Diaphragm contraction begins inspiration on the pressure-volume graph

Steps:

• Identify the start of inspiration as the point where lung volume begins increasing from baseline.
• Note that diaphragm contraction expands the thoracic cavity, increasing volume and decreasing intrapulmonary pressure.
• Locate the graph point where volume rises while pressure drops below atmospheric.
• Confirm this marks the onset of muscle contraction before air inflow.

Why A is correct:

• Point A aligns with the initiation of inspiration, where diaphragm contraction follows Boyle's Law (P∝1/V), causing volume to increase and pressure to decrease.

Why the others are wrong:

• B: Occurs mid-inspiration when volume is already rising and pressure is low.
• C: Represents peak inspiration with maximum volume and lowest pressure.
• D: Marks expiration start as volume decreases and pressure rises.

Final answer: A