<prompt> Exchange between lung capillaries and alveoli is?Facilitated diffusionPassive diffusionFiltrationActive transport  <response>: Ans. B Passive diffusionRef: Ganongs 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 