What happens to the thoracic cavity volume during inhalation?

• A. It decreases
• B. It remains the same
• C. It increases
• D. It fluctuates

Answer: (C)

During inhalation, the chest expands as the diaphragm contracts and moves downward and the external intercostal muscles lift the rib cage. This increases the thoracic cavity volume. As the space inside grows, the pressure inside the lungs falls below the outside air pressure, so air flows in to equalize the pressure. The lungs are pulled along with the expanding chest because they are wrapped in the pleural membrane, so they also enlarge as the thoracic cavity grows. This is why the thoracic cavity volume increases during inhalation. The other options don’t fit because decreasing or remaining the same would not draw air in, and fluctuating doesn’t describe the steady expansion that creates the inhalation phase.