In biological fluids and tissues, most metals and metalloids are not present as free cations. The blood level of elements is influenced by the level of current exposure–absorption, by their body burden and by excretion rate. In blood they are usually bound to red cells or to plasma proteins. Lead and cadmium are almost completely bound to red blood cells. The chemical elements bound to plasma proteins constitute the fraction available for transport into and out of the tissues. The passage of these elements from the blood into the interstitial fluid and intracellular compartment is fully dependent of on the diffusible fraction. The distribution of those metals present in the ionized and unbound form is directly related to the concentration gradient in the different compartments.