Elastic fibers enhance alveolar surface area for diffusion

Steps:

• Gas exchange occurs via diffusion across the respiratory membrane, driven by partial pressure gradients.
• Evaluate options: A misstates oxygen levels in deoxygenated blood entering lungs.
• B accurately describes diffusion path but isn't the key mechanism highlighted.
• C correctly links elastic fibers to expansion and surface area increase; D reverses CO2 gradient effect during exhalation.

Why C is correct:

• Elastic fibers enable alveolar expansion during inhalation, increasing surface area for diffusion per Fick's law (diffusion rate ∝ surface area × pressure difference).

Why the others are wrong:

• A: Blood in pulmonary capillaries (from pulmonary artery) has lower O2 than alveoli, as it's deoxygenated venous blood.
• B: While diffusion occurs across those structures, the statement overlooks the unified thin respiratory membrane as the primary barrier.
• D: Exhalation lowers alveolar CO2, increasing (not reducing) the gradient for CO2 diffusion from blood.

Final answer: C