Explain how air pressure changes occur in the chest cavity allowing exhalation to take place.

Refer to the roles of the intercostal muscles, rib cage and diaphragm.

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.