Physiological sources of variation

Physiological factors that contribute to the intra-individual variation of blood constituents have been reviewed recently (Statland and Winkel, 1977, 1981 Young, 1979 Solberg and PetitClerc, 1988). Data on the effects of physiological changes on the concentrations of trace elements in blood are scanty, but it seems inevitable that physiological factors should also have similar effects on determination of their concentrations. 

Distribution of water depends on posture standing up leads to a loss of plasma water, and to an apparent increase of approximately 10% in the concentration of nondiffusible blood constituents such as proteins or cells. Posture-dependent changes may be much more marked in disease states (e.g. Eisenberg and Wolff, 1965). 

Similar changes in water distribution take place locally, for example when a tourniquet is used for blood specimen collection (Statland and Winkel, 1981 Young, 1979 Solberg and PetitClerc, 1988). Most trace elements are transported in the blood bound to proteins or cells. Changes in posture and application of a tourniquet will therefore change their apparent concentrations. 

Exercise - even of short duration - causes changes in blood serum constituents, possibly due to leakage of intracellular components, e.g. enzymes from muscles. Continuous training may cause hemodilution this may in turn lead to apparently too low values of blood components. Changes caused by physical strain may even be seen in urinary excretion of trace elements, as exemplified by a 5-fold Increase of the urinary chromium excretion after running for 2 h (Anderson et al., 1982a). 

During pregnancy the plasma volume increases by a third this elicits changes in the concentrations of many blood components (Hytten and Leicht, 1971 Young, 1979) lac-tating may also affect serum composition, as exemplified by chromium (Anderson et al., 1993). 

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