Conservation effective passive

The major effect is on the distal tubules of nephrons, where aldosterone promotes sodium retention and potassium excretion. Under the influence of aldosterone, sodium ions are actively transported out of the distal tubular cell into blood, and this transport is coupled to passive potassium flux in the opposite direction. Consequently, intracellular [Na" ] is diminished and intracellular [K+] is elevated. This intracellular diminution of [Na+] promotes the diffusion of sodium from the filtrate into the cell, and potassium diffuses into the filtrate. Aldosterone also stimulates sodium reabsorption from salivary fluid in the salivary gland and from luminal fluid in the intestines, but these sodium-conserving actions are of minor importance. 

Based on the observed lack of bioconcentration of hexabromobenzene, hepta- and octachloronaphthalene and octachlorodioxin, it has been suggested that molecules with certain spatial dimensions are not able to cross the membrane-water interface and enter the membrane (Opperhuizen et al. 1984, 1985). These molecules, which were suggested to have minimal internal cross sections (MICS) of 0.96 nm or larger, are thus believed to have a low or zero bioconcentration factor. From a mechanistic point of view it seems conceivable that a cut-off diameter exists for passive membrane diffusion. However, what this cut-off diameter exactly is remains unclear since in a recent unpublished bioconcentration experiment, brominated biphenyls with the same MICS as hexabromobenzene were accumulated in guppies. The obvious test of this effect is to perform bioconcentration experiments with conservative chemicals varying in their spatial dimensions. 

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