Mineralizing tissues, lead distribution

Lead that enters the bloodstream of humans and experimental animals after absorption is first distributed to plasma/serum, followed by movement to erythrocyte and soft tissue binding sites with subsequent re-equilibration to longer-term binding sites in soft and mineralizing tissues. Lead entry into and removal from plasma/semm is rapid. Chamberlain et al. (1978) reported that injected radiolabeled Pb in adult volunteers entered and was removed from plasma/serum in less than 1 hour. Table 8.7 summarizes data for Pb distribution and clearance in human blood. 

Once absorbed, lead is deposited in mineral tissue such as bones and teeth, in some soft tissues such as the liver, kidneys and brain, and some is retained in the blood stream. The distribution of total body lead to the bones is about 70 per cent, increasing to 90 per cent with age. 

Mineralizing tissue in humans consists of bone and teeth. These biominerals differ in a number of ways with respect to lead deposition and lead toxicokinetics. By and large, bone is the larger repository of lead in humans and is the more complex mineralizing tissue in terms of deposited Pb. Bone Pb can be readily resorbed and serve as a source of endogenous Pb exposure long after initial transport to and deposition in the various bone subcompartments. Table 8.9 presents illustrative summaries of Pb distributions and accumulations in human mineralizing tissues. 

Until 1950, 13 mineral elements were classified as essential these comprised the major elements (calcium, phosphorus, potassium, sodium, chlorine, sulphur, magnesium) and the micro or trace elements (iron, iodine, copper, manganese, zinc and cobalt). By 1970, molybdenum, selenium, chromium and fluorine had been added to the list subsequently, arsenic, boron, lead, lithium, nickel, silicon, tin, vanadium, rubidium and aluminium have also been included, the list varying slightly according to the different authorities. Plant and animal tissues contain a further 30 mineral elements, in small quantities, for which no essential function has been found. They may be acquired from the environment, but it has been suggested that as many as 40 or more elements may have metabolic roles in mammalian tissues. Fortunately, many of these trace elements, especially those of more recent discovery, are required in such minute quantities, or are so widely distributed in foods for animals, that deficiencies are likely to be extremely rare under normal practical conditions. 

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