Diaphragm contraction begins inspiration phase

Steps:

• Identify inspiration start: diaphragm contracts to expand thoracic cavity, increasing lung volume.
• Observe graph: volume rises from baseline as pressure falls below atmospheric.
• Locate point: where volume begins increasing and pressure starts decreasing.
• Confirm: this marks muscle contraction onset before full inhalation.

Why A is correct:

• Point A aligns with Boyle's Law, where initial volume increase lowers intrapulmonary pressure to draw air in.

Why the others are wrong:

• B: Mid-inspiration; volume already rising, contraction underway.
• C: End of inspiration; volume peaks, muscles relax soon.
• D: Expiration phase; volume decreases as diaphragm relaxes.

Final answer: A