Which of the following is a feature of the thoracic cavity during inspiration?

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During inspiration, the thoracic cavity undergoes a physiological change characterized by an increase in volume. This occurs when the diaphragm contracts and moves downwards, while the intercostal muscles contract, lifting the rib cage upward and outward. This expansion of the thoracic cavity creates a negative pressure relative to the atmospheric pressure, allowing air to flow into the lungs.

The increase in volume during this process is crucial; it facilitates the inflow of air, allowing the lungs to expand and fill with oxygen. The mechanics of respiration hinge on the relationship between volume and pressure, as described by Boyle's Law, which states that at constant temperature, the pressure of a gas decreases as the volume increases. Therefore, during inspiration, the volume increase in the thoracic cavity drives the decrease in intrathoracic pressure, leading to air being drawn into the lungs.

This understanding highlights why the other options do not align with the physiological events of inspiration. Increased pressure would occur during expiration when the diaphragm relaxes, causing a decrease in volume. A decrease in volume is contrary to the actions of inspiration, and relaxation of muscles does not contribute to the active process of inhalation. Thus, the correct answer reflects the fundamental mechanics of breathing during the inspiration phase.

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