Figure 1.

Deconstruction of the dP_Na/dt—electrolyte-free water balance (EFWB) relationship. (A) The rate of change in plasma [Na⁺] or dP_Na/dt is negatively proportional to EFWB such that positive EFWB lowers plasma [Na⁺] and negative EFWB raises plasma [Na⁺]. EFWB is divided by total body water (TBW) to yield fractional dP_Na/dt and multiplied by P_[Na+K] to yield an absolute rate of change in plasma [Na⁺]. (B) Since TBW scales EFWB, a 1 L/day increase in EFWB leads to a larger fractional and absolute daily change in plasma [Na⁺] when TBW is comparatively small (10 versus 40 L).