What happens to the thoracic cavity pressure during exhalation?

• A. It becomes lower than atmospheric pressure
• B. It increases
• C. It remains constant
• D. It decreases

Answer: (B)

During exhalation, the thoracic cavity pressure increases. This occurs because the diaphragm and intercostal muscles relax, causing the chest cavity's volume to decrease. As the volume of the thoracic cavity decreases, the pressure inside it rises according to Boyle's law, which states that pressure and volume are inversely related in a closed system. When the pressure in the thoracic cavity becomes greater than the atmospheric pressure, air is pushed out of the lungs and expelled from the body, facilitating the process of breathing.

The other options do not accurately describe the physiological changes during exhalation. For instance, if the thoracic cavity pressure were to become lower than atmospheric pressure, it would lead to air being drawn into the lungs rather than expelled. Similarly, if the pressure remained constant, it would indicate that no air movement is occurring, which is not the case during exhalation. A decrease in pressure also contradicts the mechanics of exhalation, where increased pressure is essential for pushing air out.