Exhaled Air The lungs represent an important route of excretion for xenobiotics (and metabolites) that exist in a gaseous phase in the blood.  Blood gases are excreted by passive diffusion from the blood into the alveolus, following a concentration gradient.  This occurs when the concentration of the xenobiotic dissolved in capillary blood is greater than the concentration of the substance in the alveolar air.  Gases with a low solubility in blood are more rapidly eliminated than those gases with a high solubility.  Volatile liquids dissolved in the blood are also readily excreted via the expired air.  The amount of a liquid excreted by the lungs is proportional to its vapor pressure.  Exhalation is an exception to most other routes of excretion in that it can be a very efficient route of excretion for lipid soluble substances.  This is due to the very close proximity of capillary and alveolar membranes, which are thin and allow for the normal gaseous exchange that occurs in breathing.