Questions: Respiratory System Overview

Question 1

Which of the following best explains why air moves into the lungs during inspiration?

• A. The lungs actively expand by contracting their smooth muscle walls
• B. The diaphragm contracts and increases thoracic volume, lowering intrapulmonary pressure below atmospheric pressure so air flows in
• C. The trachea dilates and creates a low-pressure zone that draws air inward
• D. Surfactant in the alveoli generates a pressure gradient that pulls air into the airways

Answer: (B)

Inspiration is driven by Boyle's Law: when the diaphragm contracts and flattens, thoracic volume increases, which drops intrapulmonary pressure below atmospheric. Air then flows down its pressure gradient — from high (atmosphere) to low (lungs). The lungs themselves are passive; they expand because the chest wall pulls them outward.

Question 2

The diaphragm actively 'sucks' air into the lungs during inspiration, similar to how a vacuum draws in fluid.

• T. True
• F. False

Answer: False

The diaphragm does not suck air in — it contracts to enlarge the thoracic cavity, which reduces intrapulmonary pressure. Atmospheric pressure (external) then pushes air into the lungs to equalize the pressure difference. The mechanism is driven by a pressure gradient created by volume change, not by any active 'pulling' force from inside the lungs.

Question 3

Why does pulmonary surfactant reduce the work of breathing?

Model answer: Surfactant reduces surface tension in the alveolar fluid, which prevents alveoli from collapsing at the end of expiration and lowers the pressure needed to reinflate them during inspiration.

Without surfactant, the high surface tension of the liquid lining the alveoli would cause them to collapse (atelectasis) and require enormous effort to reopen. By lowering surface tension, surfactant makes the lungs more compliant — easier to inflate — and keeps alveoli partially open, preserving functional residual capacity between breaths.