Exhalation at rest is a passive process. The external intercostal muscles relax, and the internal intercostal muscles contract, causing the rib cage to move down and inwards. At the same time, the diaphragm relaxes and domes upwards. These actions decrease the volume of the thoracic (chest) cavity. This decrease in volume leads to an increase in air pressure inside the lungs compared to the atmospheric pressure outside. As a result, air is forced out of the lungs until the pressures equalise.
The mechanics of exhalation (breathing out) are based on the relationship between volume and pressure (Boyle's Law).
1. **Muscle Relaxation:** The process begins with the relaxation of the primary muscles of inspiration. The **diaphragm** relaxes and moves upwards into a dome shape. The **external intercostal muscles** relax, allowing the **rib cage** to fall downwards and inwards (during forced exhalation, the internal intercostals contract to pull the rib cage down more forcefully).
2. **Volume Decrease:** The combined effect of the diaphragm moving up and the rib cage moving down and in is a decrease in the volume of the thoracic cavity.
3. **Pressure Increase:** According to Boyle's Law, if the volume of a container decreases, the pressure of the gas inside it increases. Therefore, the air pressure inside the lungs becomes higher than the atmospheric pressure outside the body.
4. **Air Movement:** Air naturally moves from an area of high pressure to an area of low pressure. Consequently, air is forced out of the lungs until the pressure inside and outside is equal again.
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