expiration

• 1. the external intercostal muscles and diaphragm relax
• 2. the ribcage moves downwards and inwards, and the diaphragm becomes curved again
• 3. the volume of the thoracic cavity decreases, causing the air pressure to increase
• 4. air is forced down the pressure gradient and out of the lungs
• 5. normal expiration is a passive process - it doesn't require energy
• expiration can be forced - the external intercostal muscles relax and the internal intercostal muscles contract, pulling the ribcage further down and in. during this time, the movement of the sets of intercostal muscles is said to be antagonistic