Appendix 1
The renal handling of sodium is dictated by the response to a change in effective circulatory volume. A bedside assessment of the renal handling of body tonicity requires that only sodium (Na), potassium (K) and water be considered. Final effects on natremia (plasma sodium level) depend not only on the renal response to a change in tonicity (i.e., electrolyte and effective water clearances), but also on the rate of diuresis. P is the plasma level and U is the urinary level.
Consider: P[Na + K]and U[Na + K]
(a) U[Na + K] > P[Na + K]
Kidney is defending against a rise in tonicity, or it is responsible for a dilution of effective body tonicity.
(b) U[Na + K] < P[Na + K]
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