and the air is ejected.
When you inhale (breath), air enters your lungs, and oxygen from that air travels to your blood. A waste gas called carbon dioxide moves from your blood to your lungs at the same time and is exhaled. Numerous thoracic and abdominal muscles are contracted during active expiration. These muscles work to reduce the size of the thoracic cavity: Anterolateral abdominal wall: This increases intra-abdominal pressure, which forces the diaphragm higher into the thoracic cavity.
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