A decrease in volume or pressure is a non-osmotic stimulus for antidiuretic hormone production in the hypothalamus, which exerts its effect in the medullary collecting duct for water reabsorption. Through unknown mechanisms, activation of the sympathetic nervous system leads to enhanced proximal tubular reabsorption of salt and water, as well as urea (BUN), calcium, uric acid, and bicarbonate. The net result of these 4 mechanisms of salt and water retention is decreased output and decreased urinary excretion of sodium (< 20mEq/L). The increased reabsorption of Na leads to increased water and urea reabsorption from the proximal tubules of the kidney back into the blood. In contrast, creatinine is actually secreted in the proximal tubule. This generally leads to a BUN:Cr ratio greater than 20, a fractional excretion of Na of less than 1%, and an elevated urine osmolarity.
A decrease in volume or pressure is a non-osmotic stimulus for antidiuretic hormone production in the hypothalamus, which exerts its effect in the medullary collecting duct for water reabsorption Through unknown mechanisms, activation of the sympathetic nervous system leads to enhanced proximal tubular reabsorption of salt and water, as well as urea (BUN), calcium uric acid, and bicarbonate. The net result of these 4 mechanisms of salt and water retention is decreased output and decreased urinary excretion of sodium (< 20mEq/L). The increased reabsorption of Na leads to increased water and urea reabsorption from the proximal tubules of the kidney back into the blood. In contrast, creatinine is actually secreted in the proximal tubule. This generally leads to a BUN:Cr ratio greater than 20, a fractional excretion of Na of less than 1%, and an elevated urine osmolarity.