When the rib cage moves upward and outward and the diaphragm flattens out, what happens to breathing?

• A. Air is drawn into lungs
• B. Air pressure in the lungs rises
• C. Air is expelled from lungs
• D. There is no change in the air pressure in the lungs

Answer: (A)

When the rib cage moves upward and outward and the diaphragm flattens, the chest cavity expands and the lungs increase in volume. This larger volume lowers the air pressure inside the lungs below the outside atmospheric pressure. Air flows from where the pressure is higher (outside the body) to where it is lower (inside the lungs), so air is drawn into the lungs to begin inhalation. This mirrors the idea that increasing volume lowers pressure (Boyle’s law), driving the flow of air inward. If the lung pressure rose instead, air would be pushed out, and if there were no change in pressure, no air movement would occur.