Other Routes of Excretion
Several minor routes of excretion exist, primarily via mother's milk, sweat, saliva, tears, and semen. Excretion into milk can be important since toxicants can be passed with milk to the nursing offspring. In addition, toxic substances may be passed from cow's milk to people. Toxic substances are excreted into milk by simple diffusion. Both basic substances and lipid-soluble compounds can be excreted into milk. Basic substances can be concentrated in milk since milk is more acidic (pH ~ 6.5) than blood plasma. Since milk contains 3-4% lipids, lipid-soluble xenobiotics can diffuse along with fats from plasma into the mammary gland and thus can be present in mother's milk. Substances that are chemically similar to calcium can also be excreted into milk along with calcium. Examples of substances that can be excreted in milk are DDT, polybrominated biphenyls, and lead (which follows calcium kinetics).
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.