This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nervous System, Subject name: Physiology
Exchange between lung capillaries and alveoli is? 
Select and explain. 
A. Facilitated diffusion, B. Passive diffusion, C. Filtration, D. Active transport
B: Passive diffusion
Answer: B: Passive diffusion 
Explanation: Ans. B Passive diffusionRef: Ganong's 25th ed. pg. 634-635* Exchange of oxygen and CO2 across the lung capillary and alveoli is passive diffusion.* Diffusion of O2 occur from alveoli to capillary (towards the pressure gradient), and diffusion of CO2 is in opposite direction (from capillary to alveoli).* Facilitated diffusion requires special carrier proteins.Extra Mile* Rate of diffusion is directly proportional to:# Pressure gradient# Surface area of respiratory membrane# Solubility of gas* Rate of diffusion is inversely proportional to:# Thickness of respiratory membrane# Molecular weight of gas