Sodium chloride is primarily restricted to the extracellular space. When sodium is ingested without water the excess sodium will remain in the extracellular space, where it will raise the plasma sodium concentration and the plasma osmolality.
The increase in osmolality will result in the osmotic movement of water out of the cells into the extracellular space until the osmolality is the same in two compartments.
As a result, the glomerular filtration rate increases, and the excess sodium and water are excreted by the kidney to restore the normal extracellular volume.
A low sodium diet leads to a transient negative sodium balance, thereby reducing extracellular volume, a process that activates the sodium-conserving mechanisms of the kidney.
Because sodium is the major extracellular cation, shifts in serum sodium levels are usually inversely correlated with the hydration state of the extracellular fluid compartment.
The pathophysiology of hyponatremia then is usually expansion of body fluids leading to excess total body water. Symptomatic hyponatremia usually does not occur until the serum is below 120 to 125 mEq/L.
In the normal individual, the kidney behaves as though extracellular volume was the regulatory stimulus articulating renal excretion with salt intake.
Extra note:
Osmotic pressure is measured in milliosmoles. When the number of particles is measured per kilogram of solvent, the correct term s osmolality, when measured per liter of solvent, the correct term is osmolarity.
Sodium in extracellular fluid
