Since different tissues or organs often react differently to a given contaminant, systemic effects are often the result of selective toxicity. The systemic effects of different contaminants were discussed briefly in the preceding discussion of the physiology of the various organ systems. Specific systemic effects associated with selected contaminants were given in Table 3.1. For example, compounds that affect receptor-ligand interactions often express themselves by endocrine or nervous system impacts, and compounds that interfere with excitable membranes typically affect the nervous system. Selective toxicity can also arise from the metabolism of a contaminant, in that certain compounds can be selectively transported or metabolized in the body and thereby concentrate in certain tissues or organs. At an extreme, some compounds are selectively cytotoxic, affecting only certain types of cells. The neurotoxicity of manganese, for example, stems from its toxic effect on certain brain cells.
For radiation, this phenomenon is known as radiosensitivity, in which different organs, tissues, or cells are more or less sensitive to the deleterious impacts of radiation. This phenomenon, observed over a century ago by French scientists, has been stated as the law of Bergonie and Tribondeau (Bergonie and Tribondeau 1906): "The radiosensitivity of a tissue is directly proportional to its reproductive capacity and inversely proportional to its degree of differentiation." Updated by modern observations, four indicators of increased radiosensitivity of cells are iden tified as a "rule of thumb", because significant exceptions are extant (DOE 1999): (1) high rate of cell division, (2) high cellular metabolic rate, (3) less differentiated cells, and (4) well-nourished cells. Application of this rule explains the high radio-sensitivity of red blood cell precursors, which have all four characteristics, and the low radiosensitivity of muscular and nerve cells, which have few or none. This variable radiosensitivity of different tissues is important in determining the overall consequences from exposure to radiation.
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