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|Blood Urea Nitrogen||15 mg/dL|
|Serum Osmolarity||241 mOsm/kg|
|Urine Osmolality||405 mOsm/kg|
Hyponatremia means a lower than normal concentration of sodium in the blood, generally below 135 mEq/L.
Ok, now remember that total body water is distributed either in the intracellular compartment, meaning inside the cells, or the extracellular compartment, meaning outside the cells.
Both the intracellular and extracellular compartments have the same amount of solutes dissolved in that water - so the same osmolality, normally between 275 and 290 milliosmoles per kilogram.
Serum osmolality can be calculated using the formula: twice the concentration of sodium measured in milliequivalents per liter, plus the serum concentration of glucose divided by 18, measured in milligrams per deciliter, plus BUN, which stands for blood urea nitrogen, divided by 2.8, also in milligrams per deciliter - so if you remember one thing from this formula, it should be that sodium is the major determinant of serum osmolality.
Ok, now just to make matters more interesting, some of these solutes, like sodium and glucose, can’t freely cross cell membranes, so they generate an osmotic pressure inside the compartment.
This confers each compartment its tonicity - meaning how likely it is that compartment will draw water out of the other one.
However, small variations in solute concentrations, like having more sodium in the extracellular compartment, can alter that equilibrium, so water moves across cell membranes in order to restore the balance.
Water moves according to the rules of osmosis - or from the hypotonic compartment, where there’s more water, to the hypertonic compartment, where there’s less water. And water does that until the two compartments become once again isotonic.
That being said, hyponatremia, or low concentration of sodium in the extracellular fluid and therefore the blood, can be caused by either losing more sodium than water, or gaining more water than sodium - and it can develop acutely, over less than 48 hours, or chronically, over more than 48 hours.
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