Low blood pressure due to volume depletion The serum sodium concentration is normally regulated by ? The balance of water intake. Renal excretion of sodium and ADH mediated water conservation by distal renal tubule. These processes are mediated by ? Stimulation of thirst Secretion of ADM Feedback mechanisms of the renin angiotensin aldosterone system and variation of renal handling of .filtered sodium. Disorders in any one of the these components of sodium balance can result in sodium imbalance. Action of ADH ADH is secreted by the posterior pituitary glandQ. Its effect in kidney is mediated by the vasopressin. (V2 receptors) on the basolateral surface of the principal cells of the collecting duct. The key action of ADH in the kidney is increasing the permeability of water. It allows water to be reabsorbed from the medullary interstitium thus enhancing water reabsorption. In SIADH the A.D.IL level is inappropriately elevated The inappropriately elevated level of vasopressin enhances the reabsomtion of water thereby leading to. - Production of concentrated urine. - Inability to excrete water and consequently - Hyponatrernia Clinical features of SIADH - HyponatrenziaQ (sodium < 135 mEq/L) - Inappropriately elevated urine osmolalityQ (> 150 mosm/kg) - Excessive urine sodium excretionQ (it Nu > 30 mEq/L) - Decreased serum osmolalityQ (< 280 mosm/kg) These findings occur in - Absence of diuretic therapy - In the presence of euvulemiaQ without edemaQ - In the setting of otherwise normal cardiac, renal, adrenal, hepatic and thyroid function. Remember, The key to pathophysiological signs, symptoms and eventual treatment of SIADH is an understanding that the hyponatremia is a result of excess water and not a sodium deficiency. Laboratory features of SIADH . HyponatrentiaQ (sodium < 135 mEq/L) . B.U.N. and serum uric acid tends to falP because of plasma dilution' and increased excretion of nitrogenous products. . Serum potassium and Bicarbonate levels are normal in SIADH (hypokalemia and metabolic acidosis suggests, diuretic therapy or vomiting) . Low serum osmolalityQ. Remember, These characteristic features of SIADH - There is increase in urinary concentration of sodium in the presence of hyponatremia. - There is increase in urine osmolarity in the presence of with decrease in serum osmolarity.