Transport, Distribution, Excretion, and Storage

Metals and metal compounds taken up orally are mainly absorbed in the intestinal tract. It is assumed that absorption mainly occurs by means of diffusion processes following the concentrations gradients. In addition, there exist special transport mechanisms for certain essential metal ions like iron or calcium. Active transport, in which movements occur against an increasing concentration gradient by means of metabolic energy, is effective for sodium and potassium ions. The rate of absorption is affected by the chemical form in which the metal occurs (species) and by a number of dietary and constitutional factors. 

If metals and metal compounds are inhaled, water-soluble gases and vapors are readily dissolved in the mucous membranes of the nasopharyngeal and tracheobronchial region. Less soluble gases and vapors reach the terminal airways and the alveoli and then may pass into the bloodstream or lymph stream. Deposition of inhaled particles in the airways mainly occurs by impaction, sedimentation, and diffusion. 

Once absorbed, metal ions and compounds enter the blood, mostly bound to blood cells and/or plasma proteins, which can be very specific (transferrins, ceruloplasmin). By the bloodstream metals are usually distributed throughout the body. Metallothioneins play an important role in distribution, function, detoxification, and maybe also toxicity of heavy metals [8]. There is a blood-brain barrier which can only be crossed by lipid-soluble molecules. Liver and kidney have a high capacity to bind metals. Bones and other mineralized tissues such as teeth can serve as storage organs for metals such as Ba, Be, Tl, Pb, Sr, La, Y. A number of metals have been shown to cross the placenta and to enter the fetal blood circulation. Biotransformation includes changes in the oxidation state, methylation processes, and cleavage of metal-carbon bonds. Gastrointestinal 

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