can diffuse through a cell membrane. Other substances can be carried across by specialized transport mechanisms. Once through the cell walls at the portal of entry, they have immediate access to the circulating blood and can be transported throughout the body. The primary portals of entry for xenobiotics are the gastrointestinal tract, respiratory tract, and skin. Gastrointestinal absorption takes place primarily in the stomach and small intestine. Lipid-soluble substances and small molecules will be absorbed by