During the inhalation phase of breathing, what occurs in the thoracic cavity?

During the inhalation phase of breathing, the thoracic cavity expands as the diaphragm contracts and moves downward, while the intercostal muscles pull the ribs upward and outward. This expansion of the thoracic cavity causes the pressure within the cavity to decrease relative to the atmospheric pressure outside the body. As a result of this negative pressure, air is drawn into the lungs to equalize the pressure difference. This process is critical for effective respiration, allowing oxygen-rich air to flow into the alveoli where gas exchange occurs.

In contrast, the other options describe scenarios that do not align with the physiological mechanics of inhalation. For instance, an increase in pressure within the thoracic cavity would lead to expiration, not inhalation. Additionally, relaxation of the diaphragm would not contribute to inhalation, but rather facilitate exhalation as it rises and reduces the size of the thoracic cavity. Understanding this fundamental concept of pressure dynamics is crucial for grasping how the respiratory system functions during the inhalation phase.