Pathophysiology of hyponatremia?
Hyponatremia—defined as a serum sodium concentration of less than 135 mEq/L—is a common and important electrolyte imbalance that can be seen in isolation or, as most often is the case, as a complication of other medical illnesses (eg, heart failure, liver failure, kidney failure, pneumonia).
Hyponatremia is also classified according to volume status, as follows:
Hypovolemic hyponatremia: decrease in total body water with greater decrease in total body sodium
Euvolemic hyponatremia: normal body sodium with increase in total body water
Hypervolemic hyponatremia: increase in total body sodium with greater increase in total body water
@manami Hypo-osmolality (serum osmolality < 280 mOsm/kg) always indicates excess total body water relative to body solutes or excess water relative to solute in the extracellular fluid (ECF), as water moves freely between the intracellular and the extracellular compartments. This imbalance can be due to solute depletion, solute dilution, or a combination of both.
Under normal conditions, renal handling of water is sufficient to excrete as much as 15-20 L of free water per day. Further, the body's response to a decreased osmolality is decreased thirst. Thus, hyponatremia can occur only when some condition impairs normal free water excretion. [Generally, hyponatremia is of clinical significance only when it reflects a drop in the serum osmolality (ie, hypotonic hyponatremia), which is measured directly via osmometry or is calculated as 2(Na) mEq/L + serum glucose (mg/dL)/18 + BUN (mg/dL)/2.8. Note that urea is not an effective osmole, so when the urea levels are very high, the measured osmolality should be corrected for the contribution of urea.