Which description best describes the relationship between pulmonary edema and oxygen diffusion?

• A. Fluid in the lungs increases oxygen diffusion
• B. Cough is the only symptom
• C. Fluid in the lungs reduces oxygen diffusion
• D. Oxygen diffusion is unaffected

Answer: (C)

Oxygen must diffuse from the alveoli into the blood across the thin alveolar-capillary barrier. Pulmonary edema fills the alveoli and interstitial space with fluid, which thickens and disrupts this barrier and reduces the effective surface area available for gas exchange. That makes it harder for oxygen to move into the bloodstream, so diffusion is impaired and oxygen levels drop.

So the description that best fits is that fluid in the lungs reduces oxygen diffusion. The other ideas don’t fit because edema does not enhance diffusion, symptoms alone (like coughing) don’t describe diffusion, and diffusion is not unaffected in edema.